The Influence of Interspecific Competition and Other Factors on the Distributionof the Barnacle Chthamalus Stellatus

Joseph H Connell

Ecology Vol 42 No 4 (Oct 1961) pp 710-723

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710 JOSEPH H COXNELL Ecology Val 42 No 4

of J~itlian villagers tolvard monkeys so that ~ i io~ikeysare no longer protected as extensively as i l l forliier years ( 2 ) intensive trapping to obtain n1onLeys for export ( 3 ) changing patterns of luitl use inclueling the tleterinration of roadsicle Iial)itats ancl coni~iiercial forest managenlent

REFERLSCES

Champion H G 19311 pre1in1i11al-y survey of the forcyt ty l~cs o f India ant1 Burma 111tlia11 Fores t IZecortls (Silviculture S e r i c s ) Vol 1 No 1

Manson-Bahr P H 1954 Mallons Tropical Dis-cac 14 1(1 Iontlon Cassell a11t1 C o

Nolte A 1955 on t l ~ e daily routine 1ield o t ) s e r v a t i o ~ ~ a ~ ~ t lsocial I ~ c l ~ a v i o s of common I n t l i a ~ ~ mollkeys with sl)ecial r e i e r e ~ ~ c eto the 13onnet monkey (diacaca

radiatu Geoffroy) Jour Bombay Nat Hist Soc 5 3 177-184

Pocock R I 1929 T h e F a u n a of British India hZamtiialia Vol 1 l rimates a l ~ t l Carnivora Lon-tloli Taylor and Francis Ltd

Prakash I 1958 Thc 111-eetling season of the rhesus n~onkey2l~ri-ni-tr i~~rrltrttcl (Zinimermann) in Rajasthan Jour R o n ~ l ~ a y S a t II is t Soc 5 5 154

P r a t e r S H 1948 T h e n o o k tti 111tlian hlatnmals 1oml)ay T h e I3otnhay Satul-a1 His tory Society

Ruch T C 1959 I)icases of 1al)oratory Pr imates Philat lcl l) l~iaV I Sauntlers Company

Couthwick C H M A Beg a n d M R Siddiqi l )(Il ~)ol)ulat ion survey of rlicsus monkeys in illage t o w l ~ s a ~ ~ t l ~ ~ o r t l ~ e r ntetni~lcy of I ~ i d i a Ecology 42 538-547

THE IsrLr1scI 01 INTERSPECII~IC COIIPETITION AND OTHER F-ACTORS ON TI-IE DISTRIBUTIOS O F TI-IE B-4RNACLE

CHTHAAIALGS S7ELLA717S

] U S E I ~ I ~11 COi KELL

Uepnrtircrtt o f Biology Itlirrcrsitg o f Califorrria Snnta Burbara Goleta Califoriiia

Nost of tlie eviclence for the occurrence of interspecific coiiiletition in aninials has been gained froni lal~oratory populations Recause of tlie siiiall anioulit of direct evitlence for its oc-currelice i l l nature competition has sometimes bee11 assignet1 a niinor role in cletermining tlie coinl)obitio~i oi aninial communities

Inclirect evitlence exists liovever vhicli sug-gests that co11il)etition Iiiay sometin~es I)e re-sl)o~isil)le for tlie tlistribution of animals in na-turc The range 11i distribution of a species may I)e decreasetl in the presence of another species ivith siniilar recjuirenients (l3eauclianip and U11-yott 1932 Elitlean Kenny and Stephenson 1956) Uniform t1istril)ution is space is usually attributed to intraspecies competition (Holtne 1950 Clark and Evans 1954) IVhen aninials with similar requireme~its sucli as 2 or more closely related species are found coexisting in tlie same area careful analysis usually indicates that they are not actually conipeting with each other (Lack 1954 iLlaclrtliur 1958)

In the course of an investigation of the animals of an intertidal rocky shore I noticed that the adults of 2 species of barnacles occupied 2 sep-arate horizontal zones with a small area of over-lap whereas the young of the species from the upper zone ere found in much of the lower zone The upper species Clzthawmlus stellatz4s (Poli) thus settled but did not survive in the

lower zone I t seenied probable that this species -as eliminated by the lower one Balanus balan- oicics ( I ~ ) in a struggle for a common requisite which lvas in short supply In the rocky inter-ticlal region since for attachment aiitl groivth is often extremely limited This paper is an account of sonie observations ant1 experiments designed to test the hypothesis that tlie al~sence in the lower zone of aclults of Chtlza~tlalztswas due to inter- specific coi~ipetition lvith Balnnlrs for space Other factors lvhicli may have influei~ced the dis- tribution -ere also studied The study was made at Llillport Isle of Cumbrae Scotland

I voultl like to thank Ijrof C 12 Yonge and the staff of the hlarine Station Llillport for their help discussions ant1 encouragen~ent during the course of this vork Thanks are due to the fol- loiving for their critical reading of the manu-script C S Elton P 12 Frank G Hardin N ( Hairston Ilt Orias T Park and his students and my wife

Distribution of the species of barnacles

The upper stellati(s hasspecies C l z t l z a ~ ~ ~ a l ~ ~ s its center of d~stribution in tlie hlediterranean it reaches its northern limit in the Shetland Is-lands north of Scotland At Jlillport adults of this species occur bet~veen the levels of mean high water of neap and spring tides (hl HVN and L1HWS see Figure 5 and Table I ) In southnest England and Ireland adult Clztlln~t-

Autumn 1961 INTERSPECIFIC COMPETITION 711

alrts occur at nloderate population densities throughout the intert~dal zone more abundantly when B a l a n l ~ s ba1nnozdrs is sparse or absent (Southward and Crisp 1954 1956) - i t Mlllport the larvae settle fro111 the plankton onto the shore nlalnly In bepteml~er and Octol~er some addi-t~ona l settlement nlaq occur until December The settleinent 1s tnost abundant between MHWS ancl mean t~cle level (MT L ) in patches of rock surface left bare as a result of the mortality of Bulanzds litnpets and other sedentary organisms Few of the Cztlzntlalzts that settle below A1H I7N s u r ~ i v e so that adults are found only oc-casionally at these levels

Balaizrrc ba1anoidcs is a boreal-arctic species reaching its southern limit in northern Spain A t Alillport it occupies allnost the entire inter- tidal region from inean lo va te r of spring tides ( h l I A K S )up to the regio~i between 1IHTVN and XHWS Above l I H TN it occurs in-termingled with Clltlzal~zalltsfor a short distance Balanlts settles on the shore in pril and May often in very dense concentrations (see Table 11)

The inain purpose of this study was to de-

other barnacle species present were Balanzis cren- a tus Brug and V e r r u c a s t roe f~z ia (0F Muller) both found In small numbers only at and below hI L11S

T o measure the survival of Chthanzalus the positions of all ~ndividuals in a patch were mapl~ed Any barnacles which were empty or missing at the next examination of this patch must have died in the interval since emigration is impossible The mapping as done by plac~ng thin glass plates (lantern slide cover glasses 107 X 82 cm area 877 cm2) over a patch of barnacles and marking the position of each Clzthar~~al l tson it ith glass-marhing ink The positions of the corners of the plate vere marLed by drilling small holes in the rock Observations made 111 subsequent censuses mere noted on a paper copy of the glass map

The study areas mere chosen by searching for patches of Clztllalllcllus belom A1HWN in

termine the cause of death of those Clztlza~~lalzrs that settled belom fHYN A s t u d ~ h i c h was being carried on at this titne had revealed that physical conditions competition for pace and pretlation by the snail Tlznis lapilllts I were among the inost ~mportant causes of tnortality of Balanzts balanoides Therefore the observations antl experinlents in the present study were de-signed to detect the effects of these factors on the survival of Clz tl lat~lalus

Af ETIIODS

Tntertidal barnacles are very nea r1 ideal for the study of survival under natural conditions Their s e d e habit allons direct observation of the survival of individuals in a group whose po- sitions have 1)een mapped Their small size and dense concentrations on rocks exposed at inter-vals make experimentation feasible In addition they tnay be handled and transplanted without injury on pieces of rock since their opercular plates remain closed hen exposed to air

The experimental area was located on the Isle of Cumbrae in the Firth of Clyde Scotland Farland Point nhere the study was made com- prises the southeast tip of the island it is ex-posed to moderate mave action The shore rock cons~sts mainly of old red sandstone arranged in a series of ridges from 2 to 6 ft high oriented at right angles to the shoreline A more detailed description is given by Connell (1961) The

a stretch of shore about 50 ft long TVhen 8 patches had been found no more were looked for The onlj basis for reject1011 of an area in this search 1vas that it contained femer than 50 C1ztlla~~zalzrsin an area of about 110 m2 Each numbered area consistetl of one or more glass lnaps located in the 110 m2 They were mapped in RIarch and April 1954 before the main set-tlement of Balu~zzisbegan in late April

1ery few Clzthar~lnllts ere found to have settled belo mid-tide level Therefore pieces of rock bearing Clztllalizallts were removed fronl l e ~ e l s above 1III S and transplanted to and 11elom I TTgt A hole as drilled through each piece it as then fastened to the rock by a stainless steel screw driven into a plastic sere

anchor fitted into a hole drilled into the rock A hole in diameter and 1 deep as found to be satisfactory The screw could be removed and replaced repeatedljr and only one stone was lost in the entire period

For censusing the stones mere renloved during a low tide period l~rought to the laboratory for examination antl returned before the tide rose again The locations and arrangements of each area are given in Table I the transplanted stones are represented by areas 11 to 15

The effect of competitiotl for space on the sur- vival of Clzthal~lalzdsmas studied in the following Inaniler After the settlement of Balanzls had stopped in early June having reached densities of 49 cni on the experimental areas (Table I ) a cerlsus of the surliving Clztlzawzalzts was made on each area (see Figure 1 ) Each tnap was then divided so that about half of the number of

Autumn 1961 INTERSPECIFIC COMPETITION 711

alrts occur at nloderate population densities throughout the intert~dal zone more abundantly when B a l a n l ~ s ba1nnozdrs is sparse or absent (Southward and Crisp 1954 1956) - i t Mlllport the larvae settle fro111 the plankton onto the shore nlalnly In bepteml~er and Octol~er some addi-t~ona l settlement nlaq occur until December The settleinent 1s tnost abundant between MHWS ancl mean t~cle level (MT L ) in patches of rock surface left bare as a result of the mortality of Bulanzds litnpets and other sedentary organisms Few of the Cztlzntlalzts that settle below A1H I7N s u r ~ i v e so that adults are found only oc-casionally at these levels

Balaizrrc ba1anoidcs is a boreal-arctic species reaching its southern limit in northern Spain A t Alillport it occupies allnost the entire inter- tidal region from inean lo va te r of spring tides ( h l I A K S )up to the regio~i between 1IHTVN and XHWS Above l I H TN it occurs in-termingled with Clltlzal~zalltsfor a short distance Balanlts settles on the shore in pril and May often in very dense concentrations (see Table 11)

The inain purpose of this study was to de-

other barnacle species present were Balanzis cren- a tus Brug and V e r r u c a s t roe f~z ia (0F Muller) both found In small numbers only at and below hI L11S

T o measure the survival of Chthanzalus the positions of all ~ndividuals in a patch were mapl~ed Any barnacles which were empty or missing at the next examination of this patch must have died in the interval since emigration is impossible The mapping as done by plac~ng thin glass plates (lantern slide cover glasses 107 X 82 cm area 877 cm2) over a patch of barnacles and marking the position of each Clzthar~~al l tson it ith glass-marhing ink The positions of the corners of the plate vere marLed by drilling small holes in the rock Observations made 111 subsequent censuses mere noted on a paper copy of the glass map

The study areas mere chosen by searching for patches of Clztllalllcllus belom A1HWN in

termine the cause of death of those Clztlza~~lalzrs that settled belom fHYN A s t u d ~ h i c h was being carried on at this titne had revealed that physical conditions competition for pace and pretlation by the snail Tlznis lapilllts I were among the inost ~mportant causes of tnortality of Balanzts balanoides Therefore the observations antl experinlents in the present study were de-signed to detect the effects of these factors on the survival of Clz tl lat~lalus

Af ETIIODS

Tntertidal barnacles are very nea r1 ideal for the study of survival under natural conditions Their s e d e habit allons direct observation of the survival of individuals in a group whose po- sitions have 1)een mapped Their small size and dense concentrations on rocks exposed at inter-vals make experimentation feasible In addition they tnay be handled and transplanted without injury on pieces of rock since their opercular plates remain closed hen exposed to air

The experimental area was located on the Isle of Cumbrae in the Firth of Clyde Scotland Farland Point nhere the study was made com- prises the southeast tip of the island it is ex-posed to moderate mave action The shore rock cons~sts mainly of old red sandstone arranged in a series of ridges from 2 to 6 ft high oriented at right angles to the shoreline A more detailed description is given by Connell (1961) The

a stretch of shore about 50 ft long TVhen 8 patches had been found no more were looked for The onlj basis for reject1011 of an area in this search 1vas that it contained femer than 50 C1ztlla~~zalzrsin an area of about 110 m2 Each numbered area consistetl of one or more glass lnaps located in the 110 m2 They were mapped in RIarch and April 1954 before the main set-tlement of Balu~zzisbegan in late April

1ery few Clzthar~lnllts ere found to have settled belo mid-tide level Therefore pieces of rock bearing Clztllalizallts were removed fronl l e ~ e l s above 1III S and transplanted to and 11elom I TTgt A hole as drilled through each piece it as then fastened to the rock by a stainless steel screw driven into a plastic sere

anchor fitted into a hole drilled into the rock A hole in diameter and 1 deep as found to be satisfactory The screw could be removed and replaced repeatedljr and only one stone was lost in the entire period

For censusing the stones mere renloved during a low tide period l~rought to the laboratory for examination antl returned before the tide rose again The locations and arrangements of each area are given in Table I the transplanted stones are represented by areas 11 to 15

The effect of competitiotl for space on the sur- vival of Clzthal~lalzdsmas studied in the following Inaniler After the settlement of Balanzls had stopped in early June having reached densities of 49 cni on the experimental areas (Table I ) a cerlsus of the surliving Clztlzawzalzts was made on each area (see Figure 1 ) Each tnap was then divided so that about half of the number of

712 JOSEPH H CONNELL Ecology Vol 42 No 4

Cl~thu~~ztrluswere in cach portion One portion vas chose11 (by flipping a coin) and those Bal-tanlrs kvhich were touclii~ig or imniediately sur-ru1111tli1igeacli Cl~tha~ricll~rsere carefully removed ivith a needle the other l~ortionwas left un-touchetl 111 this vay it vas l~ossibleto measure the effect 011 tlie survival uf Clttlztl~rltrl~tsboth of intrasl~eciiic competition alone zund of competi-ti011 ith Unlnnus I t was not possible to have the 1111l1111ersor population densities of Clztiza~za-1r1s rxactly eclual 011 the 2 portions of each area This vas tlue to the fact that since Chthawmlus ofcell occurred in groups the Haltr~trrshad to he removetl from around all the ~nenlbersof a group to elisiire that 110 crowtling I)- Bnltrzlls occurred The densities of Cl~tlzu~~tt~lt tstvere very low however so that the slight differences in density

Area no

--

MHWS

I

2

MHWN

3 s 3b

4 5

6

7a 7b

11s l l b

12a 12b

l3a 13b

14a 14b

MLWN MLWS

15 7b

The letter a following an faced c~thereast or south except

bet eel1 the 2 portions of each area can probably be disregarded intraspecitic cro~vdingwas Yery seltlon~ observed Ce~isusesof the Clzthar~~c~lzts were made at intervals of 4-6 iveeks during the next year notes vere ~iiadeat each census of factors such as crowding undercutting or smoth-ering which had taken place since the last ex-~niination Vhen necessary Balanus which had grown until they threatened to touch the Clltlz-a~naluswere removed in later examinations

T o study the effects of different degrees of immersion the areas were locatetl tllroughout the tidal range either in situ or on transplanted stones as shown in Table I Area 1 had been under observation for 1 years previously T h e effects of different degrees of wave shock could no1 be studied adequately in s ~ ~ c ha small area

Height in ft from

MTL

+49

+4 2

+3 5

+31

+2

$14 +14

$10

+07

0 0

0 0

-1 0

-2 5

-30 -5 1

$10 +071

area [lumber 7a and 7b wh~ch

Itemarks

-

Vertical partly protected

Vertical wave beaten

-

Horizontal wave beaten I r

30 to vertical pattly protected u (i

Horizontal top of a boulder partly protected

Vertical protected

Vertical protcted

Horizontal immersed in tide pool 11

Vertical wave beaten I6

45 angle wave beaten U 1

--

Chthamalua of autumn 1954 set-tlement densities of Oct 19

- -- -- - -adjave~ltarea whirh x w not encloied All area

TABLEI Description of experimental areas

1

of time aub-

merged

4

9

16

21

30

38

42

44

50 u

100 100

543

71

77 96

12

POPIJLAT~OND m a r r r nocu2 I N

---

All barnacles

undisturbed portion

-

192

-

--

309 292

--

382

493 517

820 -

188 -

295 -

-

-

i-

reIei-a to a closely

JUNE1954

Chthamalus autumn

5 5I - - -indicates that this area nay enrlowd

1953

Undisturbed portion

-

22

52

-

06 0 5

19 2 4

1 1

1 3 2 3

1 O 0 2

1 2 0 8

4 9 3 1

0 7 1 0

-

32 0 37

by a cage b

settlement

Iortion witliout Balanus ---

-

-

4 2

-

0 6 0 7

0 6 1 2

1 9

2 0 20

0ti 0 3

1 2 0 9

4 1 2 4

1 1 1 0

--

-

laced 1101th

Autumn 1961 713

NOV 3 i954 MAY 131955

II( 1 4rca 711 111 tlic tllc largc I)ar~iaclcs are Ilcllnnrrs tlic sniall o l i cfirt ~~liotogral~ll c-ttcred i l l the Ilarr l~itcllC rtlrcrr~~(rllts liitr line 1111 tlie ccolitl tliitles tlic t~tltlirtc~rl)ed portion -lllc ~ ~ h o t o ~ r a [ ~ l ~ (rixlit) ~ I - ( ~ I I I I llortio11 fl-0111 llicli I ~ I ~ J I ~ ~ ~ Y ( lcft) 4 li11111rt T I ~ I ( I ~ ( I occurstlic ere r c ~ ~ ~ o v e ( l ~~7trl(r 1111

the Icft and prctlatc~rys~iails71rrris Iirpillrts arc tis~hle

of shore I ~ u t s u c l ~ c l i f f c r e ~ ~ c c is csibtctl a r e listctl in Table I

T h e effccts of tllc pretlitor snail 7hnis lnpil- l t ~ ( ~ ~ I I O I I ~ I I I O I I S rtc~Ila ~ ro p l rn ivitll o r Clench 1 9 3 7 ) vere btr~tlic(l follovs (ages of stain1ts~ stet1 wirv netting X 111ebl1cs per inch were at tacl~e(l oer some of the areas Tllis nlesli has an ope11 area of 60 anlltl ljrevious ivork (Connell 1901) hat1 shoivn that it tlitl not in-hibit groivth o r survival of the 1)arnacles T h e cages were about 3 X 6 inches tlie roof vas allout an inch a l ~ o v e the I~arnacles atld the sides were tittetl to the irregularitie of the rock They ivere heltl in place in the same manner is the tratisl)linte(l s to~ lcs Tile trlnsl)lanted stones n-ere attachetl in [lair one of each ljair I ~ e i n g e ~ ~ c l o s e t l in a cage ( T a l ~ l e I )

These ciyes Lvere effective in exclutling all but the st11illest Thais ()ccasionally smnll T h a i s J4 to 1 cnl in length enterctl the cages through gaps a t the line of juncture of netting ant1 rock surface In the concurrent study of hnlanits ( C o n -

nell 1901) sn1iII lllnis -ere cst i~i la tr t l to have occr~rretl insitle the cages a l ~ o r ~ t 3y of the time

I1 the areah inti hto~ies ere c~sta1)lishctl I~efore the scttlrment of Rn1n~irts I)egan in late ~gtril 1954 T h u s the Cliflinlrralr(s vhich hat1 settled tiaturally oti the shore yere tlletl of the 1953 vear c1iss ind all aljout 7 months oltl Some (l~tl~c~riicrlits 1954vhicIi settletl in the au tumn of -ere folloetl until the stutly ivas entletl in June 1055 In atltlition some n-hiclia d ~ ~ l t s judging from their large size ant1 the great erosion of their sliells must have settled in 1952 o r earlier ivere present on the transplantetl stones T h u s recortls ivere made of a t least 3 year-classes of Cl~tl~a~r~crlirs

T l l c c f f f r c f s of plzysical factors I n Iltigures 2 a n d 3 the tlashetl line indicates

the survival of C-I i t l za~~~alr~s without con- g r o ~ r i ~ ~ g tact with R a l a ~ ~ w T h e suffix a indicates that tlie a rea -as protected from T1lnir by a cage

714 JOSEPII H C O N N E L L Ecology V O ~ 42 No 4

In the absence of Rcrlanz4s ind Thais ant1 pro-tected 1)y the cages from damage by ivater-l~orne objects the survival of Cllthantnlus was gootl at all levels For those which had settled normally 011 the shore ( 1ig Z) the poorest survival was ( J I I the lowest area 7a ( I n the transplanted sto~les ( Fig 3 area 12) constant imnlersion i l l a title lmol resultetl in the 1)oorest sl~rvivalThe reLsolls for the trentl toward sliglltly greater ~llortility is the degree of imlllersion increasetl are u1lknou11 Ihe anlotult of att~clletltlgae on t l ~ rstones i l l the title 1)ool v~s 11lltc11greater tl1~11011 tlie other areas This h~veretlltcetlI I I ~ ~

the flax of vLter a11tl footl or havc interfered tlirectly ~vitli feetli~lgnlove~llc~ntsnother 110s-sil)le intlirect effect o f i~lcrrasetlin l~~~ers io t lis the itlc~rr~sei l l I)y the s~ l ~ i l ~)retl~tiotl Thcris Itrpillrt~ it lower levels

Clrflrir~~~irl~rs of nlttc11 greater tle-is toler~~it i

grec of it~lmersioll t lnn it nornlally ellcotu~ters T l ~ i sis shown I)y the sl~rviv~lfor L year on area 12 i t 1 L title ~ )oo l togetller with the findings of Iiscller 1102X ) atltl Iarncs ( 1050~) ivho fottnd tl1~t Clztlrtr~rlnlrt~witllstootl sul)tllersion for 12 i~l t l 22 nlo~lthsresl)cctively I t s iI)sellce 1)elow 81r1 can ~)rol)al)lyI)e iscril)ed either to i lack of illitin1 settlcnle~ltor to poor survival of llevly settletl larvae I ewis ant1 Iovrll (1960j Ilave suggestrtl that tllc survivil of Clrtl1crrrrcrlus 1 1 1 ~l)e

f~voretl 1)y increasetl light or varmtli during enlersion it1 its early life on the shore These con-tlitions woultl tend to occur higher on the shore it1 Scotla~ltlthali in southern Inglantl

