12 Conservation of Small Populations: Effective Population Sizes

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12Conservation of Small Populations: Effective Population Sizes, Inbreeding, and the 50/500 Rule Luke J. Harmon and Stanton Braude

Introduction and Background

Populationsizeisextremelyimportantinevaluatingconservationprioritiesforaspecies.Smallpopulationsareat riskofgoingextinctbecauseofdemographic stochasticityandgeneticdrift.Inthisexercise,youwilllearnaboutthreeofthemeaningsof“effectivepopu-lationsize”andhowtoestimatetwoofthem.Youwillthenlearnhowtoapplythesetech-niquestospecificconservationsituations,usingtheconceptsofinbreeding,theminimumviablepopulationsize,andthe50/500rule.

Effective Population Size

Populationsizehasamajorimpactonthedynamicsofapopulation.Forexample,inchap-ter11youusedsimulationstoseethatgeneticdriftreducesallelicdiversitymuchfasterinsmallpopulationsofwogglesthaninlargeones.Populationsizealsoinfluencesthechancesof extinction throughdemographic stochasticity, the randomchange inpopulation sizeovertimeduetorandomvariationinindividualsurvivalandreproductivesuccess.Sucheventshaveaproportionallylargeeffectinsmallpopulations.Forexample,inapopula-tionof10individuals,oneaccidentaldeathwouldreducethepopulationsizeby10%.Incontrast,ifthepopulationweremadeupof1000individuals,oneaccidentaldeathwouldreducethepopulationsizebyonly0.1%.Thus,smallpopulationsaremuchmorelikelytogoextinctduetodemographicstochasticitythanarelargepopulations.

Effectivepopulationsize(Ne)helpsusquantifyhowaparticularpopulationwillbeaf-fectedbydriftorinbreeding.Effectivesizetakesintoaccountnotonlythecurrentcensussize of a population, but also the history of the population. Effective population size isthesizeofan“idealpopulation”oforganisms(idealrefers toahypotheticalpopulationintheHardyWeinbergsensewithaconstantpopulationsize,equalsexratio,andnoim-migration,emigration,mutation,orselection)thatwouldexperiencetheeffectsofdriftorinbreedingtothesamedegreeasthepopulationwearestudying.Forexample,ifouractualpopulationof50animalsexperiencestheeffectsofdriftatthesamerateasanidealpopula-tionof20animals,thepopulationhasadrifteffectivesizeof20.

Thereisnosuchthingas“the effectivesize”ofapopulation.Differenteffectivepopula-tionsizeshelpusestimatetheimpactofdifferentforces.Theeffectivesizeyouestimatewilldependonthescientificquestionyouaretryingtoaddress(box12.1).Estimatingtheap-propriateeffectivepopulationsizeiscrucialinconservationbiology;inmost(butnotall)


126 • chapter 12

Box 12.1 Different Ways to Measure Effective Population Size

Thereareavarietyofpopulationeffectivesizesthathavedifferentmathematicalandbiologicalmeanings.Thetermsaresometimesconfusedormisunderstoodassynonymous.Suchconfusioncanhaveserious implicationsforunderstandingandmanagingpopula-tionsofendangeredorthreatenedspecies,asweseebelow.

Inbreeding effective size, Nef ,referstothesizeofanidealpopulationthatwouldallowthesameaccumulationofpedigreeinbreedingastheactualpopulationofinterest.Pedigreeinbreedingoccurswhenanoffspringinheritstwocopiesofagenefromitsparentswhichareidenticalbydescent—thatis,theyarebothdirectlydescendedfromasingleallelepres-entinoneofthefoundersofthatpopulation(perhapstheparentsarecousinsandeachinheritedtheparticularallelefromthesamegrandfather).Nef isthemeasureofeffectivepopulationsizethatemphasizestheeffectthatsmallpopulationsizehasonthechancesofrelativesmatingwitheachother.Suchmatingsleadtoalossofheterozygosityinthepopu-lation.Thus,thiseffectivesizegivesyouanindicationofthelikelylossofheterozygosityacrossallallelesinyourpopulation.