711e effects of wave action on the survival of (htlrc~r~ralltsire difficult to assess Tike the de-gree of imnlersion the effects uf ne aztion nlay act intlirectly 7lie areas 7 ant1 12 where rel-atively poor survival was foun(l vere also the ireis of Irnst rive actioti ilthough Clltllnmalus is usllally al)ttntlint on Lvavr 1)eaten areas and il)se~ltfro111sl~elteretlhays in Scotlantl Lewis and Iovcll (1900) have sliovn that in certain shel-tcretl Inys it nlay I)e very il)undatit l-Tatton ( 193) fount1 that in ~lorthernfrance settlement ~ ~ l t lgrovth rates were greater in wave-heaten ireas at ITl l ~ t at AIHVX greater in sl~cltcretlirtais

t tlie ul)l)er shore nlargins of t1istril)ution Cl~thirrr~trl~~s call cxist higher tliall I3al-evit le~~tly rzrlrrs 111~inlyas a result of its greater tolerance to heat a ~ ~ t l o rclesiccatioli Ihe evidence for this as g~inctltlltring the s l ~ r i ~ i gof 1955 Recortls frotll a title i11tl wave guage ol)erating at this tinw il)ottt one-half mile 11ortIi of tlie study area hllowetl tllnt i 1)eriotl of 11eal)titles hat1 coincitled vith ill lu l~ts l~al~)eriotlof Lvarln ctlm iveather in l)ril so that for several (lays no ~vater not even vlves reiclictl the level of rea 1 111 the 1)eriotl

BILANUS REYOVEO ---6 0 -4 0 - UNMODlFlEO

I- UNY001FlED

W2 0 ----------- - ---BALANUS REMOVED

aa BALANVS REMOVED

t75 3b r 2 2 I 2

L

----------_______ BALANUS REMOVED -------- - - - -______B A L M U S REMOVED --------------

BALAWUS REMOVED

W UNMODIFIED a 1 10 3 -Z -

-5

J J A S O N D J F M A M J L~

FIG 2 Survivorship curves of Chthutilalits stellatzrs which had settled naturally on the shore in the autumn of 1953 Areas designated a were protected from predation by cages In each area the sur-vival of Chthninolzts growing without contact with Ralunus is compared to that in the undisturbed area For each area the vertical distance in feet from MTL is shown

Autumn 1961 713

NOV 3 i954 MAY 131955

II( 1 4rca 711 111 tlic tllc largc I)ar~iaclcs are Ilcllnnrrs tlic sniall o l i cfirt ~~liotogral~ll c-ttcred i l l the Ilarr l~itcllC rtlrcrr~~(rllts liitr line 1111 tlie ccolitl tliitles tlic t~tltlirtc~rl)ed portion -lllc ~ ~ h o t o ~ r a [ ~ l ~ (rixlit) ~ I - ( ~ I I I I llortio11 fl-0111 llicli I ~ I ~ J I ~ ~ ~ Y ( lcft) 4 li11111rt T I ~ I ( I ~ ( I occurstlic ere r c ~ ~ ~ o v e ( l ~~7trl(r 1111

the Icft and prctlatc~rys~iails71rrris Iirpillrts arc tis~hle

of shore I ~ u t s u c l ~ c l i f f c r e ~ ~ c c is csibtctl a r e listctl in Table I

T h e effccts of tllc pretlitor snail 7hnis lnpil- l t ~ ( ~ ~ I I O I I ~ I I I O I I S rtc~Ila ~ ro p l rn ivitll o r Clench 1 9 3 7 ) vere btr~tlic(l follovs (ages of stain1ts~ stet1 wirv netting X 111ebl1cs per inch were at tacl~e(l oer some of the areas Tllis nlesli has an ope11 area of 60 anlltl ljrevious ivork (Connell 1901) hat1 shoivn that it tlitl not in-hibit groivth o r survival of the 1)arnacles T h e cages were about 3 X 6 inches tlie roof vas allout an inch a l ~ o v e the I~arnacles atld the sides were tittetl to the irregularitie of the rock They ivere heltl in place in the same manner is the tratisl)linte(l s to~ lcs Tile trlnsl)lanted stones n-ere attachetl in [lair one of each ljair I ~ e i n g e ~ ~ c l o s e t l in a cage ( T a l ~ l e I )

These ciyes Lvere effective in exclutling all but the st11illest Thais ()ccasionally smnll T h a i s J4 to 1 cnl in length enterctl the cages through gaps a t the line of juncture of netting ant1 rock surface In the concurrent study of hnlanits ( C o n -

nell 1901) sn1iII lllnis -ere cst i~i la tr t l to have occr~rretl insitle the cages a l ~ o r ~ t 3y of the time

I1 the areah inti hto~ies ere c~sta1)lishctl I~efore the scttlrment of Rn1n~irts I)egan in late ~gtril 1954 T h u s the Cliflinlrralr(s vhich hat1 settled tiaturally oti the shore yere tlletl of the 1953 vear c1iss ind all aljout 7 months oltl Some (l~tl~c~riicrlits 1954vhicIi settletl in the au tumn of -ere folloetl until the stutly ivas entletl in June 1055 In atltlition some n-hiclia d ~ ~ l t s judging from their large size ant1 the great erosion of their sliells must have settled in 1952 o r earlier ivere present on the transplantetl stones T h u s recortls ivere made of a t least 3 year-classes of Cl~tl~a~r~crlirs

T l l c c f f f r c f s of plzysical factors I n Iltigures 2 a n d 3 the tlashetl line indicates

the survival of C-I i t l za~~~alr~s without con- g r o ~ r i ~ ~ g tact with R a l a ~ ~ w T h e suffix a indicates that tlie a rea -as protected from T1lnir by a cage

714 JOSEPII H C O N N E L L Ecology V O ~ 42 No 4

In the absence of Rcrlanz4s ind Thais ant1 pro-tected 1)y the cages from damage by ivater-l~orne objects the survival of Cllthantnlus was gootl at all levels For those which had settled normally 011 the shore ( 1ig Z) the poorest survival was ( J I I the lowest area 7a ( I n the transplanted sto~les ( Fig 3 area 12) constant imnlersion i l l a title lmol resultetl in the 1)oorest sl~rvivalThe reLsolls for the trentl toward sliglltly greater ~llortility is the degree of imlllersion increasetl are u1lknou11 Ihe anlotult of att~clletltlgae on t l ~ rstones i l l the title 1)ool v~s 11lltc11greater tl1~11011 tlie other areas This h~veretlltcetlI I I ~ ~

the flax of vLter a11tl footl or havc interfered tlirectly ~vitli feetli~lgnlove~llc~ntsnother 110s-sil)le intlirect effect o f i~lcrrasetlin l~~~ers io t lis the itlc~rr~sei l l I)y the s~ l ~ i l ~)retl~tiotl Thcris Itrpillrt~ it lower levels

Clrflrir~~~irl~rs of nlttc11 greater tle-is toler~~it i

grec of it~lmersioll t lnn it nornlally ellcotu~ters T l ~ i sis shown I)y the sl~rviv~lfor L year on area 12 i t 1 L title ~ )oo l togetller with the findings of Iiscller 1102X ) atltl Iarncs ( 1050~) ivho fottnd tl1~t Clztlrtr~rlnlrt~witllstootl sul)tllersion for 12 i~l t l 22 nlo~lthsresl)cctively I t s iI)sellce 1)elow 81r1 can ~)rol)al)lyI)e iscril)ed either to i lack of illitin1 settlcnle~ltor to poor survival of llevly settletl larvae I ewis ant1 Iovrll (1960j Ilave suggestrtl that tllc survivil of Clrtl1crrrrcrlus 1 1 1 ~l)e

f~voretl 1)y increasetl light or varmtli during enlersion it1 its early life on the shore These con-tlitions woultl tend to occur higher on the shore it1 Scotla~ltlthali in southern Inglantl

711e effects of wave action on the survival of (htlrc~r~ralltsire difficult to assess Tike the de-gree of imnlersion the effects uf ne aztion nlay act intlirectly 7lie areas 7 ant1 12 where rel-atively poor survival was foun(l vere also the ireis of Irnst rive actioti ilthough Clltllnmalus is usllally al)ttntlint on Lvavr 1)eaten areas and il)se~ltfro111sl~elteretlhays in Scotlantl Lewis and Iovcll (1900) have sliovn that in certain shel-tcretl Inys it nlay I)e very il)undatit l-Tatton ( 193) fount1 that in ~lorthernfrance settlement ~ ~ l t lgrovth rates were greater in wave-heaten ireas at ITl l ~ t at AIHVX greater in sl~cltcretlirtais

t tlie ul)l)er shore nlargins of t1istril)ution Cl~thirrr~trl~~s call cxist higher tliall I3al-evit le~~tly rzrlrrs 111~inlyas a result of its greater tolerance to heat a ~ ~ t l o rclesiccatioli Ihe evidence for this as g~inctltlltring the s l ~ r i ~ i gof 1955 Recortls frotll a title i11tl wave guage ol)erating at this tinw il)ottt one-half mile 11ortIi of tlie study area hllowetl tllnt i 1)eriotl of 11eal)titles hat1 coincitled vith ill lu l~ts l~al~)eriotlof Lvarln ctlm iveather in l)ril so that for several (lays no ~vater not even vlves reiclictl the level of rea 1 111 the 1)eriotl

BILANUS REYOVEO ---6 0 -4 0 - UNMODlFlEO

I- UNY001FlED

W2 0 ----------- - ---BALANUS REMOVED

aa BALANVS REMOVED

t75 3b r 2 2 I 2

L

----------_______ BALANUS REMOVED -------- - - - -______B A L M U S REMOVED --------------

BALAWUS REMOVED

W UNMODIFIED a 1 10 3 -Z -

-5

J J A S O N D J F M A M J L~

FIG 2 Survivorship curves of Chthutilalits stellatzrs which had settled naturally on the shore in the autumn of 1953 Areas designated a were protected from predation by cages In each area the sur-vival of Chthninolzts growing without contact with Ralunus is compared to that in the undisturbed area For each area the vertical distance in feet from MTL is shown

714 JOSEPII H C O N N E L L Ecology V O ~ 42 No 4

In the absence of Rcrlanz4s ind Thais ant1 pro-tected 1)y the cages from damage by ivater-l~orne objects the survival of Cllthantnlus was gootl at all levels For those which had settled normally 011 the shore ( 1ig Z) the poorest survival was ( J I I the lowest area 7a ( I n the transplanted sto~les ( Fig 3 area 12) constant imnlersion i l l a title lmol resultetl in the 1)oorest sl~rvivalThe reLsolls for the trentl toward sliglltly greater ~llortility is the degree of imlllersion increasetl are u1lknou11 Ihe anlotult of att~clletltlgae on t l ~ rstones i l l the title 1)ool v~s 11lltc11greater tl1~11011 tlie other areas This h~veretlltcetlI I I ~ ~

the flax of vLter a11tl footl or havc interfered tlirectly ~vitli feetli~lgnlove~llc~ntsnother 110s-sil)le intlirect effect o f i~lcrrasetlin l~~~ers io t lis the itlc~rr~sei l l I)y the s~ l ~ i l ~)retl~tiotl Thcris Itrpillrt~ it lower levels

Clrflrir~~~irl~rs of nlttc11 greater tle-is toler~~it i

grec of it~lmersioll t lnn it nornlally ellcotu~ters T l ~ i sis shown I)y the sl~rviv~lfor L year on area 12 i t 1 L title ~ )oo l togetller with the findings of Iiscller 1102X ) atltl Iarncs ( 1050~) ivho fottnd tl1~t Clztlrtr~rlnlrt~witllstootl sul)tllersion for 12 i~l t l 22 nlo~lthsresl)cctively I t s iI)sellce 1)elow 81r1 can ~)rol)al)lyI)e iscril)ed either to i lack of illitin1 settlcnle~ltor to poor survival of llevly settletl larvae I ewis ant1 Iovrll (1960j Ilave suggestrtl that tllc survivil of Clrtl1crrrrcrlus 1 1 1 ~l)e

f~voretl 1)y increasetl light or varmtli during enlersion it1 its early life on the shore These con-tlitions woultl tend to occur higher on the shore it1 Scotla~ltlthali in southern Inglantl

711e effects of wave action on the survival of (htlrc~r~ralltsire difficult to assess Tike the de-gree of imnlersion the effects uf ne aztion nlay act intlirectly 7lie areas 7 ant1 12 where rel-atively poor survival was foun(l vere also the ireis of Irnst rive actioti ilthough Clltllnmalus is usllally al)ttntlint on Lvavr 1)eaten areas and il)se~ltfro111sl~elteretlhays in Scotlantl Lewis and Iovcll (1900) have sliovn that in certain shel-tcretl Inys it nlay I)e very il)undatit l-Tatton ( 193) fount1 that in ~lorthernfrance settlement ~ ~ l t lgrovth rates were greater in wave-heaten ireas at ITl l ~ t at AIHVX greater in sl~cltcretlirtais

t tlie ul)l)er shore nlargins of t1istril)ution Cl~thirrr~trl~~s call cxist higher tliall I3al-evit le~~tly rzrlrrs 111~inlyas a result of its greater tolerance to heat a ~ ~ t l o rclesiccatioli Ihe evidence for this as g~inctltlltring the s l ~ r i ~ i gof 1955 Recortls frotll a title i11tl wave guage ol)erating at this tinw il)ottt one-half mile 11ortIi of tlie study area hllowetl tllnt i 1)eriotl of 11eal)titles hat1 coincitled vith ill lu l~ts l~al~)eriotlof Lvarln ctlm iveather in l)ril so that for several (lays no ~vater not even vlves reiclictl the level of rea 1 111 the 1)eriotl

BILANUS REYOVEO ---6 0 -4 0 - UNMODlFlEO

I- UNY001FlED

W2 0 ----------- - ---BALANUS REMOVED

aa BALANVS REMOVED

t75 3b r 2 2 I 2

L

----------_______ BALANUS REMOVED -------- - - - -______B A L M U S REMOVED --------------

BALAWUS REMOVED

W UNMODIFIED a 1 10 3 -Z -

-5

J J A S O N D J F M A M J L~

FIG 2 Survivorship curves of Chthutilalits stellatzrs which had settled naturally on the shore in the autumn of 1953 Areas designated a were protected from predation by cages In each area the sur-vival of Chthninolzts growing without contact with Ralunus is compared to that in the undisturbed area For each area the vertical distance in feet from MTL is shown

Autumn 1961 INTERSPECIFIC COMPETITIOK

80 -60

I I O

00

120

I N TlOE POOL

130

0 A U N U S R E Y W E D

14 a- 25

40

20 BALANUS RCYWEO ---_-_ -------_-

MLANUS REMOVED

-10

+ - 2

X 5 -W

WYOOIFIED UNYOOIFIED

a 2 2 4

UNMODIFIED

P I 8 0

a 3 60

41 40 -I

Ilb 00

12 b

III TIDE POOL -I3 b

-LO

--------___--_ MUJI REMOVED

14 b -2g

C

E 20 - --P r---

0

m f 5 t

2

Frc 3 had settled

---OampLAUS REMOVED----U W O D I f S O

M U N U S I E W E O ---______----

UYOOIfIEO I A L U U S REYQIIO

---

UNYODDICO

J F MA M J I IIJISOIN J J A S O I U D J F Y A M J J ~ J A S O N D I955 I954 I955 1954 I955 1954

Survivorship curves of Chthamelus stelltitlcs on stones transplanted from high levels These in the autumn of 1953 the arrangement is the same as that of Figure 2

betneen the censu5es of February and May Bala-nus aged one year suffered a mortal~ty of 92 those 2 )ears and older 51 Over the same pe r~od the n ~ o r t a l ~ t j of Chthanml~tsaged 7 months was 6 2 2 those 1 years ant1 older 2 Rec-ords of the survival of Balallzds at sel era1 levels belou t h ~ s shoed that onlj those Balanzds in the top quarter of the Intertidal reqion suffered high mortal~ty during this titile ( C onnell 1961)

At each census notes were made for indilitlual barnacles of any crowding nhich had occurred since the last census Thus when one barnacle started to grow up over another this fact was noted and at the next census 4-6 weeks later the progress of this process was noted In this way a detailed description was built up of these grad- ually occurring events

Intraspecific competition leading to mortality in Clztlza~ltalltswas a rare event For areas 2 to 7 on the portions from which Balanus had been removed 167 deaths were recorded in a year Of these only 6 could be ascribed to crowding between individuals of Chthamahrs O n the undisturbed portions no such crowding was

observed This accords with Hattons (1938) o1)servation that he never saw crowding between individuals of Clztlzattzulrrs as contrasted to its frecluent occurrence I~etiveen individuals of Bala-11ZIS

Interspecific competition between Balanus and Chthatnalus was on the other hand a most irn-portant cause of death of Chthawtalus This is shon both by the direct observations of the process of crowding at each census and by the differences hetween the survival curves of Chtha-rnalus with and ~ i t h o u t Balanzds From the periodic observations it as noted that after the first month on the undisturbed portions of areas 3 to 7 about 10 of the Clztlztrl~lalltswere being covered as Balatzzts grew over them about 3 were being undercut and lifted by growing Bala-nzas a few had died without crovditlg By the end of the 2nd month about 20 of the Chtha-11lali4swere either wholly or partly covered by Balanus about 4 had been undercut others were surrounded by tall Balanus These processes continued a t a lower rate in the autumn and almost ceased during the later winter I n the spring Balanus resumed growth and more crowd- ing was observed

-- -- --

COX NELL Ecoiog Val 42 KO4

111 l al~ie 11 these ol)hervatiolia are sum~iiarized for the un(1istril)uted l~ortionh of all the areas l l~ovehZTI the ISalanlts tended to overgrow the Clltttr~nalltsLvhereas at tlie lower levels u~itlercutting was more conimon This same t re~id vas evitlent rvithin each groul) of areas ulidercutting I~eing more l~revalent on area 7 than on area 3 for examl~le Ihe faster growth of Ra1ants at lover levels ( 1 latton 1938 l3arnes ant1 lowell 1953) may have resultetl in more untlercutting Tlien Clrtlla111al1tsvas coml)letely coeretl 1)y Uaicrtzl(s it was recorded as dead even though death may not have occurred im-n~etl iatel~ the 1)urietl I~arnacle vas obviously not a functioliing ~ i i eml~e r of the l ~ o l ) ~ ~ l a t i o n

T ~ H I Tlie cause 11i mortality oi Llttltntrli~llrs stclatns E 1 I ~ ithe 1053 )car group un the utl(1i~turbed portions of

each area

k r a no

1 y0 1 21 1 13 1 54 ~ 8 1 0 38 l l b 10 5 1 40 0 1 0 1 6 0

14a -2 5 14b

Total 2 - 7 -

Total 11-14 1 - i J01 ) 271 I 9 1 I

I n Table TI untler the term other crotllng have been placed all instances here Clzthanlolus ere cruslied laterally between 2 o r more Balanus or where Chtlrailtallts disappeared in an interval during which a dense population of Balarllts grew rapidly Fo r example in area 7a the Balat1its which were at the high lmpulation density of 48 per cm2 had no room to expand except up- bvard and the barnacles very quickly grew into the form of tall cylinders or cones with the diameter of the opercular opening greater than

that uf tlie I~ase It as ol~vious that extreme cro~vding occurretl under these circumstances Ilut the exact cause of the mortality of the Chtlla-iritrl~iscaught in this crush vas difficult to as-certain

111 conil~aring the suriv~l curves oi I-igs 2 alid 3 ~vitliin each area it is evident tliat Clztllnll~ir-1lir 1ellt free of 13alanlts survi-etl Iletter than tliow in tlie adjacent ul~tlisturl)etl areas on all I ~ u t arcas 2 ant1 13a Area 2 Ivas in the zone here atlults of I3alanlts and yereClltl~a~l~nllrs ~iormally mixed at this high level Ralarltas evi-tlently has 110influence on the survival of Clltlla~lz-al~rs (-111 Stone 13a the survival of Cllthai~rallts ivithout Rnlan~tswas much better ~ ln t i l January vhen a Istarfisll Ast~ritrs~ I I ~ I E ) I S entered the cage and ate the barnacles

Iuch variation occurred on the other 13 areas hen the Clzthtr~llalits groving vithout contact vith Bnlanrts are compared vith those on the adjacent undisturbed portion of the area the survival ivas very much better 011 10 areas and motlerately better on 4 In all areas some Cl1fhtr~1lalltsin the undisturbed portions escaped severe crovding Someti~nes no Bnlanlcs hap-penetl to settle close to a Clztha~~zulzisor some-times those Ivhich did died soon after settlement 111 some instances Clzthatlallrs vhich Ivere being undercut by Balatltrs attached themselves to the Balnnlrs and so survi-ed Some Clztlralnnlus vere partly covered by Balatl1rs I ~ u t still survivetl I t seems 1)rol)al~le that i11 the 4 areas nos 16 l l a and 1 lb where Chfhnnmlits survived well in the presence of Halanzts a higher proportion of the Clltlltrnltrlus escaped death in one of these nays

The fate of very young Chthanzallts which set- tletl in the autumn of 1951 as folloved in de- tail in 2 instances on stone 15 and area 7b The Chtllai~~trluson stone 15 hat1 settled in an irregular space surrountletl by large Ilalanlts Most of the mortality occurred around the edges of the space as the Ralanlts undercut and lifted the small C-lltllci~~lalrcsnearl- The folloving is a tal)ula-tion of all the deaths of young Chthantnlus be-tbveen Sei)t 30 1953 and Feb 14 1955 on Stone IS vith the associated situations

Liftetl by Balanus 29 Cruslietl 1)y Bolanus 4 Smothered by Balanlts and Clltlta)talns 2 Cruslietl between Balanlis and Chthawlalzts 1 Lifted by Chtha~zulus 1 cruslietl betbveen two other Chthntilaltts 1 U1lkno-11 3

This list shows that crowding of newly settled Clltllamalus by older Balanlrs in the autumn main-

Autumn 1961 I N T E R S P E C I F I C C O h I P E T I T I O N 717

1y takes the form of undercutting rather than of s l ~ ~ o t l i e r i n ga s was the case in the spring T h e reason for thib tlifferellcc is proljal~ly that the (1rthtrlrralus a r e more tirmly attached in the spring so that the fast groving youllg llalirnus grow u p over them when they lllake contact In the autuliili tlle reverse is tlle case the Ilalanzts being firmly attached tile C-lrtlral~ztrl~tskveakly so

r l t h o l ~ ~ htlie bcttlcliicnt of Clltllt~~l~(zlrtson Stone 15 in the a u t u m n of 1954 was very tle~lse 32c1ii2 so that ~iiobtof then1 were touching all-other olily 2 of tllc 31 tlc~ths Lvcre catlsetl by intr~sl)ecitic cro~vtl ing among the Clltllal~tnltts I his i in i~ccortlit11 the tintliligs from the 1953 settlcliie~itof Clztrarrlnlrts

r I he ~i ior tal i tyrates for tlie young Clltllatnalus 011 area 71) sliouctl seasol~alvariations I3etween O c t o l ~ e r10 1934 a11tl 11 ~ y15 I955 the relative mortality rate l)er (lay X 100 was 013 on the untlisturl)etl area ant1 013 whcre I~alanushad bcen removetl ( ) v c r the next ~ n o n t h the rate increasctl to 139 on the untlisturl)ed area a n d 022 vherc Ijtrltrnrcs v~s absent T h u s the in-crease in nlortllity of young Chtlln~~~ct1rrsin late sprillg vas also associated ~vi t l ithe llresellce of Ilultrn~ts

S o ~ u eof tlic stolies t rans l~ ln~l te t lfrom high t o low 1cvels i l l the sllring of 1954 bore adul t C1rflrnlr~crlrrs O n Z stones recortls verc k e l ~ tof the s~lrvivalof these atltllts vliicl~Ii~tl scttletl in the 1uttul111 of 1952 o r i l l ~)reviot lsyears and Yere a t least 20 ~ i i o l ~ t h sold at tlle s tar t of tlie c s l ) c r i ~ i ~ e l ~ t llleir ~i ior tal i ty is sliokvn in T a l ~ l e 111 it was ~ lwaysmuch g r a t e r when Rnlnnrts - ~s~ i o t renlovetl ( In 2 of tlie 3 s t o ~ l e sthis nlortalit ra te Lvas ~ lmos ta s high a s that of the

yollllger grorll) I liese results s~ lgges t that any Clrflrn~ricrlrt~that nianagetl to survive tile conll)eti-tion for sl)ace vitli Bnlanus (luring tlie first year vo~lltl11rol)aI)ly I J V eliminated in the 211tl year