Calculation of Nef ideally requires pedigree data. However, you can estimate the in-breedingeffectivepopulationsize(Nef)bycalculatingtheharmonic mean ofthepopulationsizeovertimefromthefoundinggenerationtothepenultimategeneration.Thesymbolt representsthenumberofgenerationsforwhichwehavepopulationsizedata.N(0) isthesizeofthefoundingpopulation,N(1) isthesizeofthepopulationafteronegenerationetc.and,N(t − 1)isthesizeofthepopulationonegenerationago.

t(a)Nef = 1 1 1

+...+N(0)

+N(1) N(t −1)

Variance effective size, Nev,referstothesizeofanidealpopulationthatwouldaccumu-latethesameamountofvarianceinallelefrequenciesasthepopulationofinterest;thus,thiseffectivepopulationsizeindicateshowrapidlyallelefrequenciesarelikelytochange.Thisisimportantbecauseitalsoaffectshowrapidlyisolatedpopulationsdivergefromoneanotherundergeneticdrift.Again,thesymbolt representsthenumberofgenerationsforwhichwehavepopulationsizedata.N(1) isthesizeofthepopulationafteronegeneration,etc.,andN(t)isthesizeofthecurrentpopulation.

t (b)Nev=

1 1 1+...+

N(1)+

N(2) N(t )

Inaddition,thefollowingcorrectioncanbeusedateachgenerationifoperationalsexratiosarenot1:1.Thiscorrectedpopulationsizereflectstheincreasedeffectsofbothinbreedinganddriftwhenthesexesarenotcontributingequallytotheallelepool.

4Nm Nf (c)Ns =

Nm+Nf

(continued on following page)


population effective sizes • 127

Noticethedifferencebetweenformula(a)andformula(b).Whiletheinbreeding effective size ismoresensitivetothenumber of original founders [N(0)],thevariance effective size ismoresensitivetothenumber of offspring in the current generation [N(t)].Thisisbecause,asstatedabove,Nef focusesonthelossofheterozygosityduetopedigreeinbreed-inginthepopulation;withasmallinitialfoundingpopulation,closerelativesarelikelytomatewitheachother.Incontrast,Nev givesanindicationoftheincreaseinvarianceofallele frequenciesbetweensubpopulationsduetodrift,anddependsonthenumberofoffspringproducedbythosefoundersandbyeachsubsequentgeneration,uptothepresent-dayN(t).

Thesedifferencescanleadtolargediscrepanciesbetweenthesetwodifferenteffectivepopulationsizesinrealpopulations.Forexample, increasing populations generallyhavealargerNev thanNef ,whiledeclining populations willgenerallyhavelargerNef thanNev.Hence,apopulationcomingthroughabottleneckmayhavea lowinbreedingeffectivesize,butitcanhavealargervarianceeffectivesizeifthepopulationbouncesbackrapidly(as isthecasewiththeSouthernwhiterhinocerospopulation,whichyouwillworkoninclass).

Thereareothermeasuresofeffectivepopulationsizethatfocusondifferentpopulationgeneticparameters.Forexample,eigenvalue effective size, Neλ,focusesontherateatwhichuniqueallelesarelostfromapopulation.Forthisexercise,wewillexamineonlythetwoeffectivepopulationsizeestimatesdiscussedabove.

cases,effectivepopulationsizewillbesmallerthantheactualnumberoforganismsinthepopulation.Thinkforamomentaboutwhythisisso.AconservativeruleofthumbusedbysomebiologistsisthatNe isusuallyaboutone-fifthofthetotalpopulationsize(MaceandLande,1991).UsingsucharoughestimateisriskybecauseNe canbelarger thanthecensussizeofthepopulation,dependingonthehistoryofthepopulationandtheparticularNeunderconsideration.

Demographic stochasticity, genetic drift, and environmental variation can interacttodooma smallpopulation toextinction.This is calledanextinctionvortex, and it isduetoapositivefeedbackloop(figure12.1):thenegativeconsequencesoflowereffectivepopulationsizemakethepopulationsmaller,causingstrongernegativeeffects,leadingtoanevensmallerpopulationsize(GilpinandSoule,1986).Forexample,arandomenvi-ronmentalchangemightlowerpopulationsize,leadingtoahigherchanceofpopulationreductionduetodemographicstochasticity.Thiscouldlowerinbreedingeffectivepopu-lationsizeevenmore,leadingtosevereinbreedingdepressionandreducedfertility.Thisfurtherreducesthepopulationsize.Chainsofeventssuchasthesemeanthattheextinc-tionprobabilityforasmallpopulationcanbeextremelyhigh.Forexample,Pimmetal.(1988)showedthattheextinctionriskforbirdsonsmallislandsoffthecoastofBritainrises with decreasing numbers of nesting pairs. Conservation biologists realize that anextinctionvortexcanbeginwhenhumanscausemajorreductionsinthepopulationsizeofaspecies.

Calculating Effective Population Sizes

Considerthedataintable12.1forapopulationofEasternfencelizards,Sceloporus undulatus,atTysonResearchCenterineasternMissouri:Youcanestimatetheinbreedingeffective


128 • chapter 12

More genetic drift; less ability to adapt

More inbreeding; depression

Population more subdivided by fragmentation

More demographic variation

• Habitat destruction • Environmental degradation • Habitat fragmentation • Overharvesting • Effects of invasive species

• Environmental variation • Catastrophic events • Global climate change

Lower effective population size

(Ne) EXTINCTION

FIgure 12.1. Extinctionvortex. Populationsizedecreasesinapositivefeedbackloop,eventuallyresultingintheextinctionofthepopulation(fromPrimack,2000).

sizeforthispopulationusingformula(a)frombox12.1.Inthiscase,t =4generations;wecalculatetheinbreeding effective population size as:

Nef ≈t

1 1 1 1+ + +

N(0) N(1) N(2) N(3)(12.1)

4 4= = =68.9≈69

1 1 1 1 0.058+ + +

140 250 110 26

Trythiscalculationyourself—itcangetconfusingtakingallofthesereciprocals!Thisestimatedeffectivepopulationsize(69)meansthat,intermsofgeneticinbreeding,

thispopulation(withameancensussizeof148lizardsover5years)willaccumulatethe

Table 12.1. Sceloporus populationatTysonfrom1996to2000.

Year Populationcount

1996 140

1997 250

1998 110

1999 26

2000 180



effectsofinbreedingatthesamerateasapopulationthathadaconstantsizeofonly69individuals.

Incontrast,thevariance effective size,estimatedwithformula(b)inbox12.1,is

Nev ≈t

1 1 1 1+ + +

N(1) N(2) N(3) N(4)(12.2)

4 4= = =70.73≈71.

1 1 1 1 0.05655+ + +

250 110 26 180

Thiseffectivepopulationsize(71)meansthat,intermsofgeneticdrift,thispopulation(withameansizeof148over5years)willaccumulatetheeffectsofdriftatthesamerateasapopulationthathadaconstantsizeof71individuals.Notethat,inthisexample,themeansizeofthepopulationovertimereflectsneitherhowitwillaccumulateinbreedingnorhowitwillexperiencedrift.Althoughthedifferencebetweenvarianceeffectivesizeandinbreedingeffectivesizeappearssmallinthisexample,thepointisthatthesearenottwowaysofestimatingthe effectivesize;thesearetwodifferenteffectivepopulationsizes.Intheexamplesyouworkwithinthisexercise,youwillseehowonepopulationcanhaveverydifferenteffectivesizesthatinformusaboutverydistinctrisksforthepopulation.