Cclisuses of IIt~l(l)zltsvere not made on t h e exl)erin~elital areas I Iokvever on nlany o ther areas i l l tlie sanie stretch of shore the survival of Hnln~lrts -as Ijeing studietl dur ing the same periotl (Colinell 1901) I n Tahle IV some mor-tality ra t r s meas~lre(1in that stutly a r e l is ted the Rtrlcl~rrs -ere m e m l ~ e r s of the 1954 settlement at ~ ~ o l ) ~ ~ l a t i t ~ ~ ~densities ant1 shore levels sitnilar to those of the present stutly T h e mortality rates of Hn1n~zts -ere allout the same a s those of Chtlrn1lrnlrrs in similar situations except a t the highcst level n r ra 1 where Bnlanrts suffered much greater morttlity than Cllthalrzc~lzts M u c h of this mortality lvas carlsetl 1)y intraspecific crowding a t all levels I)eloiv area 1

T~ti11111 C O I ~ ~ ~ I ~ ~ ~ S O I Iof the mortality rates of young ant1 oltler C h t h t i ) t ~ i ~ l ~ i sctrlluttrr on transplantetl stones

No

1 0 ft below Iib LlTI

---

hfTL In a t ~ d elwol raged- -

2 5 ft below MTL caned

I 1053 year

group - -A ~

Ralanus rcnroved 51

Yumher of Chlho-rnalup prrsent In

June 1951

- ~ ---

Ralanus removed 50

l J t~d~sturbcd 60 ~ ~ - - -~-

7 morta l~tyover one year (or for 0 months for 14a) of Chlharnalua

or older

36

I A I I I E I V C ~ I I ~ ~ I I ~ ~ ~ O I Iof ~ I I I I I I L ~111ortality rates of Chthnmalus strllntirs ant1 Bulanirs hnlanoides

I

Area no

Chthamalun tella at us autumn 1053 srttlement

128 1 (tide pool) 19 Oh

Ilcight in ft from MT1

Balan~tsbalanoides spring 1954 settlement

1 ( top) 1

1fiddle C a ~ e1 1 + 2 1 1 85 1 lIiddle Cage 2 I I

25

lopulation density

nocm2 June 1954

Stone 1 - 0 9 1 20 1 X 6 Stone 2 69 i 94

76mortality in the next

year

Population densityincludes hoth species The mortality rat- of Chthamalua refer to those on thp und~sturhedpnrtioni of each arpa Thp data and arpa des~ana-tions for Balanna were taken from Connell(1961) the present area 1 is the same as that designated 1 (top) in that p a p r

In the ohserat ions made a t each census it appeared that Balnnzts was grobving faster than Chthnnznlzts hleasurelnents of growth rates of the 2 species were niatle f rom photographs of

the areas taken in J u ~ l e ant1 Sovemher 1954 Barnacles growing free of contact with each other were measured the results are given in Table V The growth rate uf Itrlntlus ivas greater than that of Clltlawzallrs in the experimental areas this agrees lvith the tintlings of I latton (1938) on the shore in 1rallce 11ltl of 1arlles ( 1956a) for collti~lr~al sul)~llerge~lce a at illillporton raft

~ A I I I ~V (irowtli r a t c of Cl r t l r i~r~~c~l~ts andcti~ll~rfrcs I~trlnriris hc~li~troitlrs lcasurcr~lcr~ts were lllatlc of un-crowtletl illdivitluala oli ~~llotogr l l )hs areasof 3a 31) and 7h lliose of Cl~fl~irrrrirlrtswcrc ni~tlrOII tlie carlle intli- vitlual or1 I)otli tlrtcs I J I llirl~rr~rrr rcl)r-csctrtctiw ~ r n l ) l e s

ere c h o w r ~

- - --- -- --- i - - -vrraqe sir ill tit ~ u t c r v a l I j

l~xlutc yrowti l rillt llcr day x 100 1 21

liter a year of cro-(ling the average 1)ol)ula- tion de~lsities of Bafan~rs a11tl Clztlzattnlus re-111ainetl i l l the assl~lle rcslativc l ) ru l )or t io~~ tlley hat1 Ijee~i at tllc thestart s i ~ ~ c c ~llortality rltes ere about the sa111c I lovevcr 1)ecluse of its faster growtll 1 ~ ~ ~ l ~ r n ~ r s Locc111)i~iI rc1~tively grc3at- er area a~l t l presunlal~ly ~)ossessetl I greater I~iomass relative to tllat oi Clltllnamp~lnlzrc after a vear

The faster grovtIi of Ilal(ltlrrs prol)al)ly lc- co1111ts for tile maliner ill vllich Clztl~nrr~nlrrs vere crowtletl by Bn1atzlrs It also accounts ior the sinuosity of the st1rviv11 curves of Clltlrtrrrrc1l~rs growing in contact it11 Ral(1nzrs The mort~lity rate of these C l z t l ~ n ~ ~ ~ a l ~ r s intlicated 1)y theas s l o l ~of the curves in Figs 2 ant1 3 greatest in suliuiier tlecreasetl in winter ant1 increased again ill spring The survival curves of Clltlrn-lrzalzrs growing vithout contact ~vi th Balntzlrs tlo not show these seasonal variatiolls which there- fore cannot he the result of the tlirect action of physical factors such as temperature vave action or rain

Seasonal variations in grovth rate of Ra1nnzlc correspond to these changes in mortality rate of Clzthamalus In Figure 1 the growth of Balanl~s throughout the year as studied on an intertidal

CON S E L L Ecology V O ~ 42 NO 4

creases in the growth rate of Balanus The correlat~on ivas tested using the Spearman rank correlation coefficient The absolute increase in diameter of Balanus in each month reat1 from the curve of growth was conllgtaretl to the per-centage mortality of C1zfhar11nllts in the same month For the 13 months in which tlata for Chthnri~rrlrrs was ava~lalde the correlation was highly significant I = 01

GROWTH OF BALANUS

M O N T H S

1-1(1 coml~arisoliof the casorial c l lange ill the groikt11 of Ilillrr~t~rsI1cilanordcr aritl i l l the survival of Cl~tl~c~rriirlrcs I ~ c i t ~ g I)y Rala)tus T h ertrllirtrir crowtlctl x r o t l ~ of 1jnlirri1rc was that of ~)ariel3 Iltar~ies and Iowcll (1953) just a l~ovc MT1- on Kcppcl Iier tilll)ort tluritig 1051-52 T h e Clztllc~tnalus were or1 area la of tlic 1)rc~crlt stutly orie-lialf mile soutli of Keppcll Iicr tlut-irig 1051-55

Iro111 ill these o1)scrvations it appears tllat tlie 1)oor sl~rvival of C-lrtlrnltzn1rrc 11elov R1 t LN is a result n ~ a i ~ l l y of crovtli~ig 11y dense ~)opula- t io~is of faster groving llnlnnlrs

t the clitl of in June 1955 the cxl)eri~lle~it tlle survivil~g Clltllc~irrnlr~s ere collected iron1 5 of tlie treis Is s l io v~~in Table V I the acrage size u as greater in the Clltllnrrcllirs a hich hatl grolvn free of contact with Bnlnillrs ill every case the tlifference -as significant (Plt 01 ~ l a ~ ~ ~ ~ - ~ l i i t ~ ~ e yV test Siege1 1956) The stir-

vivors on the undisturhetl areas Jvere often mis- shapen in some cases as a result of being lifted 011 to the sitle of an underct~tting Bnlnnr(s Thus the smaller size of these barnacles may have been tlue to disturbances in the normal pattern of grovth while they were I~eing cro detl

These Cllflza~llalzrs vere examined for the pres- ence of developing larvae in their mantle cavities I s shovn in Tahle -I in every area the pro- portion of the ~uncrovtled Cl~thn~lrnllrs with larvae

1pa11el at ilIillport Barnes and Iowell (1953) -as equal to or more often slightly greater than is compared to the survival of Clrtllnrr~nl~rsat on the crovtletl areas The reason for this may about the same intertidal level ill the present 11e related to the smaller size of the crowtled study The increased mortality of Clltlln~r~alits -as ( l~tl~a~rrnl~is is due to separation since It not foulld to occur in the same seasons as the in- Clztharrlali~s can self-fertilize ( Barnes and Crisp

--

- - -- - - - -- - - -- - - -

719 Autumn 1961 INTERSPECIFIC C O h i P E T I T I O S

TABIEL-I The effect of c r o ~ + d i n gon the size and pres- TABLI- 11 T h e effect of predation by Thois lapillus ence of ~ t c l ( ~ t f ( s on the annual mortality rate of Ci~thcl~~roll ts inlarae In C l ~ t h t l ~ ~ i t ~ l ~ ~ s collected in June stcllatirs

1955

Ievel Number I N YM 1 of lndiv~d-D I A M ~ T E I ~ of I uals whirh

4 r r a T r r a t m e t C h h - had larvae ~n1 1 i k

Baianur removed

6 U n d i s t u r b e d Baianus removed

I -i a b- b I Und~qturhed

rnaius 1 Average ( Range J m a n t ~ eravity

- - -1 0 13 11 -

O i 10

a Protected from predatlotl by a cagr

-- - ---- - --Arra

W t h Without Dif-1 Rninnts Ralanur -f e r e n ~ e

------- ------Arra i 73 (112) 25 (96) 18 Area i 90 ( 42) 47 (40) 43 Area 1 l 62 ( 21) 18 (18) 31 Arra I 2 100 ( 60) 53 (501 li Arra 1i 98 i 72) 4 ( 7 i ) 89

tlie experimental areas

mortality o i Chlhamnlus over a year (The initial Inlm-bers are given In parentheses)

-b Unproterted oprn to

predatlnn ~

W I ~ I I ( l l t i r o u t I DC Rninu Ralntius ( f e r ~ n c e

Balariu3rernoved( ( 23

1050) loore ( 1935) and th rnes (1953) have shov11 that the number of larvae in an individual of Bala~lusDala~roidcsi~zcreasesit11 increase in volunle of the parent Co~npariso~zof the cube of the diameter which is proportional to the volume of Chtha~~zalnsvitIi and without lalanus sholvs that the volume may I)e decreased to lnormal size ivllen crovtlitng occurs lssuming that the relation l~etn-eenlarval numljers and volume 1x1

C~thailralrlsis similar to that of Balt-rnus a de-crease ill I)cgtthfrecluency of occurrence ant1 abun-dance of larvae in Clltl~a~tzalusresults fro111 corn-petition bvith Balavtrls Thus the 1)rocess described in this lgtal)er satisties I)oth asl~ectsof intersl~cific competition as tlefinetl 11) IClto~i alitl l l i l ler (1954) in ~vhichone sl~ecicsaffects the pop-ulation uf another 1)y a process of interference ie 1)y reducing the reprotluctive efficiency or in-creasing the mortality of its conq)etitor

711~effrc-t of prctintiovt b y lhnis Cages which exclutled Thais liad been attached

011 6 areas (indicated l)y the letter a following tlie number of the area) rea 14 was not in-cluded in the folio-ing analysis since many star-fish ere ol~servetlfeeding on the 1)arnacles at this level one entered the cage in January 1955 and ate most of the barnacles

Tlzais ivere common in this locality feeding on barnacles and mussels and reaching average l)olx~lation densities of 2 0 0 r n V e l o w MTL

The records for 12a extend over only 10 months for purlwses of compar~sol~ the mortality rat for 12a hss been nrultiplied by 11

tTide pool

at loiver levels greater outside Densities of Tlzais tend to be greater at and below h1TL so that this trentl in the mortality rates of Chthawialus may he ascril~edto an increase in predation by Thais it lower levels

Mortality of Clzthavttalz4s in the absence of lialaltus was appreciably greater outside than insitle the cage only on area 13 In the other 4 areas it seems evident that few Chthat+zalus were 1)eing eaten by Thais In a concurrent study of the I)ehavior of Thais in feeding on Balanzts bala~~oidrsit was foulid that Tlzais selected the larger i~ldividualsas prey ( Couuell 1961) Since Bnlalrlls after a few months growth was usually larger thali Chthanzalzcs it might he expected that 7hnis would feed on Balanus in preference to Clltllc~ir~alusIn a later study (unpublished) made at Santa Uarbara California Thais entargin-ata Ueshayes were enclosed in cages on the shore vith mixed pol)ulations of Balanus ylandula Dar-vin and Clztha~wzllrsfissus Darwin These species Yere each of the salne size range as the corre-sl~ontlingsl~eciesat Millport I t was fount1 that Tlzais emarginata fed on Balanus glandula in preference to Chthamulus fissus

As has k e n indicated much of the ~nortality of Clzfl~amalusgrowing naturally intermingled with Bala~tuswas a result of direct crowding by Ralanrts I t therefore seemed reasonable to take the difference between the ~nortality rates of Clztl~art~al~tswith and without Balanus as an index

(Connell 1961) The mortality rates for Clztlza~rt- of the degree of c ~ m ~ t i t i o nbetween the species nllrs in cages and on adjacent areas outside cages (indicated l ~ ythe letter h after the number) are shovn on Ta l~ leVII

If the mortality rates of Clztllanznllcs growing vitliout contact with Balanz4s are compared in and out of the cages it can be seen that at the upper levels mortality is greater inside the cages

This difference was calculated for each area and is included in Table VI I If these differences are compared between each pair of adjacent areas in and out of a cage it appears that the difference and therefore the degree of competition was greater outside the cages at the upper shore levels and less outside the cages at the lower levels

720 JOSEIII 11

I h u h as l)rc(litio~l illcreased at lover levels the degrce of competition decreased This result ivoultl Ilave heen expected i f Tllais had fed upon Ba1a~lrs in preference to Clltha~tlnllrsThe gen- eral effect of lIretlatio11 by Thais seenls to have Iwen to lessen the interspecific com11etition I~elow 1 1TI gt

ltllough animal communities appear cluali-tatively to be constructetl as if competition ivere regulating their structure even in the I~est studied cases there are nearly al~vays difficulties antl un-exl~loretl possil~ilities ( Hutchinson 1957)

In the present study direct ol~servations at in-

COX SELL Ecology Vol 42 No 4

a~lt l 3 ) and these survivors n-ere oftell misshapen antl smaller than those vhicli had not been crowdetl (Table V I ) rldults on the transplanted stones had suffered high mortality in the previous year (Table 111)

nother diftictllty was tliat C71tllai1zal~tswas rarely found to have settled belov mid tide level at hlilll~ort Tlie reasons for this are unknown it survived lvell if transplanted I)elov this level ill the absence of Balanz~s I n other areas of the British Isles (in soutlivest 1ngland and Ireland ior example) it occurs I)elov mid tide level

The 1)ossi11ility that ClztJznlitall(s might affect hala111tsdeleteriously remains to be considered I t is unlikely that Cl1tlzair1alitscould cause much

tervals shovetl tliat couipetitio~i vas o c c u r r i ~ ~ gmortality of Ilalanl(s 1- direct crovding its untler natural co~itlitions I n addition the evi-dence is strollg that the observed competition with Ilalnll(s vas the l~rincipal factor determining the local distril~ution of Chtl1irirltrlz(sC l l t l ~ a ~ ~ ~ a l ~ r s thrived at lower levels when it vas not grobving in contact witli Balnn14s

lIowever there remain unexplored possibilities The elitilitiation of Clztlzainal~(srequires a dense 1101~1lation of lialon~rsyet the settlenlent of Ra-lirnlis varied from year to year It Alilllmrt the settlement density of Hnlarz~ts bnlirttoitirs vas nleasuretl for 9 years I)etveen 1914 a~lt l 195s ( Iiarnes 195hl) Co~inell 1961 ) Settlemelit vas light in 2 years 19-16 antl 1958 I11 tlie 3 seasons of Hala~l~(ssettlemelit stutlietl in tlctail 1C)jZ-55 there vas a vast oversul)l)ly of larvae reatly for settlement It thus seelils 1rol)al)le that most of the Chtlzai~nlz~svhich survivetl in a year of poor settlement of Haln~zltsivould he liilletl in competi- tion with a normal settlement the folloving ear A succession of years vitli poor settlemelits of Bnln~lcsis a possil)lc I~u t it~ll~robable occurrence at Millport judging from the past record A very light settlement is lgtrol)ably the result of a chance cotubination of unfavorable veather cir-cumstances (luring the l~lanktonic lxriod (Uarnes 1956h) A-41so after a light settlement survival 011 the shore is iti~ljroved oving l)rit~cipally to the reductio~l in intraspecitic crobvdiug (Connell 1961) this ivoultl tend to favor a normal settle- ment the folloiving year since 1)arnacles are stim- ulated to settle 1)y the presence of niemhers of their ovn species already attached 011 the suriace (Knight- Jones 1953)

Tlie fate of those Chtllniilalts I-liich had sur-vived a year on tlie undisturbed areas is not knobvn since the experiment ended at that titile It is probable Iobvever that most of them ivould have been eli t~~inated within 6 tl~ontlis the mor-tality rate had increased in the spring (Figs 2

growth is n~ucli sloiver and crovtling I~etween intlivitluals of Clitllainalrrs seltlom resulted in tleath dense ~)olj~ilation of Chthirigtzallts tnight delxive larvae of Balanzts of space for settle-ment ilso Clztlznirtallls might feed on the plank- tonic larvae of Bnlanus however this vould oc-cur in March and April vhen 110th the sea vater teml~erature and rate of cirral activity (pre-sunia1)ly correlated with feeding activity) vould he near their minima (Southard 1955)

Tlie itldication from the caging experiments that predation decreased interspecific competi-tion suggests that the action of such additional factors tentls to reduce the intensity of such interactions in natural conditio~ls I11 adtlitional suggestion in this regartl timy 1)e made con-cerni~lg parasitism Crisp ( 1960) found that the growth rate of Balanzcs bnlnfzoidcs was de-creased i f indivitluals were infected with the iso- ~ ) o d parasite IIc~t~ionisrzrsbalnni ( Spence Rate) In I3ritain this l~arasite has not l~een reported from Chtl~n~lzal~rsstcllntlrs Thus i f this parasite Yere l~resellt 110th the groivth rate of Balnnlrs ant1 its al~ility to eliminate Clztlzarl~alltswould I)e decreasetl ivith a corresl)onding lessening of the tlegree of coml~etition hetveen the species

The evitlence prese~ited in this paper indicates that tlie lower limit of the intertitlal zone of Chtlln~lrnlus stcllntlls at A1illl)ort vas tletermined 1)y intel-specific coml)etition for space it11 Bnln-~ z t s Onlnnoidrs Bnlnnzts by virtue of its greater po~~ula t iondensity and faster grovth eliminated niost of tlie Clzthnnlnllts by directing crovding

At the upper limits of the zones of these species no interaction bvas observed Chtllnii~nl~ts e~ idet~t lycan exist higher on the shore than Bnla)llrs mainly as a result of its greater toleratice to heat andor desiccation

Autumn 1961 INTERSPECIFIC COMPETITIOS 721

The u ~ l ~ r of most intertidallimits animals are 1xo11al)ly determined 11y physical factors such as these Since growth rates usually decrease ~v i th increasing height on the shore it bvould be less likely that a sessile species occupying a higher zone could by competition for space prevent a lover one from extending up-ards 12ilieaise there has heen as far as the author is aivare no study made -hich sholvs that predation land species tletermines the upper limit of an intertidal animal In one of the most thorough of such studies Ilrinnan ( 1957) indicated that intense predation I)y 1)irtls accounted for an an-nual mortality of 225 of cockles (cardi lr~~ledrrlc Igtj in sand flats hvhere their total tniortality ivas 74 per year

I n regard to the lover l i~nits of an animals zone it is evident that physical factors may act directly to determine this boundary For example some active arnphipods from the upper levels of sandy beaches die if kept submerged However evidence is accumulating that the lower limits of tlistribution of intertidal animals are deter-mined mainly by biotic factors

Connell ( 1961 ) found that the shorter length of life of Bczlnn~rs balanoides at low shore levels could 1)e accounted for by selective predation l)y Thnis lapillzts and increased intraspecific corn-petition for sllace The results of the experiments in the present study c o ~ ~ f i r m suggestions of the other authors that lower limits may be due to in- terspecific competition for space Knox (1954) suggested that competition determined the dis-triljution of 2 species of I~arnacles in New Zealand Iltntlean Kenny and Stephenson ( 1956) gave indirect evidence that competition with a colonial polychaete norm ((aleolaritr) may have de-termined the lower limit of a I~arnacle (Tetra-clitcz) in Queensland Australia In turn the loner limit of (trlcolczricz appeared to he deter-~nined by competition with a tunicate Pyztra or ~vi th dense algal mats

lrith regard to the 2 species of 1)arnacles in the present paper some interesting ol)servations have heen made concernitng changes in their abundance in Britain l roore (1936) found that in southtvest England in 1933 Chtha~~lal~cs stella-trts Lvas most dense at MHVN decreasing in numbers toward MTL while Balanlrs halanoidrs increased in numbers below MH l r S - I t the same localities in 1951 Southward and Crisp (1953) fount1 that Balanl(s had alrnost disap-peared and that Clztl~a~~ral~ts bothhad increased ahove and below hIHVN Chtlla~nallrshad not reached the former densities of Bala~titsexcept

at one locality Erixham After 1951 Bnlan~as I~egan to return in numl~ers although by 1954 it had not reached the densities of 1934 Clztha- ~rznlrrshad declined 1)ut again not to its former densities (Southward and Crisp 1956)

Since Cl~thairlalus increased in al~mndance a t the lower levels vacated by Balanlrs it may lxeviously have I~een excluded by con~l~eti t ion with Balarllrs The grolvth rate of Balanl4s is greater than Clztlln~izalusboth north and south (Hatton 1938) of this location so that Balanus would be likely to -in in con~petition with Chthn~italrrsHowever changes in other environ- mental factors such as temperature may have influenced the abundance of these species in a reciprocal manner I n its return to southwest Ingland after 1951 the maxinlum density of settlement of Balanlrs vas 12 per cm2 compe-tition of the degree observed at llillport would not he expected to occur at this density A t a higher population density Balattns in southern England nould prohaldy eliminate Chtha~~talzts at low shore levels in the same manner as it did at llillport

I n Ioch Sween on the Argyll Peninsula Scot- land Igtevis and Powell (1960) have described an unusual pattern of zonation of Clzthamalz~sstel-latlts On the outer coast of the Argyll Penin-sula Clztlza~~tal~rshas a distribution sitnilar to that at YIillport 111 the tnore sheltered waters of 1och Sxveen holvever Chtlza~+znlusoccurs from above YI1T-S to about hITT2 judging the distril~ution I)y its relationship to other organisms finlatlrrs htrlanoides is scarce above h1TL in Loch S~veen so that there appears to he no pos- sil~ility of competition with Chtl~a~nalus assuch that occurring at Millport between the levels of lIT1 and 1IH lTN

In Figure 5 an attempt has been made to summarize the distribution of adults and newly settled larvae in relation to the main factors which appear to determine this distribution For Ba-lanzrs the estimates were based on the findings of a previous study (Connell 1961) intraspecific conllxtition Lvas severe at the lower levels during the first year after which predation increased in importance fith Chthamalzas it appears that avoidance of settlement or early mortality of those larvae which settled at levels below MTL and elimination by competition with Balatzzts of those which settled between MTL and MH WN were the principal causes for the ab-sence of adults below MHlTN at Millport This distribution appears to be typical for much of western Scotland

-- -- --

COX NELL Ecoiog Val 42 KO4

111 l al~ie 11 these ol)hervatiolia are sum~iiarized for the un(1istril)uted l~ortionh of all the areas l l~ovehZTI the ISalanlts tended to overgrow the Clltttr~nalltsLvhereas at tlie lower levels u~itlercutting was more conimon This same t re~id vas evitlent rvithin each groul) of areas ulidercutting I~eing more l~revalent on area 7 than on area 3 for examl~le Ihe faster growth of Ra1ants at lover levels ( 1 latton 1938 l3arnes ant1 lowell 1953) may have resultetl in more untlercutting Tlien Clrtlla111al1tsvas coml)letely coeretl 1)y Uaicrtzl(s it was recorded as dead even though death may not have occurred im-n~etl iatel~ the 1)urietl I~arnacle vas obviously not a functioliing ~ i i eml~e r of the l ~ o l ) ~ ~ l a t i o n

T ~ H I Tlie cause 11i mortality oi Llttltntrli~llrs stclatns E 1 I ~ ithe 1053 )car group un the utl(1i~turbed portions of

each area

k r a no

1 y0 1 21 1 13 1 54 ~ 8 1 0 38 l l b 10 5 1 40 0 1 0 1 6 0

14a -2 5 14b

Total 2 - 7 -

Total 11-14 1 - i J01 ) 271 I 9 1 I

I n Table TI untler the term other crotllng have been placed all instances here Clzthanlolus ere cruslied laterally between 2 o r more Balanus or where Chtlrailtallts disappeared in an interval during which a dense population of Balarllts grew rapidly Fo r example in area 7a the Balat1its which were at the high lmpulation density of 48 per cm2 had no room to expand except up- bvard and the barnacles very quickly grew into the form of tall cylinders or cones with the diameter of the opercular opening greater than

that uf tlie I~ase It as ol~vious that extreme cro~vding occurretl under these circumstances Ilut the exact cause of the mortality of the Chtlla-iritrl~iscaught in this crush vas difficult to as-certain

111 conil~aring the suriv~l curves oi I-igs 2 alid 3 ~vitliin each area it is evident tliat Clztllnll~ir-1lir 1ellt free of 13alanlts survi-etl Iletter than tliow in tlie adjacent ul~tlisturl)etl areas on all I ~ u t arcas 2 ant1 13a Area 2 Ivas in the zone here atlults of I3alanlts and yereClltl~a~l~nllrs ~iormally mixed at this high level Ralarltas evi-tlently has 110influence on the survival of Clltlla~lz-al~rs (-111 Stone 13a the survival of Cllthai~rallts ivithout Rnlan~tswas much better ~ ln t i l January vhen a Istarfisll Ast~ritrs~ I I ~ I E ) I S entered the cage and ate the barnacles

Iuch variation occurred on the other 13 areas hen the Clzthtr~llalits groving vithout contact vith Bnlanrts are compared vith those on the adjacent undisturbed portion of the area the survival ivas very much better 011 10 areas and motlerately better on 4 In all areas some Cl1fhtr~1lalltsin the undisturbed portions escaped severe crovding Someti~nes no Bnlanlcs hap-penetl to settle close to a Clztha~~zulzisor some-times those Ivhich did died soon after settlement 111 some instances Clzthatlallrs vhich Ivere being undercut by Balatltrs attached themselves to the Balnnlrs and so survi-ed Some Clztlralnnlus vere partly covered by Balatl1rs I ~ u t still survivetl I t seems 1)rol)al~le that i11 the 4 areas nos 16 l l a and 1 lb where Chfhnnmlits survived well in the presence of Halanzts a higher proportion of the Clltlltrnltrlus escaped death in one of these nays

The fate of very young Chthanzallts which set- tletl in the autumn of 1951 as folloved in de- tail in 2 instances on stone 15 and area 7b The Chtllai~~trluson stone 15 hat1 settled in an irregular space surrountletl by large Ilalanlts Most of the mortality occurred around the edges of the space as the Ralanlts undercut and lifted the small C-lltllci~~lalrcsnearl- The folloving is a tal)ula-tion of all the deaths of young Chthantnlus be-tbveen Sei)t 30 1953 and Feb 14 1955 on Stone IS vith the associated situations

Liftetl by Balanus 29 Cruslietl 1)y Bolanus 4 Smothered by Balanlts and Clltlta)talns 2 Cruslietl between Balanlis and Chthawlalzts 1 Lifted by Chtha~zulus 1 cruslietl betbveen two other Chthntilaltts 1 U1lkno-11 3

This list shows that crowding of newly settled Clltllamalus by older Balanlrs in the autumn main-

Autumn 1961 I N T E R S P E C I F I C C O h I P E T I T I O N 717

1y takes the form of undercutting rather than of s l ~ ~ o t l i e r i n ga s was the case in the spring T h e reason for thib tlifferellcc is proljal~ly that the (1rthtrlrralus a r e more tirmly attached in the spring so that the fast groving youllg llalirnus grow u p over them when they lllake contact In the autuliili tlle reverse is tlle case the Ilalanzts being firmly attached tile C-lrtlral~ztrl~tskveakly so

r l t h o l ~ ~ htlie bcttlcliicnt of Clltllt~~l~(zlrtson Stone 15 in the a u t u m n of 1954 was very tle~lse 32c1ii2 so that ~iiobtof then1 were touching all-other olily 2 of tllc 31 tlc~ths Lvcre catlsetl by intr~sl)ecitic cro~vtl ing among the Clltllal~tnltts I his i in i~ccortlit11 the tintliligs from the 1953 settlcliie~itof Clztrarrlnlrts

r I he ~i ior tal i tyrates for tlie young Clltllatnalus 011 area 71) sliouctl seasol~alvariations I3etween O c t o l ~ e r10 1934 a11tl 11 ~ y15 I955 the relative mortality rate l)er (lay X 100 was 013 on the untlisturl)etl area ant1 013 whcre I~alanushad bcen removetl ( ) v c r the next ~ n o n t h the rate increasctl to 139 on the untlisturl)ed area a n d 022 vherc Ijtrltrnrcs v~s absent T h u s the in-crease in nlortllity of young Chtlln~~~ct1rrsin late sprillg vas also associated ~vi t l ithe llresellce of Ilultrn~ts

S o ~ u eof tlic stolies t rans l~ ln~l te t lfrom high t o low 1cvels i l l the sllring of 1954 bore adul t C1rflrnlr~crlrrs O n Z stones recortls verc k e l ~ tof the s~lrvivalof these atltllts vliicl~Ii~tl scttletl in the 1uttul111 of 1952 o r i l l ~)reviot lsyears and Yere a t least 20 ~ i i o l ~ t h sold at tlle s tar t of tlie c s l ) c r i ~ i ~ e l ~ t llleir ~i ior tal i ty is sliokvn in T a l ~ l e 111 it was ~ lwaysmuch g r a t e r when Rnlnnrts - ~s~ i o t renlovetl ( In 2 of tlie 3 s t o ~ l e sthis nlortalit ra te Lvas ~ lmos ta s high a s that of the

yollllger grorll) I liese results s~ lgges t that any Clrflrn~ricrlrt~that nianagetl to survive tile conll)eti-tion for sl)ace vitli Bnlanus (luring tlie first year vo~lltl11rol)aI)ly I J V eliminated in the 211tl year

Cclisuses of IIt~l(l)zltsvere not made on t h e exl)erin~elital areas I Iokvever on nlany o ther areas i l l tlie sanie stretch of shore the survival of Hnln~lrts -as Ijeing studietl dur ing the same periotl (Colinell 1901) I n Tahle IV some mor-tality ra t r s meas~lre(1in that stutly a r e l is ted the Rtrlcl~rrs -ere m e m l ~ e r s of the 1954 settlement at ~ ~ o l ) ~ ~ l a t i t ~ ~ ~densities ant1 shore levels sitnilar to those of the present stutly T h e mortality rates of Hn1n~zts -ere allout the same a s those of Chtlrn1lrnlrrs in similar situations except a t the highcst level n r ra 1 where Bnlanrts suffered much greater morttlity than Cllthalrzc~lzts M u c h of this mortality lvas carlsetl 1)y intraspecific crowding a t all levels I)eloiv area 1

T~ti11111 C O I ~ ~ ~ I ~ ~ ~ S O I Iof the mortality rates of young ant1 oltler C h t h t i ) t ~ i ~ l ~ i sctrlluttrr on transplantetl stones

No

1 0 ft below Iib LlTI

---

hfTL In a t ~ d elwol raged- -

2 5 ft below MTL caned

I 1053 year

group - -A ~

Ralanus rcnroved 51

Yumher of Chlho-rnalup prrsent In

June 1951

- ~ ---

Ralanus removed 50

l J t~d~sturbcd 60 ~ ~ - - -~-

7 morta l~tyover one year (or for 0 months for 14a) of Chlharnalua

or older

36

I A I I I E I V C ~ I I ~ ~ I I ~ ~ ~ O I Iof ~ I I I I I I L ~111ortality rates of Chthnmalus strllntirs ant1 Bulanirs hnlanoides

I

Area no

Chthamalun tella at us autumn 1053 srttlement

128 1 (tide pool) 19 Oh

Ilcight in ft from MT1

Balan~tsbalanoides spring 1954 settlement

1 ( top) 1

1fiddle C a ~ e1 1 + 2 1 1 85 1 lIiddle Cage 2 I I

25

lopulation density

nocm2 June 1954

Stone 1 - 0 9 1 20 1 X 6 Stone 2 69 i 94

76mortality in the next

year

Population densityincludes hoth species The mortality rat- of Chthamalua refer to those on thp und~sturhedpnrtioni of each arpa Thp data and arpa des~ana-tions for Balanna were taken from Connell(1961) the present area 1 is the same as that designated 1 (top) in that p a p r

In the ohserat ions made a t each census it appeared that Balnnzts was grobving faster than Chthnnznlzts hleasurelnents of growth rates of the 2 species were niatle f rom photographs of

the areas taken in J u ~ l e ant1 Sovemher 1954 Barnacles growing free of contact with each other were measured the results are given in Table V The growth rate uf Itrlntlus ivas greater than that of Clltlawzallrs in the experimental areas this agrees lvith the tintlings of I latton (1938) on the shore in 1rallce 11ltl of 1arlles ( 1956a) for collti~lr~al sul)~llerge~lce a at illillporton raft

~ A I I I ~V (irowtli r a t c of Cl r t l r i~r~~c~l~ts andcti~ll~rfrcs I~trlnriris hc~li~troitlrs lcasurcr~lcr~ts were lllatlc of un-crowtletl illdivitluala oli ~~llotogr l l )hs areasof 3a 31) and 7h lliose of Cl~fl~irrrrirlrtswcrc ni~tlrOII tlie carlle intli- vitlual or1 I)otli tlrtcs I J I llirl~rr~rrr rcl)r-csctrtctiw ~ r n l ) l e s

ere c h o w r ~

- - --- -- --- i - - -vrraqe sir ill tit ~ u t c r v a l I j

l~xlutc yrowti l rillt llcr day x 100 1 21

liter a year of cro-(ling the average 1)ol)ula- tion de~lsities of Bafan~rs a11tl Clztlzattnlus re-111ainetl i l l the assl~lle rcslativc l ) ru l )or t io~~ tlley hat1 Ijee~i at tllc thestart s i ~ ~ c c ~llortality rltes ere about the sa111c I lovevcr 1)ecluse of its faster growtll 1 ~ ~ ~ l ~ r n ~ r s Locc111)i~iI rc1~tively grc3at- er area a~l t l presunlal~ly ~)ossessetl I greater I~iomass relative to tllat oi Clltllnamp~lnlzrc after a vear

The faster grovtIi of Ilal(ltlrrs prol)al)ly lc- co1111ts for tile maliner ill vllich Clztl~nrr~nlrrs vere crowtletl by Bn1atzlrs It also accounts ior the sinuosity of the st1rviv11 curves of Clltlrtrrrrc1l~rs growing in contact it11 Ral(1nzrs The mort~lity rate of these C l z t l ~ n ~ ~ ~ a l ~ r s intlicated 1)y theas s l o l ~of the curves in Figs 2 ant1 3 greatest in suliuiier tlecreasetl in winter ant1 increased again ill spring The survival curves of Clltlrn-lrzalzrs growing vithout contact ~vi th Balntzlrs tlo not show these seasonal variatiolls which there- fore cannot he the result of the tlirect action of physical factors such as temperature vave action or rain

Seasonal variations in grovth rate of Ra1nnzlc correspond to these changes in mortality rate of Clzthamalus In Figure 1 the growth of Balanl~s throughout the year as studied on an intertidal

CON S E L L Ecology V O ~ 42 NO 4

creases in the growth rate of Balanus The correlat~on ivas tested using the Spearman rank correlation coefficient The absolute increase in diameter of Balanus in each month reat1 from the curve of growth was conllgtaretl to the per-centage mortality of C1zfhar11nllts in the same month For the 13 months in which tlata for Chthnri~rrlrrs was ava~lalde the correlation was highly significant I = 01

GROWTH OF BALANUS

M O N T H S

1-1(1 coml~arisoliof the casorial c l lange ill the groikt11 of Ilillrr~t~rsI1cilanordcr aritl i l l the survival of Cl~tl~c~rriirlrcs I ~ c i t ~ g I)y Rala)tus T h ertrllirtrir crowtlctl x r o t l ~ of 1jnlirri1rc was that of ~)ariel3 Iltar~ies and Iowcll (1953) just a l~ovc MT1- on Kcppcl Iier tilll)ort tluritig 1051-52 T h e Clztllc~tnalus were or1 area la of tlic 1)rc~crlt stutly orie-lialf mile soutli of Keppcll Iicr tlut-irig 1051-55

Iro111 ill these o1)scrvations it appears tllat tlie 1)oor sl~rvival of C-lrtlrnltzn1rrc 11elov R1 t LN is a result n ~ a i ~ l l y of crovtli~ig 11y dense ~)opula- t io~is of faster groving llnlnnlrs

t the clitl of in June 1955 the cxl)eri~lle~it tlle survivil~g Clltllc~irrnlr~s ere collected iron1 5 of tlie treis Is s l io v~~in Table V I the acrage size u as greater in the Clltllnrrcllirs a hich hatl grolvn free of contact with Bnlnillrs ill every case the tlifference -as significant (Plt 01 ~ l a ~ ~ ~ ~ - ~ l i i t ~ ~ e yV test Siege1 1956) The stir-

vivors on the undisturhetl areas Jvere often mis- shapen in some cases as a result of being lifted 011 to the sitle of an underct~tting Bnlnnr(s Thus the smaller size of these barnacles may have been tlue to disturbances in the normal pattern of grovth while they were I~eing cro detl

These Cllflza~llalzrs vere examined for the pres- ence of developing larvae in their mantle cavities I s shovn in Tahle -I in every area the pro- portion of the ~uncrovtled Cl~thn~lrnllrs with larvae

1pa11el at ilIillport Barnes and Iowell (1953) -as equal to or more often slightly greater than is compared to the survival of Clrtllnrr~nl~rsat on the crovtletl areas The reason for this may about the same intertidal level ill the present 11e related to the smaller size of the crowtled study The increased mortality of Clltlln~r~alits -as ( l~tl~a~rrnl~is is due to separation since It not foulld to occur in the same seasons as the in- Clztharrlali~s can self-fertilize ( Barnes and Crisp

--

- - -- - - - -- - - -- - - -

719 Autumn 1961 INTERSPECIFIC C O h i P E T I T I O S

TABIEL-I The effect of c r o ~ + d i n gon the size and pres- TABLI- 11 T h e effect of predation by Thois lapillus ence of ~ t c l ( ~ t f ( s on the annual mortality rate of Ci~thcl~~roll ts inlarae In C l ~ t h t l ~ ~ i t ~ l ~ ~ s collected in June stcllatirs

1955

Ievel Number I N YM 1 of lndiv~d-D I A M ~ T E I ~ of I uals whirh

4 r r a T r r a t m e t C h h - had larvae ~n1 1 i k

Baianur removed

6 U n d i s t u r b e d Baianus removed

I -i a b- b I Und~qturhed

rnaius 1 Average ( Range J m a n t ~ eravity

- - -1 0 13 11 -

O i 10

a Protected from predatlotl by a cagr

-- - ---- - --Arra

W t h Without Dif-1 Rninnts Ralanur -f e r e n ~ e

------- ------Arra i 73 (112) 25 (96) 18 Area i 90 ( 42) 47 (40) 43 Area 1 l 62 ( 21) 18 (18) 31 Arra I 2 100 ( 60) 53 (501 li Arra 1i 98 i 72) 4 ( 7 i ) 89

tlie experimental areas

mortality o i Chlhamnlus over a year (The initial Inlm-bers are given In parentheses)

-b Unproterted oprn to

predatlnn ~

W I ~ I I ( l l t i r o u t I DC Rninu Ralntius ( f e r ~ n c e

Balariu3rernoved( ( 23

1050) loore ( 1935) and th rnes (1953) have shov11 that the number of larvae in an individual of Bala~lusDala~roidcsi~zcreasesit11 increase in volunle of the parent Co~npariso~zof the cube of the diameter which is proportional to the volume of Chtha~~zalnsvitIi and without lalanus sholvs that the volume may I)e decreased to lnormal size ivllen crovtlitng occurs lssuming that the relation l~etn-eenlarval numljers and volume 1x1

C~thailralrlsis similar to that of Balt-rnus a de-crease ill I)cgtthfrecluency of occurrence ant1 abun-dance of larvae in Clltl~a~tzalusresults fro111 corn-petition bvith Balavtrls Thus the 1)rocess described in this lgtal)er satisties I)oth asl~ectsof intersl~cific competition as tlefinetl 11) IClto~i alitl l l i l ler (1954) in ~vhichone sl~ecicsaffects the pop-ulation uf another 1)y a process of interference ie 1)y reducing the reprotluctive efficiency or in-creasing the mortality of its conq)etitor

711~effrc-t of prctintiovt b y lhnis Cages which exclutled Thais liad been attached

011 6 areas (indicated l)y the letter a following tlie number of the area) rea 14 was not in-cluded in the folio-ing analysis since many star-fish ere ol~servetlfeeding on the 1)arnacles at this level one entered the cage in January 1955 and ate most of the barnacles

Tlzais ivere common in this locality feeding on barnacles and mussels and reaching average l)olx~lation densities of 2 0 0 r n V e l o w MTL

The records for 12a extend over only 10 months for purlwses of compar~sol~ the mortality rat for 12a hss been nrultiplied by 11

tTide pool

at loiver levels greater outside Densities of Tlzais tend to be greater at and below h1TL so that this trentl in the mortality rates of Chthawialus may he ascril~edto an increase in predation by Thais it lower levels

Mortality of Clzthavttalz4s in the absence of lialaltus was appreciably greater outside than insitle the cage only on area 13 In the other 4 areas it seems evident that few Chthat+zalus were 1)eing eaten by Thais In a concurrent study of the I)ehavior of Thais in feeding on Balanzts bala~~oidrsit was foulid that Tlzais selected the larger i~ldividualsas prey ( Couuell 1961) Since Bnlalrlls after a few months growth was usually larger thali Chthanzalzcs it might he expected that 7hnis would feed on Balanus in preference to Clltllc~ir~alusIn a later study (unpublished) made at Santa Uarbara California Thais entargin-ata Ueshayes were enclosed in cages on the shore vith mixed pol)ulations of Balanus ylandula Dar-vin and Clztha~wzllrsfissus Darwin These species Yere each of the salne size range as the corre-sl~ontlingsl~eciesat Millport I t was fount1 that Tlzais emarginata fed on Balanus glandula in preference to Chthamulus fissus

As has k e n indicated much of the ~nortality of Clzfl~amalusgrowing naturally intermingled with Bala~tuswas a result of direct crowding by Ralanrts I t therefore seemed reasonable to take the difference between the ~nortality rates of Clztl~art~al~tswith and without Balanus as an index

(Connell 1961) The mortality rates for Clztlza~rt- of the degree of c ~ m ~ t i t i o nbetween the species nllrs in cages and on adjacent areas outside cages (indicated l ~ ythe letter h after the number) are shovn on Ta l~ leVII

If the mortality rates of Clztllanznllcs growing vitliout contact with Balanz4s are compared in and out of the cages it can be seen that at the upper levels mortality is greater inside the cages

This difference was calculated for each area and is included in Table VI I If these differences are compared between each pair of adjacent areas in and out of a cage it appears that the difference and therefore the degree of competition was greater outside the cages at the upper shore levels and less outside the cages at the lower levels

720 JOSEIII 11

I h u h as l)rc(litio~l illcreased at lover levels the degrce of competition decreased This result ivoultl Ilave heen expected i f Tllais had fed upon Ba1a~lrs in preference to Clltha~tlnllrsThe gen- eral effect of lIretlatio11 by Thais seenls to have Iwen to lessen the interspecific com11etition I~elow 1 1TI gt

ltllough animal communities appear cluali-tatively to be constructetl as if competition ivere regulating their structure even in the I~est studied cases there are nearly al~vays difficulties antl un-exl~loretl possil~ilities ( Hutchinson 1957)

In the present study direct ol~servations at in-

COX SELL Ecology Vol 42 No 4

a~lt l 3 ) and these survivors n-ere oftell misshapen antl smaller than those vhicli had not been crowdetl (Table V I ) rldults on the transplanted stones had suffered high mortality in the previous year (Table 111)

nother diftictllty was tliat C71tllai1zal~tswas rarely found to have settled belov mid tide level at hlilll~ort Tlie reasons for this are unknown it survived lvell if transplanted I)elov this level ill the absence of Balanz~s I n other areas of the British Isles (in soutlivest 1ngland and Ireland ior example) it occurs I)elov mid tide level

The 1)ossi11ility that ClztJznlitall(s might affect hala111tsdeleteriously remains to be considered I t is unlikely that Cl1tlzair1alitscould cause much

tervals shovetl tliat couipetitio~i vas o c c u r r i ~ ~ gmortality of Ilalanl(s 1- direct crovding its untler natural co~itlitions I n addition the evi-dence is strollg that the observed competition with Ilalnll(s vas the l~rincipal factor determining the local distril~ution of Chtl1irirltrlz(sC l l t l ~ a ~ ~ ~ a l ~ r s thrived at lower levels when it vas not grobving in contact witli Balnn14s

lIowever there remain unexplored possibilities The elitilitiation of Clztlzainal~(srequires a dense 1101~1lation of lialon~rsyet the settlenlent of Ra-lirnlis varied from year to year It Alilllmrt the settlement density of Hnlarz~ts bnlirttoitirs vas nleasuretl for 9 years I)etveen 1914 a~lt l 195s ( Iiarnes 195hl) Co~inell 1961 ) Settlemelit vas light in 2 years 19-16 antl 1958 I11 tlie 3 seasons of Hala~l~(ssettlemelit stutlietl in tlctail 1C)jZ-55 there vas a vast oversul)l)ly of larvae reatly for settlement It thus seelils 1rol)al)le that most of the Chtlzai~nlz~svhich survivetl in a year of poor settlement of Haln~zltsivould he liilletl in competi- tion with a normal settlement the folloving ear A succession of years vitli poor settlemelits of Bnln~lcsis a possil)lc I~u t it~ll~robable occurrence at Millport judging from the past record A very light settlement is lgtrol)ably the result of a chance cotubination of unfavorable veather cir-cumstances (luring the l~lanktonic lxriod (Uarnes 1956h) A-41so after a light settlement survival 011 the shore is iti~ljroved oving l)rit~cipally to the reductio~l in intraspecitic crobvdiug (Connell 1961) this ivoultl tend to favor a normal settle- ment the folloiving year since 1)arnacles are stim- ulated to settle 1)y the presence of niemhers of their ovn species already attached 011 the suriace (Knight- Jones 1953)

Tlie fate of those Chtllniilalts I-liich had sur-vived a year on tlie undisturbed areas is not knobvn since the experiment ended at that titile It is probable Iobvever that most of them ivould have been eli t~~inated within 6 tl~ontlis the mor-tality rate had increased in the spring (Figs 2

growth is n~ucli sloiver and crovtling I~etween intlivitluals of Clitllainalrrs seltlom resulted in tleath dense ~)olj~ilation of Chthirigtzallts tnight delxive larvae of Balanzts of space for settle-ment ilso Clztlznirtallls might feed on the plank- tonic larvae of Bnlanus however this vould oc-cur in March and April vhen 110th the sea vater teml~erature and rate of cirral activity (pre-sunia1)ly correlated with feeding activity) vould he near their minima (Southard 1955)

Tlie itldication from the caging experiments that predation decreased interspecific competi-tion suggests that the action of such additional factors tentls to reduce the intensity of such interactions in natural conditio~ls I11 adtlitional suggestion in this regartl timy 1)e made con-cerni~lg parasitism Crisp ( 1960) found that the growth rate of Balanzcs bnlnfzoidcs was de-creased i f indivitluals were infected with the iso- ~ ) o d parasite IIc~t~ionisrzrsbalnni ( Spence Rate) In I3ritain this l~arasite has not l~een reported from Chtl~n~lzal~rsstcllntlrs Thus i f this parasite Yere l~resellt 110th the groivth rate of Balnnlrs ant1 its al~ility to eliminate Clztlzarl~alltswould I)e decreasetl ivith a corresl)onding lessening of the tlegree of coml~etition hetveen the species

The evitlence prese~ited in this paper indicates that tlie lower limit of the intertitlal zone of Chtlln~lrnlus stcllntlls at A1illl)ort vas tletermined 1)y intel-specific coml)etition for space it11 Bnln-~ z t s Onlnnoidrs Bnlnnzts by virtue of its greater po~~ula t iondensity and faster grovth eliminated niost of tlie Clzthnnlnllts by directing crovding

At the upper limits of the zones of these species no interaction bvas observed Chtllnii~nl~ts e~ idet~t lycan exist higher on the shore than Bnla)llrs mainly as a result of its greater toleratice to heat andor desiccation

Autumn 1961 INTERSPECIFIC COMPETITIOS 721

The u ~ l ~ r of most intertidallimits animals are 1xo11al)ly determined 11y physical factors such as these Since growth rates usually decrease ~v i th increasing height on the shore it bvould be less likely that a sessile species occupying a higher zone could by competition for space prevent a lover one from extending up-ards 12ilieaise there has heen as far as the author is aivare no study made -hich sholvs that predation land species tletermines the upper limit of an intertidal animal In one of the most thorough of such studies Ilrinnan ( 1957) indicated that intense predation I)y 1)irtls accounted for an an-nual mortality of 225 of cockles (cardi lr~~ledrrlc Igtj in sand flats hvhere their total tniortality ivas 74 per year

I n regard to the lover l i~nits of an animals zone it is evident that physical factors may act directly to determine this boundary For example some active arnphipods from the upper levels of sandy beaches die if kept submerged However evidence is accumulating that the lower limits of tlistribution of intertidal animals are deter-mined mainly by biotic factors

Connell ( 1961 ) found that the shorter length of life of Bczlnn~rs balanoides at low shore levels could 1)e accounted for by selective predation l)y Thnis lapillzts and increased intraspecific corn-petition for sllace The results of the experiments in the present study c o ~ ~ f i r m suggestions of the other authors that lower limits may be due to in- terspecific competition for space Knox (1954) suggested that competition determined the dis-triljution of 2 species of I~arnacles in New Zealand Iltntlean Kenny and Stephenson ( 1956) gave indirect evidence that competition with a colonial polychaete norm ((aleolaritr) may have de-termined the lower limit of a I~arnacle (Tetra-clitcz) in Queensland Australia In turn the loner limit of (trlcolczricz appeared to he deter-~nined by competition with a tunicate Pyztra or ~vi th dense algal mats

lrith regard to the 2 species of 1)arnacles in the present paper some interesting ol)servations have heen made concernitng changes in their abundance in Britain l roore (1936) found that in southtvest England in 1933 Chtha~~lal~cs stella-trts Lvas most dense at MHVN decreasing in numbers toward MTL while Balanlrs halanoidrs increased in numbers below MH l r S - I t the same localities in 1951 Southward and Crisp (1953) fount1 that Balanl(s had alrnost disap-peared and that Clztl~a~~ral~ts bothhad increased ahove and below hIHVN Chtlla~nallrshad not reached the former densities of Bala~titsexcept

at one locality Erixham After 1951 Bnlan~as I~egan to return in numl~ers although by 1954 it had not reached the densities of 1934 Clztha- ~rznlrrshad declined 1)ut again not to its former densities (Southward and Crisp 1956)

Since Cl~thairlalus increased in al~mndance a t the lower levels vacated by Balanlrs it may lxeviously have I~een excluded by con~l~eti t ion with Balarllrs The grolvth rate of Balanl4s is greater than Clztlln~izalusboth north and south (Hatton 1938) of this location so that Balanus would be likely to -in in con~petition with Chthn~italrrsHowever changes in other environ- mental factors such as temperature may have influenced the abundance of these species in a reciprocal manner I n its return to southwest Ingland after 1951 the maxinlum density of settlement of Balanlrs vas 12 per cm2 compe-tition of the degree observed at llillport would not he expected to occur at this density A t a higher population density Balattns in southern England nould prohaldy eliminate Chtha~~talzts at low shore levels in the same manner as it did at llillport

I n Ioch Sween on the Argyll Peninsula Scot- land Igtevis and Powell (1960) have described an unusual pattern of zonation of Clzthamalz~sstel-latlts On the outer coast of the Argyll Penin-sula Clztlza~~tal~rshas a distribution sitnilar to that at YIillport 111 the tnore sheltered waters of 1och Sxveen holvever Chtlza~+znlusoccurs from above YI1T-S to about hITT2 judging the distril~ution I)y its relationship to other organisms finlatlrrs htrlanoides is scarce above h1TL in Loch S~veen so that there appears to he no pos- sil~ility of competition with Chtl~a~nalus assuch that occurring at Millport between the levels of lIT1 and 1IH lTN

In Figure 5 an attempt has been made to summarize the distribution of adults and newly settled larvae in relation to the main factors which appear to determine this distribution For Ba-lanzrs the estimates were based on the findings of a previous study (Connell 1961) intraspecific conllxtition Lvas severe at the lower levels during the first year after which predation increased in importance fith Chthamalzas it appears that avoidance of settlement or early mortality of those larvae which settled at levels below MTL and elimination by competition with Balatzzts of those which settled between MTL and MH WN were the principal causes for the ab-sence of adults below MHlTN at Millport This distribution appears to be typical for much of western Scotland

Autumn 1961 I N T E R S P E C I F I C C O h I P E T I T I O N 717

1y takes the form of undercutting rather than of s l ~ ~ o t l i e r i n ga s was the case in the spring T h e reason for thib tlifferellcc is proljal~ly that the (1rthtrlrralus a r e more tirmly attached in the spring so that the fast groving youllg llalirnus grow u p over them when they lllake contact In the autuliili tlle reverse is tlle case the Ilalanzts being firmly attached tile C-lrtlral~ztrl~tskveakly so

r l t h o l ~ ~ htlie bcttlcliicnt of Clltllt~~l~(zlrtson Stone 15 in the a u t u m n of 1954 was very tle~lse 32c1ii2 so that ~iiobtof then1 were touching all-other olily 2 of tllc 31 tlc~ths Lvcre catlsetl by intr~sl)ecitic cro~vtl ing among the Clltllal~tnltts I his i in i~ccortlit11 the tintliligs from the 1953 settlcliie~itof Clztrarrlnlrts

r I he ~i ior tal i tyrates for tlie young Clltllatnalus 011 area 71) sliouctl seasol~alvariations I3etween O c t o l ~ e r10 1934 a11tl 11 ~ y15 I955 the relative mortality rate l)er (lay X 100 was 013 on the untlisturl)etl area ant1 013 whcre I~alanushad bcen removetl ( ) v c r the next ~ n o n t h the rate increasctl to 139 on the untlisturl)ed area a n d 022 vherc Ijtrltrnrcs v~s absent T h u s the in-crease in nlortllity of young Chtlln~~~ct1rrsin late sprillg vas also associated ~vi t l ithe llresellce of Ilultrn~ts

S o ~ u eof tlic stolies t rans l~ ln~l te t lfrom high t o low 1cvels i l l the sllring of 1954 bore adul t C1rflrnlr~crlrrs O n Z stones recortls verc k e l ~ tof the s~lrvivalof these atltllts vliicl~Ii~tl scttletl in the 1uttul111 of 1952 o r i l l ~)reviot lsyears and Yere a t least 20 ~ i i o l ~ t h sold at tlle s tar t of tlie c s l ) c r i ~ i ~ e l ~ t llleir ~i ior tal i ty is sliokvn in T a l ~ l e 111 it was ~ lwaysmuch g r a t e r when Rnlnnrts - ~s~ i o t renlovetl ( In 2 of tlie 3 s t o ~ l e sthis nlortalit ra te Lvas ~ lmos ta s high a s that of the

yollllger grorll) I liese results s~ lgges t that any Clrflrn~ricrlrt~that nianagetl to survive tile conll)eti-tion for sl)ace vitli Bnlanus (luring tlie first year vo~lltl11rol)aI)ly I J V eliminated in the 211tl year

Cclisuses of IIt~l(l)zltsvere not made on t h e exl)erin~elital areas I Iokvever on nlany o ther areas i l l tlie sanie stretch of shore the survival of Hnln~lrts -as Ijeing studietl dur ing the same periotl (Colinell 1901) I n Tahle IV some mor-tality ra t r s meas~lre(1in that stutly a r e l is ted the Rtrlcl~rrs -ere m e m l ~ e r s of the 1954 settlement at ~ ~ o l ) ~ ~ l a t i t ~ ~ ~densities ant1 shore levels sitnilar to those of the present stutly T h e mortality rates of Hn1n~zts -ere allout the same a s those of Chtlrn1lrnlrrs in similar situations except a t the highcst level n r ra 1 where Bnlanrts suffered much greater morttlity than Cllthalrzc~lzts M u c h of this mortality lvas carlsetl 1)y intraspecific crowding a t all levels I)eloiv area 1

T~ti11111 C O I ~ ~ ~ I ~ ~ ~ S O I Iof the mortality rates of young ant1 oltler C h t h t i ) t ~ i ~ l ~ i sctrlluttrr on transplantetl stones

No

1 0 ft below Iib LlTI

---

hfTL In a t ~ d elwol raged- -

2 5 ft below MTL caned

I 1053 year

group - -A ~

Ralanus rcnroved 51

Yumher of Chlho-rnalup prrsent In

June 1951

- ~ ---

Ralanus removed 50

l J t~d~sturbcd 60 ~ ~ - - -~-

7 morta l~tyover one year (or for 0 months for 14a) of Chlharnalua

or older

36

I A I I I E I V C ~ I I ~ ~ I I ~ ~ ~ O I Iof ~ I I I I I I L ~111ortality rates of Chthnmalus strllntirs ant1 Bulanirs hnlanoides

I

Area no

Chthamalun tella at us autumn 1053 srttlement

128 1 (tide pool) 19 Oh

Ilcight in ft from MT1

Balan~tsbalanoides spring 1954 settlement

1 ( top) 1

1fiddle C a ~ e1 1 + 2 1 1 85 1 lIiddle Cage 2 I I

25

lopulation density

nocm2 June 1954

Stone 1 - 0 9 1 20 1 X 6 Stone 2 69 i 94

76mortality in the next

year

Population densityincludes hoth species The mortality rat- of Chthamalua refer to those on thp und~sturhedpnrtioni of each arpa Thp data and arpa des~ana-tions for Balanna were taken from Connell(1961) the present area 1 is the same as that designated 1 (top) in that p a p r

In the ohserat ions made a t each census it appeared that Balnnzts was grobving faster than Chthnnznlzts hleasurelnents of growth rates of the 2 species were niatle f rom photographs of

the areas taken in J u ~ l e ant1 Sovemher 1954 Barnacles growing free of contact with each other were measured the results are given in Table V The growth rate uf Itrlntlus ivas greater than that of Clltlawzallrs in the experimental areas this agrees lvith the tintlings of I latton (1938) on the shore in 1rallce 11ltl of 1arlles ( 1956a) for collti~lr~al sul)~llerge~lce a at illillporton raft

~ A I I I ~V (irowtli r a t c of Cl r t l r i~r~~c~l~ts andcti~ll~rfrcs I~trlnriris hc~li~troitlrs lcasurcr~lcr~ts were lllatlc of un-crowtletl illdivitluala oli ~~llotogr l l )hs areasof 3a 31) and 7h lliose of Cl~fl~irrrrirlrtswcrc ni~tlrOII tlie carlle intli- vitlual or1 I)otli tlrtcs I J I llirl~rr~rrr rcl)r-csctrtctiw ~ r n l ) l e s

ere c h o w r ~

- - --- -- --- i - - -vrraqe sir ill tit ~ u t c r v a l I j

l~xlutc yrowti l rillt llcr day x 100 1 21

liter a year of cro-(ling the average 1)ol)ula- tion de~lsities of Bafan~rs a11tl Clztlzattnlus re-111ainetl i l l the assl~lle rcslativc l ) ru l )or t io~~ tlley hat1 Ijee~i at tllc thestart s i ~ ~ c c ~llortality rltes ere about the sa111c I lovevcr 1)ecluse of its faster growtll 1 ~ ~ ~ l ~ r n ~ r s Locc111)i~iI rc1~tively grc3at- er area a~l t l presunlal~ly ~)ossessetl I greater I~iomass relative to tllat oi Clltllnamp~lnlzrc after a vear

The faster grovtIi of Ilal(ltlrrs prol)al)ly lc- co1111ts for tile maliner ill vllich Clztl~nrr~nlrrs vere crowtletl by Bn1atzlrs It also accounts ior the sinuosity of the st1rviv11 curves of Clltlrtrrrrc1l~rs growing in contact it11 Ral(1nzrs The mort~lity rate of these C l z t l ~ n ~ ~ ~ a l ~ r s intlicated 1)y theas s l o l ~of the curves in Figs 2 ant1 3 greatest in suliuiier tlecreasetl in winter ant1 increased again ill spring The survival curves of Clltlrn-lrzalzrs growing vithout contact ~vi th Balntzlrs tlo not show these seasonal variatiolls which there- fore cannot he the result of the tlirect action of physical factors such as temperature vave action or rain

Seasonal variations in grovth rate of Ra1nnzlc correspond to these changes in mortality rate of Clzthamalus In Figure 1 the growth of Balanl~s throughout the year as studied on an intertidal

CON S E L L Ecology V O ~ 42 NO 4

creases in the growth rate of Balanus The correlat~on ivas tested using the Spearman rank correlation coefficient The absolute increase in diameter of Balanus in each month reat1 from the curve of growth was conllgtaretl to the per-centage mortality of C1zfhar11nllts in the same month For the 13 months in which tlata for Chthnri~rrlrrs was ava~lalde the correlation was highly significant I = 01

GROWTH OF BALANUS

M O N T H S

1-1(1 coml~arisoliof the casorial c l lange ill the groikt11 of Ilillrr~t~rsI1cilanordcr aritl i l l the survival of Cl~tl~c~rriirlrcs I ~ c i t ~ g I)y Rala)tus T h ertrllirtrir crowtlctl x r o t l ~ of 1jnlirri1rc was that of ~)ariel3 Iltar~ies and Iowcll (1953) just a l~ovc MT1- on Kcppcl Iier tilll)ort tluritig 1051-52 T h e Clztllc~tnalus were or1 area la of tlic 1)rc~crlt stutly orie-lialf mile soutli of Keppcll Iicr tlut-irig 1051-55

Iro111 ill these o1)scrvations it appears tllat tlie 1)oor sl~rvival of C-lrtlrnltzn1rrc 11elov R1 t LN is a result n ~ a i ~ l l y of crovtli~ig 11y dense ~)opula- t io~is of faster groving llnlnnlrs

t the clitl of in June 1955 the cxl)eri~lle~it tlle survivil~g Clltllc~irrnlr~s ere collected iron1 5 of tlie treis Is s l io v~~in Table V I the acrage size u as greater in the Clltllnrrcllirs a hich hatl grolvn free of contact with Bnlnillrs ill every case the tlifference -as significant (Plt 01 ~ l a ~ ~ ~ ~ - ~ l i i t ~ ~ e yV test Siege1 1956) The stir-

vivors on the undisturhetl areas Jvere often mis- shapen in some cases as a result of being lifted 011 to the sitle of an underct~tting Bnlnnr(s Thus the smaller size of these barnacles may have been tlue to disturbances in the normal pattern of grovth while they were I~eing cro detl

These Cllflza~llalzrs vere examined for the pres- ence of developing larvae in their mantle cavities I s shovn in Tahle -I in every area the pro- portion of the ~uncrovtled Cl~thn~lrnllrs with larvae

1pa11el at ilIillport Barnes and Iowell (1953) -as equal to or more often slightly greater than is compared to the survival of Clrtllnrr~nl~rsat on the crovtletl areas The reason for this may about the same intertidal level ill the present 11e related to the smaller size of the crowtled study The increased mortality of Clltlln~r~alits -as ( l~tl~a~rrnl~is is due to separation since It not foulld to occur in the same seasons as the in- Clztharrlali~s can self-fertilize ( Barnes and Crisp

--

- - -- - - - -- - - -- - - -

719 Autumn 1961 INTERSPECIFIC C O h i P E T I T I O S

TABIEL-I The effect of c r o ~ + d i n gon the size and pres- TABLI- 11 T h e effect of predation by Thois lapillus ence of ~ t c l ( ~ t f ( s on the annual mortality rate of Ci~thcl~~roll ts inlarae In C l ~ t h t l ~ ~ i t ~ l ~ ~ s collected in June stcllatirs

1955

Ievel Number I N YM 1 of lndiv~d-D I A M ~ T E I ~ of I uals whirh

4 r r a T r r a t m e t C h h - had larvae ~n1 1 i k

Baianur removed

6 U n d i s t u r b e d Baianus removed

I -i a b- b I Und~qturhed

rnaius 1 Average ( Range J m a n t ~ eravity

- - -1 0 13 11 -

O i 10

a Protected from predatlotl by a cagr

-- - ---- - --Arra

W t h Without Dif-1 Rninnts Ralanur -f e r e n ~ e

------- ------Arra i 73 (112) 25 (96) 18 Area i 90 ( 42) 47 (40) 43 Area 1 l 62 ( 21) 18 (18) 31 Arra I 2 100 ( 60) 53 (501 li Arra 1i 98 i 72) 4 ( 7 i ) 89

tlie experimental areas

mortality o i Chlhamnlus over a year (The initial Inlm-bers are given In parentheses)

-b Unproterted oprn to

predatlnn ~

W I ~ I I ( l l t i r o u t I DC Rninu Ralntius ( f e r ~ n c e

Balariu3rernoved( ( 23

1050) loore ( 1935) and th rnes (1953) have shov11 that the number of larvae in an individual of Bala~lusDala~roidcsi~zcreasesit11 increase in volunle of the parent Co~npariso~zof the cube of the diameter which is proportional to the volume of Chtha~~zalnsvitIi and without lalanus sholvs that the volume may I)e decreased to lnormal size ivllen crovtlitng occurs lssuming that the relation l~etn-eenlarval numljers and volume 1x1

C~thailralrlsis similar to that of Balt-rnus a de-crease ill I)cgtthfrecluency of occurrence ant1 abun-dance of larvae in Clltl~a~tzalusresults fro111 corn-petition bvith Balavtrls Thus the 1)rocess described in this lgtal)er satisties I)oth asl~ectsof intersl~cific competition as tlefinetl 11) IClto~i alitl l l i l ler (1954) in ~vhichone sl~ecicsaffects the pop-ulation uf another 1)y a process of interference ie 1)y reducing the reprotluctive efficiency or in-creasing the mortality of its conq)etitor

711~effrc-t of prctintiovt b y lhnis Cages which exclutled Thais liad been attached

011 6 areas (indicated l)y the letter a following tlie number of the area) rea 14 was not in-cluded in the folio-ing analysis since many star-fish ere ol~servetlfeeding on the 1)arnacles at this level one entered the cage in January 1955 and ate most of the barnacles

Tlzais ivere common in this locality feeding on barnacles and mussels and reaching average l)olx~lation densities of 2 0 0 r n V e l o w MTL

The records for 12a extend over only 10 months for purlwses of compar~sol~ the mortality rat for 12a hss been nrultiplied by 11

tTide pool

at loiver levels greater outside Densities of Tlzais tend to be greater at and below h1TL so that this trentl in the mortality rates of Chthawialus may he ascril~edto an increase in predation by Thais it lower levels

Mortality of Clzthavttalz4s in the absence of lialaltus was appreciably greater outside than insitle the cage only on area 13 In the other 4 areas it seems evident that few Chthat+zalus were 1)eing eaten by Thais In a concurrent study of the I)ehavior of Thais in feeding on Balanzts bala~~oidrsit was foulid that Tlzais selected the larger i~ldividualsas prey ( Couuell 1961) Since Bnlalrlls after a few months growth was usually larger thali Chthanzalzcs it might he expected that 7hnis would feed on Balanus in preference to Clltllc~ir~alusIn a later study (unpublished) made at Santa Uarbara California Thais entargin-ata Ueshayes were enclosed in cages on the shore vith mixed pol)ulations of Balanus ylandula Dar-vin and Clztha~wzllrsfissus Darwin These species Yere each of the salne size range as the corre-sl~ontlingsl~eciesat Millport I t was fount1 that Tlzais emarginata fed on Balanus glandula in preference to Chthamulus fissus

As has k e n indicated much of the ~nortality of Clzfl~amalusgrowing naturally intermingled with Bala~tuswas a result of direct crowding by Ralanrts I t therefore seemed reasonable to take the difference between the ~nortality rates of Clztl~art~al~tswith and without Balanus as an index

(Connell 1961) The mortality rates for Clztlza~rt- of the degree of c ~ m ~ t i t i o nbetween the species nllrs in cages and on adjacent areas outside cages (indicated l ~ ythe letter h after the number) are shovn on Ta l~ leVII

If the mortality rates of Clztllanznllcs growing vitliout contact with Balanz4s are compared in and out of the cages it can be seen that at the upper levels mortality is greater inside the cages

This difference was calculated for each area and is included in Table VI I If these differences are compared between each pair of adjacent areas in and out of a cage it appears that the difference and therefore the degree of competition was greater outside the cages at the upper shore levels and less outside the cages at the lower levels

720 JOSEIII 11

I h u h as l)rc(litio~l illcreased at lover levels the degrce of competition decreased This result ivoultl Ilave heen expected i f Tllais had fed upon Ba1a~lrs in preference to Clltha~tlnllrsThe gen- eral effect of lIretlatio11 by Thais seenls to have Iwen to lessen the interspecific com11etition I~elow 1 1TI gt

ltllough animal communities appear cluali-tatively to be constructetl as if competition ivere regulating their structure even in the I~est studied cases there are nearly al~vays difficulties antl un-exl~loretl possil~ilities ( Hutchinson 1957)

In the present study direct ol~servations at in-

COX SELL Ecology Vol 42 No 4

a~lt l 3 ) and these survivors n-ere oftell misshapen antl smaller than those vhicli had not been crowdetl (Table V I ) rldults on the transplanted stones had suffered high mortality in the previous year (Table 111)

nother diftictllty was tliat C71tllai1zal~tswas rarely found to have settled belov mid tide level at hlilll~ort Tlie reasons for this are unknown it survived lvell if transplanted I)elov this level ill the absence of Balanz~s I n other areas of the British Isles (in soutlivest 1ngland and Ireland ior example) it occurs I)elov mid tide level

The 1)ossi11ility that ClztJznlitall(s might affect hala111tsdeleteriously remains to be considered I t is unlikely that Cl1tlzair1alitscould cause much

tervals shovetl tliat couipetitio~i vas o c c u r r i ~ ~ gmortality of Ilalanl(s 1- direct crovding its untler natural co~itlitions I n addition the evi-dence is strollg that the observed competition with Ilalnll(s vas the l~rincipal factor determining the local distril~ution of Chtl1irirltrlz(sC l l t l ~ a ~ ~ ~ a l ~ r s thrived at lower levels when it vas not grobving in contact witli Balnn14s

lIowever there remain unexplored possibilities The elitilitiation of Clztlzainal~(srequires a dense 1101~1lation of lialon~rsyet the settlenlent of Ra-lirnlis varied from year to year It Alilllmrt the settlement density of Hnlarz~ts bnlirttoitirs vas nleasuretl for 9 years I)etveen 1914 a~lt l 195s ( Iiarnes 195hl) Co~inell 1961 ) Settlemelit vas light in 2 years 19-16 antl 1958 I11 tlie 3 seasons of Hala~l~(ssettlemelit stutlietl in tlctail 1C)jZ-55 there vas a vast oversul)l)ly of larvae reatly for settlement It thus seelils 1rol)al)le that most of the Chtlzai~nlz~svhich survivetl in a year of poor settlement of Haln~zltsivould he liilletl in competi- tion with a normal settlement the folloving ear A succession of years vitli poor settlemelits of Bnln~lcsis a possil)lc I~u t it~ll~robable occurrence at Millport judging from the past record A very light settlement is lgtrol)ably the result of a chance cotubination of unfavorable veather cir-cumstances (luring the l~lanktonic lxriod (Uarnes 1956h) A-41so after a light settlement survival 011 the shore is iti~ljroved oving l)rit~cipally to the reductio~l in intraspecitic crobvdiug (Connell 1961) this ivoultl tend to favor a normal settle- ment the folloiving year since 1)arnacles are stim- ulated to settle 1)y the presence of niemhers of their ovn species already attached 011 the suriace (Knight- Jones 1953)

Tlie fate of those Chtllniilalts I-liich had sur-vived a year on tlie undisturbed areas is not knobvn since the experiment ended at that titile It is probable Iobvever that most of them ivould have been eli t~~inated within 6 tl~ontlis the mor-tality rate had increased in the spring (Figs 2

growth is n~ucli sloiver and crovtling I~etween intlivitluals of Clitllainalrrs seltlom resulted in tleath dense ~)olj~ilation of Chthirigtzallts tnight delxive larvae of Balanzts of space for settle-ment ilso Clztlznirtallls might feed on the plank- tonic larvae of Bnlanus however this vould oc-cur in March and April vhen 110th the sea vater teml~erature and rate of cirral activity (pre-sunia1)ly correlated with feeding activity) vould he near their minima (Southard 1955)

Tlie itldication from the caging experiments that predation decreased interspecific competi-tion suggests that the action of such additional factors tentls to reduce the intensity of such interactions in natural conditio~ls I11 adtlitional suggestion in this regartl timy 1)e made con-cerni~lg parasitism Crisp ( 1960) found that the growth rate of Balanzcs bnlnfzoidcs was de-creased i f indivitluals were infected with the iso- ~ ) o d parasite IIc~t~ionisrzrsbalnni ( Spence Rate) In I3ritain this l~arasite has not l~een reported from Chtl~n~lzal~rsstcllntlrs Thus i f this parasite Yere l~resellt 110th the groivth rate of Balnnlrs ant1 its al~ility to eliminate Clztlzarl~alltswould I)e decreasetl ivith a corresl)onding lessening of the tlegree of coml~etition hetveen the species

The evitlence prese~ited in this paper indicates that tlie lower limit of the intertitlal zone of Chtlln~lrnlus stcllntlls at A1illl)ort vas tletermined 1)y intel-specific coml)etition for space it11 Bnln-~ z t s Onlnnoidrs Bnlnnzts by virtue of its greater po~~ula t iondensity and faster grovth eliminated niost of tlie Clzthnnlnllts by directing crovding

At the upper limits of the zones of these species no interaction bvas observed Chtllnii~nl~ts e~ idet~t lycan exist higher on the shore than Bnla)llrs mainly as a result of its greater toleratice to heat andor desiccation

Autumn 1961 INTERSPECIFIC COMPETITIOS 721

The u ~ l ~ r of most intertidallimits animals are 1xo11al)ly determined 11y physical factors such as these Since growth rates usually decrease ~v i th increasing height on the shore it bvould be less likely that a sessile species occupying a higher zone could by competition for space prevent a lover one from extending up-ards 12ilieaise there has heen as far as the author is aivare no study made -hich sholvs that predation land species tletermines the upper limit of an intertidal animal In one of the most thorough of such studies Ilrinnan ( 1957) indicated that intense predation I)y 1)irtls accounted for an an-nual mortality of 225 of cockles (cardi lr~~ledrrlc Igtj in sand flats hvhere their total tniortality ivas 74 per year

I n regard to the lover l i~nits of an animals zone it is evident that physical factors may act directly to determine this boundary For example some active arnphipods from the upper levels of sandy beaches die if kept submerged However evidence is accumulating that the lower limits of tlistribution of intertidal animals are deter-mined mainly by biotic factors

Connell ( 1961 ) found that the shorter length of life of Bczlnn~rs balanoides at low shore levels could 1)e accounted for by selective predation l)y Thnis lapillzts and increased intraspecific corn-petition for sllace The results of the experiments in the present study c o ~ ~ f i r m suggestions of the other authors that lower limits may be due to in- terspecific competition for space Knox (1954) suggested that competition determined the dis-triljution of 2 species of I~arnacles in New Zealand Iltntlean Kenny and Stephenson ( 1956) gave indirect evidence that competition with a colonial polychaete norm ((aleolaritr) may have de-termined the lower limit of a I~arnacle (Tetra-clitcz) in Queensland Australia In turn the loner limit of (trlcolczricz appeared to he deter-~nined by competition with a tunicate Pyztra or ~vi th dense algal mats

lrith regard to the 2 species of 1)arnacles in the present paper some interesting ol)servations have heen made concernitng changes in their abundance in Britain l roore (1936) found that in southtvest England in 1933 Chtha~~lal~cs stella-trts Lvas most dense at MHVN decreasing in numbers toward MTL while Balanlrs halanoidrs increased in numbers below MH l r S - I t the same localities in 1951 Southward and Crisp (1953) fount1 that Balanl(s had alrnost disap-peared and that Clztl~a~~ral~ts bothhad increased ahove and below hIHVN Chtlla~nallrshad not reached the former densities of Bala~titsexcept

at one locality Erixham After 1951 Bnlan~as I~egan to return in numl~ers although by 1954 it had not reached the densities of 1934 Clztha- ~rznlrrshad declined 1)ut again not to its former densities (Southward and Crisp 1956)

Since Cl~thairlalus increased in al~mndance a t the lower levels vacated by Balanlrs it may lxeviously have I~een excluded by con~l~eti t ion with Balarllrs The grolvth rate of Balanl4s is greater than Clztlln~izalusboth north and south (Hatton 1938) of this location so that Balanus would be likely to -in in con~petition with Chthn~italrrsHowever changes in other environ- mental factors such as temperature may have influenced the abundance of these species in a reciprocal manner I n its return to southwest Ingland after 1951 the maxinlum density of settlement of Balanlrs vas 12 per cm2 compe-tition of the degree observed at llillport would not he expected to occur at this density A t a higher population density Balattns in southern England nould prohaldy eliminate Chtha~~talzts at low shore levels in the same manner as it did at llillport

I n Ioch Sween on the Argyll Peninsula Scot- land Igtevis and Powell (1960) have described an unusual pattern of zonation of Clzthamalz~sstel-latlts On the outer coast of the Argyll Penin-sula Clztlza~~tal~rshas a distribution sitnilar to that at YIillport 111 the tnore sheltered waters of 1och Sxveen holvever Chtlza~+znlusoccurs from above YI1T-S to about hITT2 judging the distril~ution I)y its relationship to other organisms finlatlrrs htrlanoides is scarce above h1TL in Loch S~veen so that there appears to he no pos- sil~ility of competition with Chtl~a~nalus assuch that occurring at Millport between the levels of lIT1 and 1IH lTN

In Figure 5 an attempt has been made to summarize the distribution of adults and newly settled larvae in relation to the main factors which appear to determine this distribution For Ba-lanzrs the estimates were based on the findings of a previous study (Connell 1961) intraspecific conllxtition Lvas severe at the lower levels during the first year after which predation increased in importance fith Chthamalzas it appears that avoidance of settlement or early mortality of those larvae which settled at levels below MTL and elimination by competition with Balatzzts of those which settled between MTL and MH WN were the principal causes for the ab-sence of adults below MHlTN at Millport This distribution appears to be typical for much of western Scotland

the areas taken in J u ~ l e ant1 Sovemher 1954 Barnacles growing free of contact with each other were measured the results are given in Table V The growth rate uf Itrlntlus ivas greater than that of Clltlawzallrs in the experimental areas this agrees lvith the tintlings of I latton (1938) on the shore in 1rallce 11ltl of 1arlles ( 1956a) for collti~lr~al sul)~llerge~lce a at illillporton raft

~ A I I I ~V (irowtli r a t c of Cl r t l r i~r~~c~l~ts andcti~ll~rfrcs I~trlnriris hc~li~troitlrs lcasurcr~lcr~ts were lllatlc of un-crowtletl illdivitluala oli ~~llotogr l l )hs areasof 3a 31) and 7h lliose of Cl~fl~irrrrirlrtswcrc ni~tlrOII tlie carlle intli- vitlual or1 I)otli tlrtcs I J I llirl~rr~rrr rcl)r-csctrtctiw ~ r n l ) l e s

ere c h o w r ~

- - --- -- --- i - - -vrraqe sir ill tit ~ u t c r v a l I j

l~xlutc yrowti l rillt llcr day x 100 1 21

liter a year of cro-(ling the average 1)ol)ula- tion de~lsities of Bafan~rs a11tl Clztlzattnlus re-111ainetl i l l the assl~lle rcslativc l ) ru l )or t io~~ tlley hat1 Ijee~i at tllc thestart s i ~ ~ c c ~llortality rltes ere about the sa111c I lovevcr 1)ecluse of its faster growtll 1 ~ ~ ~ l ~ r n ~ r s Locc111)i~iI rc1~tively grc3at- er area a~l t l presunlal~ly ~)ossessetl I greater I~iomass relative to tllat oi Clltllnamp~lnlzrc after a vear

The faster grovtIi of Ilal(ltlrrs prol)al)ly lc- co1111ts for tile maliner ill vllich Clztl~nrr~nlrrs vere crowtletl by Bn1atzlrs It also accounts ior the sinuosity of the st1rviv11 curves of Clltlrtrrrrc1l~rs growing in contact it11 Ral(1nzrs The mort~lity rate of these C l z t l ~ n ~ ~ ~ a l ~ r s intlicated 1)y theas s l o l ~of the curves in Figs 2 ant1 3 greatest in suliuiier tlecreasetl in winter ant1 increased again ill spring The survival curves of Clltlrn-lrzalzrs growing vithout contact ~vi th Balntzlrs tlo not show these seasonal variatiolls which there- fore cannot he the result of the tlirect action of physical factors such as temperature vave action or rain

Seasonal variations in grovth rate of Ra1nnzlc correspond to these changes in mortality rate of Clzthamalus In Figure 1 the growth of Balanl~s throughout the year as studied on an intertidal

CON S E L L Ecology V O ~ 42 NO 4

creases in the growth rate of Balanus The correlat~on ivas tested using the Spearman rank correlation coefficient The absolute increase in diameter of Balanus in each month reat1 from the curve of growth was conllgtaretl to the per-centage mortality of C1zfhar11nllts in the same month For the 13 months in which tlata for Chthnri~rrlrrs was ava~lalde the correlation was highly significant I = 01

GROWTH OF BALANUS

M O N T H S

1-1(1 coml~arisoliof the casorial c l lange ill the groikt11 of Ilillrr~t~rsI1cilanordcr aritl i l l the survival of Cl~tl~c~rriirlrcs I ~ c i t ~ g I)y Rala)tus T h ertrllirtrir crowtlctl x r o t l ~ of 1jnlirri1rc was that of ~)ariel3 Iltar~ies and Iowcll (1953) just a l~ovc MT1- on Kcppcl Iier tilll)ort tluritig 1051-52 T h e Clztllc~tnalus were or1 area la of tlic 1)rc~crlt stutly orie-lialf mile soutli of Keppcll Iicr tlut-irig 1051-55

Iro111 ill these o1)scrvations it appears tllat tlie 1)oor sl~rvival of C-lrtlrnltzn1rrc 11elov R1 t LN is a result n ~ a i ~ l l y of crovtli~ig 11y dense ~)opula- t io~is of faster groving llnlnnlrs

t the clitl of in June 1955 the cxl)eri~lle~it tlle survivil~g Clltllc~irrnlr~s ere collected iron1 5 of tlie treis Is s l io v~~in Table V I the acrage size u as greater in the Clltllnrrcllirs a hich hatl grolvn free of contact with Bnlnillrs ill every case the tlifference -as significant (Plt 01 ~ l a ~ ~ ~ ~ - ~ l i i t ~ ~ e yV test Siege1 1956) The stir-

vivors on the undisturhetl areas Jvere often mis- shapen in some cases as a result of being lifted 011 to the sitle of an underct~tting Bnlnnr(s Thus the smaller size of these barnacles may have been tlue to disturbances in the normal pattern of grovth while they were I~eing cro detl

These Cllflza~llalzrs vere examined for the pres- ence of developing larvae in their mantle cavities I s shovn in Tahle -I in every area the pro- portion of the ~uncrovtled Cl~thn~lrnllrs with larvae

1pa11el at ilIillport Barnes and Iowell (1953) -as equal to or more often slightly greater than is compared to the survival of Clrtllnrr~nl~rsat on the crovtletl areas The reason for this may about the same intertidal level ill the present 11e related to the smaller size of the crowtled study The increased mortality of Clltlln~r~alits -as ( l~tl~a~rrnl~is is due to separation since It not foulld to occur in the same seasons as the in- Clztharrlali~s can self-fertilize ( Barnes and Crisp

--

- - -- - - - -- - - -- - - -

719 Autumn 1961 INTERSPECIFIC C O h i P E T I T I O S

TABIEL-I The effect of c r o ~ + d i n gon the size and pres- TABLI- 11 T h e effect of predation by Thois lapillus ence of ~ t c l ( ~ t f ( s on the annual mortality rate of Ci~thcl~~roll ts inlarae In C l ~ t h t l ~ ~ i t ~ l ~ ~ s collected in June stcllatirs

1955

Ievel Number I N YM 1 of lndiv~d-D I A M ~ T E I ~ of I uals whirh

4 r r a T r r a t m e t C h h - had larvae ~n1 1 i k

Baianur removed

6 U n d i s t u r b e d Baianus removed

I -i a b- b I Und~qturhed

rnaius 1 Average ( Range J m a n t ~ eravity

- - -1 0 13 11 -

O i 10

a Protected from predatlotl by a cagr

-- - ---- - --Arra

W t h Without Dif-1 Rninnts Ralanur -f e r e n ~ e

------- ------Arra i 73 (112) 25 (96) 18 Area i 90 ( 42) 47 (40) 43 Area 1 l 62 ( 21) 18 (18) 31 Arra I 2 100 ( 60) 53 (501 li Arra 1i 98 i 72) 4 ( 7 i ) 89

tlie experimental areas

mortality o i Chlhamnlus over a year (The initial Inlm-bers are given In parentheses)

-b Unproterted oprn to

predatlnn ~

W I ~ I I ( l l t i r o u t I DC Rninu Ralntius ( f e r ~ n c e

Balariu3rernoved( ( 23

1050) loore ( 1935) and th rnes (1953) have shov11 that the number of larvae in an individual of Bala~lusDala~roidcsi~zcreasesit11 increase in volunle of the parent Co~npariso~zof the cube of the diameter which is proportional to the volume of Chtha~~zalnsvitIi and without lalanus sholvs that the volume may I)e decreased to lnormal size ivllen crovtlitng occurs lssuming that the relation l~etn-eenlarval numljers and volume 1x1

C~thailralrlsis similar to that of Balt-rnus a de-crease ill I)cgtthfrecluency of occurrence ant1 abun-dance of larvae in Clltl~a~tzalusresults fro111 corn-petition bvith Balavtrls Thus the 1)rocess described in this lgtal)er satisties I)oth asl~ectsof intersl~cific competition as tlefinetl 11) IClto~i alitl l l i l ler (1954) in ~vhichone sl~ecicsaffects the pop-ulation uf another 1)y a process of interference ie 1)y reducing the reprotluctive efficiency or in-creasing the mortality of its conq)etitor

711~effrc-t of prctintiovt b y lhnis Cages which exclutled Thais liad been attached

011 6 areas (indicated l)y the letter a following tlie number of the area) rea 14 was not in-cluded in the folio-ing analysis since many star-fish ere ol~servetlfeeding on the 1)arnacles at this level one entered the cage in January 1955 and ate most of the barnacles

Tlzais ivere common in this locality feeding on barnacles and mussels and reaching average l)olx~lation densities of 2 0 0 r n V e l o w MTL

The records for 12a extend over only 10 months for purlwses of compar~sol~ the mortality rat for 12a hss been nrultiplied by 11

tTide pool

at loiver levels greater outside Densities of Tlzais tend to be greater at and below h1TL so that this trentl in the mortality rates of Chthawialus may he ascril~edto an increase in predation by Thais it lower levels

Mortality of Clzthavttalz4s in the absence of lialaltus was appreciably greater outside than insitle the cage only on area 13 In the other 4 areas it seems evident that few Chthat+zalus were 1)eing eaten by Thais In a concurrent study of the I)ehavior of Thais in feeding on Balanzts bala~~oidrsit was foulid that Tlzais selected the larger i~ldividualsas prey ( Couuell 1961) Since Bnlalrlls after a few months growth was usually larger thali Chthanzalzcs it might he expected that 7hnis would feed on Balanus in preference to Clltllc~ir~alusIn a later study (unpublished) made at Santa Uarbara California Thais entargin-ata Ueshayes were enclosed in cages on the shore vith mixed pol)ulations of Balanus ylandula Dar-vin and Clztha~wzllrsfissus Darwin These species Yere each of the salne size range as the corre-sl~ontlingsl~eciesat Millport I t was fount1 that Tlzais emarginata fed on Balanus glandula in preference to Chthamulus fissus

As has k e n indicated much of the ~nortality of Clzfl~amalusgrowing naturally intermingled with Bala~tuswas a result of direct crowding by Ralanrts I t therefore seemed reasonable to take the difference between the ~nortality rates of Clztl~art~al~tswith and without Balanus as an index

(Connell 1961) The mortality rates for Clztlza~rt- of the degree of c ~ m ~ t i t i o nbetween the species nllrs in cages and on adjacent areas outside cages (indicated l ~ ythe letter h after the number) are shovn on Ta l~ leVII

If the mortality rates of Clztllanznllcs growing vitliout contact with Balanz4s are compared in and out of the cages it can be seen that at the upper levels mortality is greater inside the cages

This difference was calculated for each area and is included in Table VI I If these differences are compared between each pair of adjacent areas in and out of a cage it appears that the difference and therefore the degree of competition was greater outside the cages at the upper shore levels and less outside the cages at the lower levels

720 JOSEIII 11

I h u h as l)rc(litio~l illcreased at lover levels the degrce of competition decreased This result ivoultl Ilave heen expected i f Tllais had fed upon Ba1a~lrs in preference to Clltha~tlnllrsThe gen- eral effect of lIretlatio11 by Thais seenls to have Iwen to lessen the interspecific com11etition I~elow 1 1TI gt

ltllough animal communities appear cluali-tatively to be constructetl as if competition ivere regulating their structure even in the I~est studied cases there are nearly al~vays difficulties antl un-exl~loretl possil~ilities ( Hutchinson 1957)

In the present study direct ol~servations at in-

COX SELL Ecology Vol 42 No 4

a~lt l 3 ) and these survivors n-ere oftell misshapen antl smaller than those vhicli had not been crowdetl (Table V I ) rldults on the transplanted stones had suffered high mortality in the previous year (Table 111)

nother diftictllty was tliat C71tllai1zal~tswas rarely found to have settled belov mid tide level at hlilll~ort Tlie reasons for this are unknown it survived lvell if transplanted I)elov this level ill the absence of Balanz~s I n other areas of the British Isles (in soutlivest 1ngland and Ireland ior example) it occurs I)elov mid tide level

The 1)ossi11ility that ClztJznlitall(s might affect hala111tsdeleteriously remains to be considered I t is unlikely that Cl1tlzair1alitscould cause much

tervals shovetl tliat couipetitio~i vas o c c u r r i ~ ~ gmortality of Ilalanl(s 1- direct crovding its untler natural co~itlitions I n addition the evi-dence is strollg that the observed competition with Ilalnll(s vas the l~rincipal factor determining the local distril~ution of Chtl1irirltrlz(sC l l t l ~ a ~ ~ ~ a l ~ r s thrived at lower levels when it vas not grobving in contact witli Balnn14s

lIowever there remain unexplored possibilities The elitilitiation of Clztlzainal~(srequires a dense 1101~1lation of lialon~rsyet the settlenlent of Ra-lirnlis varied from year to year It Alilllmrt the settlement density of Hnlarz~ts bnlirttoitirs vas nleasuretl for 9 years I)etveen 1914 a~lt l 195s ( Iiarnes 195hl) Co~inell 1961 ) Settlemelit vas light in 2 years 19-16 antl 1958 I11 tlie 3 seasons of Hala~l~(ssettlemelit stutlietl in tlctail 1C)jZ-55 there vas a vast oversul)l)ly of larvae reatly for settlement It thus seelils 1rol)al)le that most of the Chtlzai~nlz~svhich survivetl in a year of poor settlement of Haln~zltsivould he liilletl in competi- tion with a normal settlement the folloving ear A succession of years vitli poor settlemelits of Bnln~lcsis a possil)lc I~u t it~ll~robable occurrence at Millport judging from the past record A very light settlement is lgtrol)ably the result of a chance cotubination of unfavorable veather cir-cumstances (luring the l~lanktonic lxriod (Uarnes 1956h) A-41so after a light settlement survival 011 the shore is iti~ljroved oving l)rit~cipally to the reductio~l in intraspecitic crobvdiug (Connell 1961) this ivoultl tend to favor a normal settle- ment the folloiving year since 1)arnacles are stim- ulated to settle 1)y the presence of niemhers of their ovn species already attached 011 the suriace (Knight- Jones 1953)

Tlie fate of those Chtllniilalts I-liich had sur-vived a year on tlie undisturbed areas is not knobvn since the experiment ended at that titile It is probable Iobvever that most of them ivould have been eli t~~inated within 6 tl~ontlis the mor-tality rate had increased in the spring (Figs 2

growth is n~ucli sloiver and crovtling I~etween intlivitluals of Clitllainalrrs seltlom resulted in tleath dense ~)olj~ilation of Chthirigtzallts tnight delxive larvae of Balanzts of space for settle-ment ilso Clztlznirtallls might feed on the plank- tonic larvae of Bnlanus however this vould oc-cur in March and April vhen 110th the sea vater teml~erature and rate of cirral activity (pre-sunia1)ly correlated with feeding activity) vould he near their minima (Southard 1955)

Tlie itldication from the caging experiments that predation decreased interspecific competi-tion suggests that the action of such additional factors tentls to reduce the intensity of such interactions in natural conditio~ls I11 adtlitional suggestion in this regartl timy 1)e made con-cerni~lg parasitism Crisp ( 1960) found that the growth rate of Balanzcs bnlnfzoidcs was de-creased i f indivitluals were infected with the iso- ~ ) o d parasite IIc~t~ionisrzrsbalnni ( Spence Rate) In I3ritain this l~arasite has not l~een reported from Chtl~n~lzal~rsstcllntlrs Thus i f this parasite Yere l~resellt 110th the groivth rate of Balnnlrs ant1 its al~ility to eliminate Clztlzarl~alltswould I)e decreasetl ivith a corresl)onding lessening of the tlegree of coml~etition hetveen the species

The evitlence prese~ited in this paper indicates that tlie lower limit of the intertitlal zone of Chtlln~lrnlus stcllntlls at A1illl)ort vas tletermined 1)y intel-specific coml)etition for space it11 Bnln-~ z t s Onlnnoidrs Bnlnnzts by virtue of its greater po~~ula t iondensity and faster grovth eliminated niost of tlie Clzthnnlnllts by directing crovding

At the upper limits of the zones of these species no interaction bvas observed Chtllnii~nl~ts e~ idet~t lycan exist higher on the shore than Bnla)llrs mainly as a result of its greater toleratice to heat andor desiccation

Autumn 1961 INTERSPECIFIC COMPETITIOS 721

The u ~ l ~ r of most intertidallimits animals are 1xo11al)ly determined 11y physical factors such as these Since growth rates usually decrease ~v i th increasing height on the shore it bvould be less likely that a sessile species occupying a higher zone could by competition for space prevent a lover one from extending up-ards 12ilieaise there has heen as far as the author is aivare no study made -hich sholvs that predation land species tletermines the upper limit of an intertidal animal In one of the most thorough of such studies Ilrinnan ( 1957) indicated that intense predation I)y 1)irtls accounted for an an-nual mortality of 225 of cockles (cardi lr~~ledrrlc Igtj in sand flats hvhere their total tniortality ivas 74 per year

I n regard to the lover l i~nits of an animals zone it is evident that physical factors may act directly to determine this boundary For example some active arnphipods from the upper levels of sandy beaches die if kept submerged However evidence is accumulating that the lower limits of tlistribution of intertidal animals are deter-mined mainly by biotic factors

Connell ( 1961 ) found that the shorter length of life of Bczlnn~rs balanoides at low shore levels could 1)e accounted for by selective predation l)y Thnis lapillzts and increased intraspecific corn-petition for sllace The results of the experiments in the present study c o ~ ~ f i r m suggestions of the other authors that lower limits may be due to in- terspecific competition for space Knox (1954) suggested that competition determined the dis-triljution of 2 species of I~arnacles in New Zealand Iltntlean Kenny and Stephenson ( 1956) gave indirect evidence that competition with a colonial polychaete norm ((aleolaritr) may have de-termined the lower limit of a I~arnacle (Tetra-clitcz) in Queensland Australia In turn the loner limit of (trlcolczricz appeared to he deter-~nined by competition with a tunicate Pyztra or ~vi th dense algal mats

lrith regard to the 2 species of 1)arnacles in the present paper some interesting ol)servations have heen made concernitng changes in their abundance in Britain l roore (1936) found that in southtvest England in 1933 Chtha~~lal~cs stella-trts Lvas most dense at MHVN decreasing in numbers toward MTL while Balanlrs halanoidrs increased in numbers below MH l r S - I t the same localities in 1951 Southward and Crisp (1953) fount1 that Balanl(s had alrnost disap-peared and that Clztl~a~~ral~ts bothhad increased ahove and below hIHVN Chtlla~nallrshad not reached the former densities of Bala~titsexcept

at one locality Erixham After 1951 Bnlan~as I~egan to return in numl~ers although by 1954 it had not reached the densities of 1934 Clztha- ~rznlrrshad declined 1)ut again not to its former densities (Southward and Crisp 1956)

Since Cl~thairlalus increased in al~mndance a t the lower levels vacated by Balanlrs it may lxeviously have I~een excluded by con~l~eti t ion with Balarllrs The grolvth rate of Balanl4s is greater than Clztlln~izalusboth north and south (Hatton 1938) of this location so that Balanus would be likely to -in in con~petition with Chthn~italrrsHowever changes in other environ- mental factors such as temperature may have influenced the abundance of these species in a reciprocal manner I n its return to southwest Ingland after 1951 the maxinlum density of settlement of Balanlrs vas 12 per cm2 compe-tition of the degree observed at llillport would not he expected to occur at this density A t a higher population density Balattns in southern England nould prohaldy eliminate Chtha~~talzts at low shore levels in the same manner as it did at llillport

I n Ioch Sween on the Argyll Peninsula Scot- land Igtevis and Powell (1960) have described an unusual pattern of zonation of Clzthamalz~sstel-latlts On the outer coast of the Argyll Penin-sula Clztlza~~tal~rshas a distribution sitnilar to that at YIillport 111 the tnore sheltered waters of 1och Sxveen holvever Chtlza~+znlusoccurs from above YI1T-S to about hITT2 judging the distril~ution I)y its relationship to other organisms finlatlrrs htrlanoides is scarce above h1TL in Loch S~veen so that there appears to he no pos- sil~ility of competition with Chtl~a~nalus assuch that occurring at Millport between the levels of lIT1 and 1IH lTN

In Figure 5 an attempt has been made to summarize the distribution of adults and newly settled larvae in relation to the main factors which appear to determine this distribution For Ba-lanzrs the estimates were based on the findings of a previous study (Connell 1961) intraspecific conllxtition Lvas severe at the lower levels during the first year after which predation increased in importance fith Chthamalzas it appears that avoidance of settlement or early mortality of those larvae which settled at levels below MTL and elimination by competition with Balatzzts of those which settled between MTL and MH WN were the principal causes for the ab-sence of adults below MHlTN at Millport This distribution appears to be typical for much of western Scotland

--

- - -- - - - -- - - -- - - -

719 Autumn 1961 INTERSPECIFIC C O h i P E T I T I O S

TABIEL-I The effect of c r o ~ + d i n gon the size and pres- TABLI- 11 T h e effect of predation by Thois lapillus ence of ~ t c l ( ~ t f ( s on the annual mortality rate of Ci~thcl~~roll ts inlarae In C l ~ t h t l ~ ~ i t ~ l ~ ~ s collected in June stcllatirs

1955

Ievel Number I N YM 1 of lndiv~d-D I A M ~ T E I ~ of I uals whirh

4 r r a T r r a t m e t C h h - had larvae ~n1 1 i k

Baianur removed

6 U n d i s t u r b e d Baianus removed

I -i a b- b I Und~qturhed

rnaius 1 Average ( Range J m a n t ~ eravity

- - -1 0 13 11 -

O i 10

a Protected from predatlotl by a cagr

-- - ---- - --Arra

W t h Without Dif-1 Rninnts Ralanur -f e r e n ~ e

------- ------Arra i 73 (112) 25 (96) 18 Area i 90 ( 42) 47 (40) 43 Area 1 l 62 ( 21) 18 (18) 31 Arra I 2 100 ( 60) 53 (501 li Arra 1i 98 i 72) 4 ( 7 i ) 89

tlie experimental areas

mortality o i Chlhamnlus over a year (The initial Inlm-bers are given In parentheses)

-b Unproterted oprn to

predatlnn ~

W I ~ I I ( l l t i r o u t I DC Rninu Ralntius ( f e r ~ n c e

Balariu3rernoved( ( 23

1050) loore ( 1935) and th rnes (1953) have shov11 that the number of larvae in an individual of Bala~lusDala~roidcsi~zcreasesit11 increase in volunle of the parent Co~npariso~zof the cube of the diameter which is proportional to the volume of Chtha~~zalnsvitIi and without lalanus sholvs that the volume may I)e decreased to lnormal size ivllen crovtlitng occurs lssuming that the relation l~etn-eenlarval numljers and volume 1x1

C~thailralrlsis similar to that of Balt-rnus a de-crease ill I)cgtthfrecluency of occurrence ant1 abun-dance of larvae in Clltl~a~tzalusresults fro111 corn-petition bvith Balavtrls Thus the 1)rocess described in this lgtal)er satisties I)oth asl~ectsof intersl~cific competition as tlefinetl 11) IClto~i alitl l l i l ler (1954) in ~vhichone sl~ecicsaffects the pop-ulation uf another 1)y a process of interference ie 1)y reducing the reprotluctive efficiency or in-creasing the mortality of its conq)etitor

711~effrc-t of prctintiovt b y lhnis Cages which exclutled Thais liad been attached

011 6 areas (indicated l)y the letter a following tlie number of the area) rea 14 was not in-cluded in the folio-ing analysis since many star-fish ere ol~servetlfeeding on the 1)arnacles at this level one entered the cage in January 1955 and ate most of the barnacles

Tlzais ivere common in this locality feeding on barnacles and mussels and reaching average l)olx~lation densities of 2 0 0 r n V e l o w MTL

The records for 12a extend over only 10 months for purlwses of compar~sol~ the mortality rat for 12a hss been nrultiplied by 11

tTide pool

at loiver levels greater outside Densities of Tlzais tend to be greater at and below h1TL so that this trentl in the mortality rates of Chthawialus may he ascril~edto an increase in predation by Thais it lower levels

Mortality of Clzthavttalz4s in the absence of lialaltus was appreciably greater outside than insitle the cage only on area 13 In the other 4 areas it seems evident that few Chthat+zalus were 1)eing eaten by Thais In a concurrent study of the I)ehavior of Thais in feeding on Balanzts bala~~oidrsit was foulid that Tlzais selected the larger i~ldividualsas prey ( Couuell 1961) Since Bnlalrlls after a few months growth was usually larger thali Chthanzalzcs it might he expected that 7hnis would feed on Balanus in preference to Clltllc~ir~alusIn a later study (unpublished) made at Santa Uarbara California Thais entargin-ata Ueshayes were enclosed in cages on the shore vith mixed pol)ulations of Balanus ylandula Dar-vin and Clztha~wzllrsfissus Darwin These species Yere each of the salne size range as the corre-sl~ontlingsl~eciesat Millport I t was fount1 that Tlzais emarginata fed on Balanus glandula in preference to Chthamulus fissus

As has k e n indicated much of the ~nortality of Clzfl~amalusgrowing naturally intermingled with Bala~tuswas a result of direct crowding by Ralanrts I t therefore seemed reasonable to take the difference between the ~nortality rates of Clztl~art~al~tswith and without Balanus as an index

(Connell 1961) The mortality rates for Clztlza~rt- of the degree of c ~ m ~ t i t i o nbetween the species nllrs in cages and on adjacent areas outside cages (indicated l ~ ythe letter h after the number) are shovn on Ta l~ leVII

If the mortality rates of Clztllanznllcs growing vitliout contact with Balanz4s are compared in and out of the cages it can be seen that at the upper levels mortality is greater inside the cages

This difference was calculated for each area and is included in Table VI I If these differences are compared between each pair of adjacent areas in and out of a cage it appears that the difference and therefore the degree of competition was greater outside the cages at the upper shore levels and less outside the cages at the lower levels

720 JOSEIII 11

I h u h as l)rc(litio~l illcreased at lover levels the degrce of competition decreased This result ivoultl Ilave heen expected i f Tllais had fed upon Ba1a~lrs in preference to Clltha~tlnllrsThe gen- eral effect of lIretlatio11 by Thais seenls to have Iwen to lessen the interspecific com11etition I~elow 1 1TI gt

ltllough animal communities appear cluali-tatively to be constructetl as if competition ivere regulating their structure even in the I~est studied cases there are nearly al~vays difficulties antl un-exl~loretl possil~ilities ( Hutchinson 1957)

In the present study direct ol~servations at in-

COX SELL Ecology Vol 42 No 4

a~lt l 3 ) and these survivors n-ere oftell misshapen antl smaller than those vhicli had not been crowdetl (Table V I ) rldults on the transplanted stones had suffered high mortality in the previous year (Table 111)

nother diftictllty was tliat C71tllai1zal~tswas rarely found to have settled belov mid tide level at hlilll~ort Tlie reasons for this are unknown it survived lvell if transplanted I)elov this level ill the absence of Balanz~s I n other areas of the British Isles (in soutlivest 1ngland and Ireland ior example) it occurs I)elov mid tide level

The 1)ossi11ility that ClztJznlitall(s might affect hala111tsdeleteriously remains to be considered I t is unlikely that Cl1tlzair1alitscould cause much

tervals shovetl tliat couipetitio~i vas o c c u r r i ~ ~ gmortality of Ilalanl(s 1- direct crovding its untler natural co~itlitions I n addition the evi-dence is strollg that the observed competition with Ilalnll(s vas the l~rincipal factor determining the local distril~ution of Chtl1irirltrlz(sC l l t l ~ a ~ ~ ~ a l ~ r s thrived at lower levels when it vas not grobving in contact witli Balnn14s

lIowever there remain unexplored possibilities The elitilitiation of Clztlzainal~(srequires a dense 1101~1lation of lialon~rsyet the settlenlent of Ra-lirnlis varied from year to year It Alilllmrt the settlement density of Hnlarz~ts bnlirttoitirs vas nleasuretl for 9 years I)etveen 1914 a~lt l 195s ( Iiarnes 195hl) Co~inell 1961 ) Settlemelit vas light in 2 years 19-16 antl 1958 I11 tlie 3 seasons of Hala~l~(ssettlemelit stutlietl in tlctail 1C)jZ-55 there vas a vast oversul)l)ly of larvae reatly for settlement It thus seelils 1rol)al)le that most of the Chtlzai~nlz~svhich survivetl in a year of poor settlement of Haln~zltsivould he liilletl in competi- tion with a normal settlement the folloving ear A succession of years vitli poor settlemelits of Bnln~lcsis a possil)lc I~u t it~ll~robable occurrence at Millport judging from the past record A very light settlement is lgtrol)ably the result of a chance cotubination of unfavorable veather cir-cumstances (luring the l~lanktonic lxriod (Uarnes 1956h) A-41so after a light settlement survival 011 the shore is iti~ljroved oving l)rit~cipally to the reductio~l in intraspecitic crobvdiug (Connell 1961) this ivoultl tend to favor a normal settle- ment the folloiving year since 1)arnacles are stim- ulated to settle 1)y the presence of niemhers of their ovn species already attached 011 the suriace (Knight- Jones 1953)

Tlie fate of those Chtllniilalts I-liich had sur-vived a year on tlie undisturbed areas is not knobvn since the experiment ended at that titile It is probable Iobvever that most of them ivould have been eli t~~inated within 6 tl~ontlis the mor-tality rate had increased in the spring (Figs 2

growth is n~ucli sloiver and crovtling I~etween intlivitluals of Clitllainalrrs seltlom resulted in tleath dense ~)olj~ilation of Chthirigtzallts tnight delxive larvae of Balanzts of space for settle-ment ilso Clztlznirtallls might feed on the plank- tonic larvae of Bnlanus however this vould oc-cur in March and April vhen 110th the sea vater teml~erature and rate of cirral activity (pre-sunia1)ly correlated with feeding activity) vould he near their minima (Southard 1955)

Tlie itldication from the caging experiments that predation decreased interspecific competi-tion suggests that the action of such additional factors tentls to reduce the intensity of such interactions in natural conditio~ls I11 adtlitional suggestion in this regartl timy 1)e made con-cerni~lg parasitism Crisp ( 1960) found that the growth rate of Balanzcs bnlnfzoidcs was de-creased i f indivitluals were infected with the iso- ~ ) o d parasite IIc~t~ionisrzrsbalnni ( Spence Rate) In I3ritain this l~arasite has not l~een reported from Chtl~n~lzal~rsstcllntlrs Thus i f this parasite Yere l~resellt 110th the groivth rate of Balnnlrs ant1 its al~ility to eliminate Clztlzarl~alltswould I)e decreasetl ivith a corresl)onding lessening of the tlegree of coml~etition hetveen the species

The evitlence prese~ited in this paper indicates that tlie lower limit of the intertitlal zone of Chtlln~lrnlus stcllntlls at A1illl)ort vas tletermined 1)y intel-specific coml)etition for space it11 Bnln-~ z t s Onlnnoidrs Bnlnnzts by virtue of its greater po~~ula t iondensity and faster grovth eliminated niost of tlie Clzthnnlnllts by directing crovding

At the upper limits of the zones of these species no interaction bvas observed Chtllnii~nl~ts e~ idet~t lycan exist higher on the shore than Bnla)llrs mainly as a result of its greater toleratice to heat andor desiccation

Autumn 1961 INTERSPECIFIC COMPETITIOS 721

The u ~ l ~ r of most intertidallimits animals are 1xo11al)ly determined 11y physical factors such as these Since growth rates usually decrease ~v i th increasing height on the shore it bvould be less likely that a sessile species occupying a higher zone could by competition for space prevent a lover one from extending up-ards 12ilieaise there has heen as far as the author is aivare no study made -hich sholvs that predation land species tletermines the upper limit of an intertidal animal In one of the most thorough of such studies Ilrinnan ( 1957) indicated that intense predation I)y 1)irtls accounted for an an-nual mortality of 225 of cockles (cardi lr~~ledrrlc Igtj in sand flats hvhere their total tniortality ivas 74 per year

I n regard to the lover l i~nits of an animals zone it is evident that physical factors may act directly to determine this boundary For example some active arnphipods from the upper levels of sandy beaches die if kept submerged However evidence is accumulating that the lower limits of tlistribution of intertidal animals are deter-mined mainly by biotic factors

Connell ( 1961 ) found that the shorter length of life of Bczlnn~rs balanoides at low shore levels could 1)e accounted for by selective predation l)y Thnis lapillzts and increased intraspecific corn-petition for sllace The results of the experiments in the present study c o ~ ~ f i r m suggestions of the other authors that lower limits may be due to in- terspecific competition for space Knox (1954) suggested that competition determined the dis-triljution of 2 species of I~arnacles in New Zealand Iltntlean Kenny and Stephenson ( 1956) gave indirect evidence that competition with a colonial polychaete norm ((aleolaritr) may have de-termined the lower limit of a I~arnacle (Tetra-clitcz) in Queensland Australia In turn the loner limit of (trlcolczricz appeared to he deter-~nined by competition with a tunicate Pyztra or ~vi th dense algal mats

lrith regard to the 2 species of 1)arnacles in the present paper some interesting ol)servations have heen made concernitng changes in their abundance in Britain l roore (1936) found that in southtvest England in 1933 Chtha~~lal~cs stella-trts Lvas most dense at MHVN decreasing in numbers toward MTL while Balanlrs halanoidrs increased in numbers below MH l r S - I t the same localities in 1951 Southward and Crisp (1953) fount1 that Balanl(s had alrnost disap-peared and that Clztl~a~~ral~ts bothhad increased ahove and below hIHVN Chtlla~nallrshad not reached the former densities of Bala~titsexcept

at one locality Erixham After 1951 Bnlan~as I~egan to return in numl~ers although by 1954 it had not reached the densities of 1934 Clztha- ~rznlrrshad declined 1)ut again not to its former densities (Southward and Crisp 1956)

Since Cl~thairlalus increased in al~mndance a t the lower levels vacated by Balanlrs it may lxeviously have I~een excluded by con~l~eti t ion with Balarllrs The grolvth rate of Balanl4s is greater than Clztlln~izalusboth north and south (Hatton 1938) of this location so that Balanus would be likely to -in in con~petition with Chthn~italrrsHowever changes in other environ- mental factors such as temperature may have influenced the abundance of these species in a reciprocal manner I n its return to southwest Ingland after 1951 the maxinlum density of settlement of Balanlrs vas 12 per cm2 compe-tition of the degree observed at llillport would not he expected to occur at this density A t a higher population density Balattns in southern England nould prohaldy eliminate Chtha~~talzts at low shore levels in the same manner as it did at llillport

I n Ioch Sween on the Argyll Peninsula Scot- land Igtevis and Powell (1960) have described an unusual pattern of zonation of Clzthamalz~sstel-latlts On the outer coast of the Argyll Penin-sula Clztlza~~tal~rshas a distribution sitnilar to that at YIillport 111 the tnore sheltered waters of 1och Sxveen holvever Chtlza~+znlusoccurs from above YI1T-S to about hITT2 judging the distril~ution I)y its relationship to other organisms finlatlrrs htrlanoides is scarce above h1TL in Loch S~veen so that there appears to he no pos- sil~ility of competition with Chtl~a~nalus assuch that occurring at Millport between the levels of lIT1 and 1IH lTN

In Figure 5 an attempt has been made to summarize the distribution of adults and newly settled larvae in relation to the main factors which appear to determine this distribution For Ba-lanzrs the estimates were based on the findings of a previous study (Connell 1961) intraspecific conllxtition Lvas severe at the lower levels during the first year after which predation increased in importance fith Chthamalzas it appears that avoidance of settlement or early mortality of those larvae which settled at levels below MTL and elimination by competition with Balatzzts of those which settled between MTL and MH WN were the principal causes for the ab-sence of adults below MHlTN at Millport This distribution appears to be typical for much of western Scotland

720 JOSEIII 11

I h u h as l)rc(litio~l illcreased at lover levels the degrce of competition decreased This result ivoultl Ilave heen expected i f Tllais had fed upon Ba1a~lrs in preference to Clltha~tlnllrsThe gen- eral effect of lIretlatio11 by Thais seenls to have Iwen to lessen the interspecific com11etition I~elow 1 1TI gt

ltllough animal communities appear cluali-tatively to be constructetl as if competition ivere regulating their structure even in the I~est studied cases there are nearly al~vays difficulties antl un-exl~loretl possil~ilities ( Hutchinson 1957)

In the present study direct ol~servations at in-

COX SELL Ecology Vol 42 No 4

a~lt l 3 ) and these survivors n-ere oftell misshapen antl smaller than those vhicli had not been crowdetl (Table V I ) rldults on the transplanted stones had suffered high mortality in the previous year (Table 111)

nother diftictllty was tliat C71tllai1zal~tswas rarely found to have settled belov mid tide level at hlilll~ort Tlie reasons for this are unknown it survived lvell if transplanted I)elov this level ill the absence of Balanz~s I n other areas of the British Isles (in soutlivest 1ngland and Ireland ior example) it occurs I)elov mid tide level

The 1)ossi11ility that ClztJznlitall(s might affect hala111tsdeleteriously remains to be considered I t is unlikely that Cl1tlzair1alitscould cause much

tervals shovetl tliat couipetitio~i vas o c c u r r i ~ ~ gmortality of Ilalanl(s 1- direct crovding its untler natural co~itlitions I n addition the evi-dence is strollg that the observed competition with Ilalnll(s vas the l~rincipal factor determining the local distril~ution of Chtl1irirltrlz(sC l l t l ~ a ~ ~ ~ a l ~ r s thrived at lower levels when it vas not grobving in contact witli Balnn14s

lIowever there remain unexplored possibilities The elitilitiation of Clztlzainal~(srequires a dense 1101~1lation of lialon~rsyet the settlenlent of Ra-lirnlis varied from year to year It Alilllmrt the settlement density of Hnlarz~ts bnlirttoitirs vas nleasuretl for 9 years I)etveen 1914 a~lt l 195s ( Iiarnes 195hl) Co~inell 1961 ) Settlemelit vas light in 2 years 19-16 antl 1958 I11 tlie 3 seasons of Hala~l~(ssettlemelit stutlietl in tlctail 1C)jZ-55 there vas a vast oversul)l)ly of larvae reatly for settlement It thus seelils 1rol)al)le that most of the Chtlzai~nlz~svhich survivetl in a year of poor settlement of Haln~zltsivould he liilletl in competi- tion with a normal settlement the folloving ear A succession of years vitli poor settlemelits of Bnln~lcsis a possil)lc I~u t it~ll~robable occurrence at Millport judging from the past record A very light settlement is lgtrol)ably the result of a chance cotubination of unfavorable veather cir-cumstances (luring the l~lanktonic lxriod (Uarnes 1956h) A-41so after a light settlement survival 011 the shore is iti~ljroved oving l)rit~cipally to the reductio~l in intraspecitic crobvdiug (Connell 1961) this ivoultl tend to favor a normal settle- ment the folloiving year since 1)arnacles are stim- ulated to settle 1)y the presence of niemhers of their ovn species already attached 011 the suriace (Knight- Jones 1953)

Tlie fate of those Chtllniilalts I-liich had sur-vived a year on tlie undisturbed areas is not knobvn since the experiment ended at that titile It is probable Iobvever that most of them ivould have been eli t~~inated within 6 tl~ontlis the mor-tality rate had increased in the spring (Figs 2

growth is n~ucli sloiver and crovtling I~etween intlivitluals of Clitllainalrrs seltlom resulted in tleath dense ~)olj~ilation of Chthirigtzallts tnight delxive larvae of Balanzts of space for settle-ment ilso Clztlznirtallls might feed on the plank- tonic larvae of Bnlanus however this vould oc-cur in March and April vhen 110th the sea vater teml~erature and rate of cirral activity (pre-sunia1)ly correlated with feeding activity) vould he near their minima (Southard 1955)

Tlie itldication from the caging experiments that predation decreased interspecific competi-tion suggests that the action of such additional factors tentls to reduce the intensity of such interactions in natural conditio~ls I11 adtlitional suggestion in this regartl timy 1)e made con-cerni~lg parasitism Crisp ( 1960) found that the growth rate of Balanzcs bnlnfzoidcs was de-creased i f indivitluals were infected with the iso- ~ ) o d parasite IIc~t~ionisrzrsbalnni ( Spence Rate) In I3ritain this l~arasite has not l~een reported from Chtl~n~lzal~rsstcllntlrs Thus i f this parasite Yere l~resellt 110th the groivth rate of Balnnlrs ant1 its al~ility to eliminate Clztlzarl~alltswould I)e decreasetl ivith a corresl)onding lessening of the tlegree of coml~etition hetveen the species

The evitlence prese~ited in this paper indicates that tlie lower limit of the intertitlal zone of Chtlln~lrnlus stcllntlls at A1illl)ort vas tletermined 1)y intel-specific coml)etition for space it11 Bnln-~ z t s Onlnnoidrs Bnlnnzts by virtue of its greater po~~ula t iondensity and faster grovth eliminated niost of tlie Clzthnnlnllts by directing crovding

At the upper limits of the zones of these species no interaction bvas observed Chtllnii~nl~ts e~ idet~t lycan exist higher on the shore than Bnla)llrs mainly as a result of its greater toleratice to heat andor desiccation

Autumn 1961 INTERSPECIFIC COMPETITIOS 721

The u ~ l ~ r of most intertidallimits animals are 1xo11al)ly determined 11y physical factors such as these Since growth rates usually decrease ~v i th increasing height on the shore it bvould be less likely that a sessile species occupying a higher zone could by competition for space prevent a lover one from extending up-ards 12ilieaise there has heen as far as the author is aivare no study made -hich sholvs that predation land species tletermines the upper limit of an intertidal animal In one of the most thorough of such studies Ilrinnan ( 1957) indicated that intense predation I)y 1)irtls accounted for an an-nual mortality of 225 of cockles (cardi lr~~ledrrlc Igtj in sand flats hvhere their total tniortality ivas 74 per year

I n regard to the lover l i~nits of an animals zone it is evident that physical factors may act directly to determine this boundary For example some active arnphipods from the upper levels of sandy beaches die if kept submerged However evidence is accumulating that the lower limits of tlistribution of intertidal animals are deter-mined mainly by biotic factors

Connell ( 1961 ) found that the shorter length of life of Bczlnn~rs balanoides at low shore levels could 1)e accounted for by selective predation l)y Thnis lapillzts and increased intraspecific corn-petition for sllace The results of the experiments in the present study c o ~ ~ f i r m suggestions of the other authors that lower limits may be due to in- terspecific competition for space Knox (1954) suggested that competition determined the dis-triljution of 2 species of I~arnacles in New Zealand Iltntlean Kenny and Stephenson ( 1956) gave indirect evidence that competition with a colonial polychaete norm ((aleolaritr) may have de-termined the lower limit of a I~arnacle (Tetra-clitcz) in Queensland Australia In turn the loner limit of (trlcolczricz appeared to he deter-~nined by competition with a tunicate Pyztra or ~vi th dense algal mats

lrith regard to the 2 species of 1)arnacles in the present paper some interesting ol)servations have heen made concernitng changes in their abundance in Britain l roore (1936) found that in southtvest England in 1933 Chtha~~lal~cs stella-trts Lvas most dense at MHVN decreasing in numbers toward MTL while Balanlrs halanoidrs increased in numbers below MH l r S - I t the same localities in 1951 Southward and Crisp (1953) fount1 that Balanl(s had alrnost disap-peared and that Clztl~a~~ral~ts bothhad increased ahove and below hIHVN Chtlla~nallrshad not reached the former densities of Bala~titsexcept

at one locality Erixham After 1951 Bnlan~as I~egan to return in numl~ers although by 1954 it had not reached the densities of 1934 Clztha- ~rznlrrshad declined 1)ut again not to its former densities (Southward and Crisp 1956)

Since Cl~thairlalus increased in al~mndance a t the lower levels vacated by Balanlrs it may lxeviously have I~een excluded by con~l~eti t ion with Balarllrs The grolvth rate of Balanl4s is greater than Clztlln~izalusboth north and south (Hatton 1938) of this location so that Balanus would be likely to -in in con~petition with Chthn~italrrsHowever changes in other environ- mental factors such as temperature may have influenced the abundance of these species in a reciprocal manner I n its return to southwest Ingland after 1951 the maxinlum density of settlement of Balanlrs vas 12 per cm2 compe-tition of the degree observed at llillport would not he expected to occur at this density A t a higher population density Balattns in southern England nould prohaldy eliminate Chtha~~talzts at low shore levels in the same manner as it did at llillport

I n Ioch Sween on the Argyll Peninsula Scot- land Igtevis and Powell (1960) have described an unusual pattern of zonation of Clzthamalz~sstel-latlts On the outer coast of the Argyll Penin-sula Clztlza~~tal~rshas a distribution sitnilar to that at YIillport 111 the tnore sheltered waters of 1och Sxveen holvever Chtlza~+znlusoccurs from above YI1T-S to about hITT2 judging the distril~ution I)y its relationship to other organisms finlatlrrs htrlanoides is scarce above h1TL in Loch S~veen so that there appears to he no pos- sil~ility of competition with Chtl~a~nalus assuch that occurring at Millport between the levels of lIT1 and 1IH lTN

In Figure 5 an attempt has been made to summarize the distribution of adults and newly settled larvae in relation to the main factors which appear to determine this distribution For Ba-lanzrs the estimates were based on the findings of a previous study (Connell 1961) intraspecific conllxtition Lvas severe at the lower levels during the first year after which predation increased in importance fith Chthamalzas it appears that avoidance of settlement or early mortality of those larvae which settled at levels below MTL and elimination by competition with Balatzzts of those which settled between MTL and MH WN were the principal causes for the ab-sence of adults below MHlTN at Millport This distribution appears to be typical for much of western Scotland

Autumn 1961 INTERSPECIFIC COMPETITIOS 721

The u ~ l ~ r of most intertidallimits animals are 1xo11al)ly determined 11y physical factors such as these Since growth rates usually decrease ~v i th increasing height on the shore it bvould be less likely that a sessile species occupying a higher zone could by competition for space prevent a lover one from extending up-ards 12ilieaise there has heen as far as the author is aivare no study made -hich sholvs that predation land species tletermines the upper limit of an intertidal animal In one of the most thorough of such studies Ilrinnan ( 1957) indicated that intense predation I)y 1)irtls accounted for an an-nual mortality of 225 of cockles (cardi lr~~ledrrlc Igtj in sand flats hvhere their total tniortality ivas 74 per year

I n regard to the lover l i~nits of an animals zone it is evident that physical factors may act directly to determine this boundary For example some active arnphipods from the upper levels of sandy beaches die if kept submerged However evidence is accumulating that the lower limits of tlistribution of intertidal animals are deter-mined mainly by biotic factors

Connell ( 1961 ) found that the shorter length of life of Bczlnn~rs balanoides at low shore levels could 1)e accounted for by selective predation l)y Thnis lapillzts and increased intraspecific corn-petition for sllace The results of the experiments in the present study c o ~ ~ f i r m suggestions of the other authors that lower limits may be due to in- terspecific competition for space Knox (1954) suggested that competition determined the dis-triljution of 2 species of I~arnacles in New Zealand Iltntlean Kenny and Stephenson ( 1956) gave indirect evidence that competition with a colonial polychaete norm ((aleolaritr) may have de-termined the lower limit of a I~arnacle (Tetra-clitcz) in Queensland Australia In turn the loner limit of (trlcolczricz appeared to he deter-~nined by competition with a tunicate Pyztra or ~vi th dense algal mats

lrith regard to the 2 species of 1)arnacles in the present paper some interesting ol)servations have heen made concernitng changes in their abundance in Britain l roore (1936) found that in southtvest England in 1933 Chtha~~lal~cs stella-trts Lvas most dense at MHVN decreasing in numbers toward MTL while Balanlrs halanoidrs increased in numbers below MH l r S - I t the same localities in 1951 Southward and Crisp (1953) fount1 that Balanl(s had alrnost disap-peared and that Clztl~a~~ral~ts bothhad increased ahove and below hIHVN Chtlla~nallrshad not reached the former densities of Bala~titsexcept

at one locality Erixham After 1951 Bnlan~as I~egan to return in numl~ers although by 1954 it had not reached the densities of 1934 Clztha- ~rznlrrshad declined 1)ut again not to its former densities (Southward and Crisp 1956)

Since Cl~thairlalus increased in al~mndance a t the lower levels vacated by Balanlrs it may lxeviously have I~een excluded by con~l~eti t ion with Balarllrs The grolvth rate of Balanl4s is greater than Clztlln~izalusboth north and south (Hatton 1938) of this location so that Balanus would be likely to -in in con~petition with Chthn~italrrsHowever changes in other environ- mental factors such as temperature may have influenced the abundance of these species in a reciprocal manner I n its return to southwest Ingland after 1951 the maxinlum density of settlement of Balanlrs vas 12 per cm2 compe-tition of the degree observed at llillport would not he expected to occur at this density A t a higher population density Balattns in southern England nould prohaldy eliminate Chtha~~talzts at low shore levels in the same manner as it did at llillport

I n Ioch Sween on the Argyll Peninsula Scot- land Igtevis and Powell (1960) have described an unusual pattern of zonation of Clzthamalz~sstel-latlts On the outer coast of the Argyll Penin-sula Clztlza~~tal~rshas a distribution sitnilar to that at YIillport 111 the tnore sheltered waters of 1och Sxveen holvever Chtlza~+znlusoccurs from above YI1T-S to about hITT2 judging the distril~ution I)y its relationship to other organisms finlatlrrs htrlanoides is scarce above h1TL in Loch S~veen so that there appears to he no pos- sil~ility of competition with Chtl~a~nalus assuch that occurring at Millport between the levels of lIT1 and 1IH lTN

In Figure 5 an attempt has been made to summarize the distribution of adults and newly settled larvae in relation to the main factors which appear to determine this distribution For Ba-lanzrs the estimates were based on the findings of a previous study (Connell 1961) intraspecific conllxtition Lvas severe at the lower levels during the first year after which predation increased in importance fith Chthamalzas it appears that avoidance of settlement or early mortality of those larvae which settled at levels below MTL and elimination by competition with Balatzzts of those which settled between MTL and MH WN were the principal causes for the ab-sence of adults below MHlTN at Millport This distribution appears to be typical for much of western Scotland

JOSEPII H CONKELL Ecology Vol 42 No 4

BALANUS CHTHAMALUSI

AWLTS LARVAE XSlCCATlON INTRASPECIFIC COMPETITION- PREDATION

ADULTS LARVAE DfSICCATION INTERSPECIFIC g COMPETITION

PREDATION WITH

BY -BALANUS THAIS

RELATIVE EFFECTS OF THESE FACTORS

of Balanus bnlanoides and relative effects of the princi-

-BY THIS

lt Y

DISTRIBUTION RELATIVE OF THESE

F-IG 5 T h c intertidal distribution of Cl~tl~czr~~i~lzcsstclltitus a t Millport it11 a pal limiting factors

SL-Jl Jl UY

J

EFFECTS IFACTORS D1sTRBUTIO

adults and newly settlctl larvae diagrammatic representation of the

u~itlercut or crushed the C l r t h a ~ ~ ~ a l u s the greatest occurred the

tnarine i~itertidal i l l a zone al~ove that of another seasons of most rapid gro~vth of Balanzts Even barnacle Itrlarr~rs l~czlczt~oitlcs 1-ou~igC l ~ t h a i ~ ~ a l t t solder Clltlltr~~rczl~rs

Atlults of Cl~fllczrrrulrcc stcllatz(s occur in the ~nortality of Cl~tlza~~~alzrs during

transplanted to low levels were settle in tlie Ralc~ t~ l t szone I)ut evitlently seldom survive since fen adults are fount1 there

The survival of Clrtlrn~rrnl~rslvhich had settled at various leels in the Rn1nt1lrs zone vas fol-lovetl for a year 11y successive censuses of rnal~ped indivitluals Some ere kept freeC l ~ t l n ~ ~ ~ i ~ l ~ r s of contact vith ljnlnj~rcs These surived cry

-ell at all intertidal leels indicating that in-creased time of sul~merg-ence as not the factor resl~onsil~lefor elimination of Chtllnl~ralztsat low shore levels Comparison of the surival of un-lx-otectetl 1~1pulations vith others l~rotectecl by enclosure in cages from predation I) the snail Tllnis lnpilllrs shovetl tliat Tllnis as not greatly affecting the survival of C l ~ t l ~ n ~ ~ r n l ~ ~ s

Comparison of the survial of untlisturbed pop- ulations of Chtha~rrcrl~csith those kept free of contact vith Hnln)lrrs intlicatetl tliat Rnlnl~zrs could cause great mortality of Clltlrn~llnlrrs 1ja- lanlts settled in greater p o l ~ ~ ~ l a t i o n densities and grew faster that1 Chthn l~ra l~rs Direct observations at each census shovetl that Ralant(s smothered

killetl 11y I1nlnnrls in this vay Predation l ~ y 7lznis tended to decrease the severity of this interspecific competition

Survivors of Clzthniirnl~tsafter a year of crowd- ing I) Rnlnj11rs ere smaller than uncrowded ones Since smaller barnacles l~rotluce fewer off- gtIring coml~etition tentletl to reduce reproductive efficiency in addition to increasing mortality

llortality as a result of intraspecies competition for sllace I)et-een indivitluals of Cl1thalrzallts was only rare l~ ol~servetl

The evidence of this and other studies indicates that tlie lover limit of tlistribution of intertidal organisnls is mainly tleterminecl by the action of biotic factors such as cornl~etition for space or pretlation The ul1l)er limit is prol~ahly more often set 1)y phsical factors

References

Barnes H 1953 Size variat ions in the cyprids of some common barnacles J M a r Biol Ass C K 32 297-304

Autumn 1961 AIETABOLIC C l l h K A C T E R I S T I C S 0 1 11 RO i l k ( 13 723

--- 1956a T h e groth rate of Chthai~zalus stel- Iatlts ( P o l i ) J M a r Biol Ass U K 35 355-361

- 1956b Balartlts balaloides (L) in the F i r th of Clyde T h e development and annual variat ion of the larval Donulation and the causat ive factors I h i m Ecol 25 72-84

and H T Powell 1953 T h e growth of Ba-la~~zdsl~alarloidcs ( L ) and B crerzatus Brug under varying conditions of suljtnersion J M a r Biol Ass U K 32 107-128

---- and D J Crisp 1956 Evidence of self-fer-tilization in certain species of barnacles IM a r Biol Ass U K 35 631-639

Beauchamp R S A and P Ullyott 1932 Competi-tive relationships between certain species of fresh-water Triclads J Ecol 20 200-208

Clark P J and F C Evans 1954 Distance to nearest neighbor a s a measure of spat ial relationships in populations Ecology 35 445-453

Clench W J 1947 T h e genera I I I ~ ~ I I Y L IThaisand in the western Atlantic Johnsonia 2 S o 23 61-92

Connell J H 1961 T h e effects of competition preda- tion by 7Jtais lapillzls and other factors on natural populations of the I~arnacle Balanlts baltrnoidcs Ecol Man 31 61-104

Crisp D J 1960 Fac tors influencing growth-rate in Balatlns balanoidcs J Anim Ecol 29 95-116

Drinnan R E 1957 T h e winter feeding of the oyster- catcher (I-lac~rrntop~ls ostralcgzts) 011 the edible cockle (Cardiltrt~ cdrtlt~) J Anim Ecol 26 441-469

Elton Charles and R S Miller 1954 T h e ecological survey of animal c o m n ~ u n i t i e s with a practical scheme of classifying habitats by structural char -acters J Ecol 42 460-496

Endean R R Kenny and W Stephenson 1956 T h e ecology and distrihutiotl of intert idal organisms on the rocky shores of the Queensland mainland Aust J m a r freshw Res 7 88-146

Fischer E 1928 S u r la distr ihution geographique de

Hatton H 1938 Essa is d e bionomie explicative s u r quelques especes intercotidales dalgues e t d animaux Ann Inst Oceanogr Monaco 17 241-348

Holme N A 1950 Population-dispersion in Tcllina totrris D a Costa J Liar Biol Ass U K 29 267-280

Hutchinson G E 1957 Concluding remarks Cold Spr ing H a r b o r Symposium on Quant Biol 22 415- 4 L

Knight-Jones E W 1953 Laboratory experiments on gregariousness during setting in Balar~rts balun- oidcs and other harnacles J Exp Biol 30 584-598

Knox G A 1954 T h e intert idal flora and fauna of the Chatham Islands N a t u r e Lond 174 871-873

Lack D 1954 T h e na tura l regulat ion of an imal num- bers Oxford Clarendon Press

Lewis J R and H T Powell 1960 spects of the intert idal ecology of rocky shores in Argyll Scot-land I (enera1 description of the a rea 11 T h e distribution of Chtlrat~alzrs stcllntrrs and Rala~~rrs l i~~ltr~loidrs T r a n s Soc Edin 64 in Kintyre Roy 45-100

MacArthur R H 1958 Populat ion ecology of some warblers of northeastern coniferous forests Ecology 39 599-619

Moore H B 1935 T h e biology of Bal~lzts balan- oidcs 111 T h e soft parts J Mar Biol Ass U K 20 263-277

-- 1936 T h e biology of Balar~zts ~aaitoidcs 1 r)is tr ihution in the Plymouth a rea J M a r Biol ss U Ilt 20 701-716

Siegel S 1956 Nonparametric statistics New York l lcGraw Hil l

Southward A J 1955 O n the behavior of barnacles I T h e relat ion of c i r ra l and other activities to temperature J Liar Biol Ass U K34 403-422

-- and D J Crisp 1954 Recent changes in the distribution of the intertidal harnacles CJttJtamallts s t t~l lat~tsPoli and Ralatrzts h(11a1roidcs L in tie Iritish Isles J Anim Ecol 23 163-177

cluelc~ues organismes de rocher le long des cotes - 1956 Fluctuations in the distr ibution a n d de la Manche T r a v Lab Xlus His t Nat St-Servan 2 1-16

1NTRODUCTION

Because of its rich taxonomic diversity the deer mouse P P Y O I I I ~ S C ~ I S has heen used for many stud- ies of speciation and natural selection These have focused mainly on the evolutionary iml~lications of variations in morphology pelage color hehavior and reproductive biology Progress along these lines has been reviewed I)y Blair (1950) The ex~)eriments relmrtetl here are a contril~ution to the investigation of physioogical aspects of evo-luction within this genus R4easurements of meta-

abundance of intert idal harnacles J M a r Biol ss U k35 211-229

holic rate body temperature and pelage insula-tion were made in order to evaluate the selective roles that might be played 11y the enviromental variables oxygen tension temperature and arid- ity 11any of the data were organized in accord- ance with the model of Scholander ct al (1950) which formulates the expected relationships be-tween ambient temperature body temperature metabolism and insulation These relationships in non-laboratory mammals have also been studied by Griffin ct al (1953) Har t and Heroux ( 1953 )

Autumn 1961 AIETABOLIC C l l h K A C T E R I S T I C S 0 1 11 RO i l k ( 13 723

--- 1956a T h e groth rate of Chthai~zalus stel- Iatlts ( P o l i ) J M a r Biol Ass U K 35 355-361

- 1956b Balartlts balaloides (L) in the F i r th of Clyde T h e development and annual variat ion of the larval Donulation and the causat ive factors I h i m Ecol 25 72-84

and H T Powell 1953 T h e growth of Ba-la~~zdsl~alarloidcs ( L ) and B crerzatus Brug under varying conditions of suljtnersion J M a r Biol Ass U K 32 107-128

---- and D J Crisp 1956 Evidence of self-fer-tilization in certain species of barnacles IM a r Biol Ass U K 35 631-639

Beauchamp R S A and P Ullyott 1932 Competi-tive relationships between certain species of fresh-water Triclads J Ecol 20 200-208

Clark P J and F C Evans 1954 Distance to nearest neighbor a s a measure of spat ial relationships in populations Ecology 35 445-453

Clench W J 1947 T h e genera I I I ~ ~ I I Y L IThaisand in the western Atlantic Johnsonia 2 S o 23 61-92

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1NTRODUCTION

Because of its rich taxonomic diversity the deer mouse P P Y O I I I ~ S C ~ I S has heen used for many stud- ies of speciation and natural selection These have focused mainly on the evolutionary iml~lications of variations in morphology pelage color hehavior and reproductive biology Progress along these lines has been reviewed I)y Blair (1950) The ex~)eriments relmrtetl here are a contril~ution to the investigation of physioogical aspects of evo-luction within this genus R4easurements of meta-

abundance of intert idal harnacles J M a r Biol ss U k35 211-229

holic rate body temperature and pelage insula-tion were made in order to evaluate the selective roles that might be played 11y the enviromental variables oxygen tension temperature and arid- ity 11any of the data were organized in accord- ance with the model of Scholander ct al (1950) which formulates the expected relationships be-tween ambient temperature body temperature metabolism and insulation These relationships in non-laboratory mammals have also been studied by Griffin ct al (1953) Har t and Heroux ( 1953 )