Effective Population Size, Inbreeding, and Extinction

Justasthereareanumberofdifferentmeaningsofeffectivepopulationsize,thereareanumberofdifferentmeaningsofinbreeding.Becausebreedingwithcloserelativestypicallyreducesthepoolofgenescontributingtothenextgeneration,onemeasureofinbreedingisF,inbreedingasameasureofdrift.Inadditiontoincreasingtheimpactofdrift,inbreedingcanincreasetheproportionofdeleterioushomozygousgenecombinationsinapopulation,whichleadstolowersurvivalofyoungandthuslowerreproductiveoutput;thisiscalledinbreedingdepression.Tobegintoevaluatethepotentialimpactsofsmallpopulationsizeoninbreeding,wecanusethefollowingestimation(fromSoule,1980):

1ΔF=1−1− t. (12.3)

2Nef

Inthisequation,ΔF istheincreaseintheinbreedingcoefficientovertime;Nef isthein-breeding effective population size; and t is the number of generations. We can use thisequationtopredicthowmuchtheinbreedingcoefficientinasmallpopulationwillincreaseover time. IfΔF is0.6orhigher, the fecundityof individuals in thepopulationmaybereduced,andthepopulationcouldbeathigherriskforextinction.Forexample,wecancalculatetheincreaseintheinbreedingcoefficientafter100generationsforthepopulationdescribedabove.Forthefencelizards,Nefwas69.ThismeansthatΔF =1−(1−1/138)100=0.52.Wewouldconcludethatafter100generationsthefencelizardsareexperiencingsomeinbreeding;however,becauseΔF <0.60itmaynotthreatenthepopulation.Ofcourse,theinbreedingcoefficientwillcontinuetoincreaseovertime,possiblytodangerouslevels,ifthepopulationremainssmallandisolated.NotethattheestimateofΔF ignoresgeneflow—andevenmoderategeneflowcangreatlyreducetheeffectofgeneticdrift,slowingtherateofincreaseintheinbreedingcoefficientovertime.Soevenmoderategeneflowcanhelpmaintaingeneticdiversity.Thisisonereasonwhymanyconservationbiologistsadvocatethemaintenanceofcorridorsconnectingsmall,isolatedpopulationsofaspecies.


130 • chapter 12

Minimum Viable Populations and the 50/500 Rule

Youknowthatdemographicstochasticityandgeneticdriftcannegativelyaffectsmallpop-ulations.Demographicstochasticityleadstotherandomextinctionofsmallpopulations,whilegeneticdriftcancauseareductionofgeneticdiversitywithinapopulation.Thesefactorscaninteractinanextinctionvortex(figure12.1discussedabove)thatcaneventuallyleadtotheextinctionofapopulation.

Todecidewhenthese factorsmightbe important forapopulationofanendangeredspecies,Shaffer(1981)proposedtheconceptoftheminimumviablepopulation(MVP).HedefinedtheMVPasthesmallestisolatedpopulation(ofagivenspeciesinagivenhabi-tat)havinga99%chanceofremaininginexistencefor1,000years,despitetheforeseeableeffects of demographic stochasticity, genetic drift, environmental stochasticity (randomchangesintheenvironment),andnaturalcatastrophes(Shaffer,1981).Shafferchosethepercentage and time scale to represent what most scientists consider a good chance forsurvivalofaspecies.QuantitativeobjectivesliketheMVPprovidespecificguidelinesforgaugingthesuccessofconservationprograms(Fooseetal.1995).PopulationssmallerthantheMVPareconsideredtobeatsignificantriskofenteringintotheextinctionvortexandbecomingextinct,soaconservationprogramcanbeconsideredsuccessfulonlyifitraisestheeffectivepopulationsizeabovetheMVP.

Arelatedconceptisthe50/500rule,proposedbyFranklin(1980).The“50”partofthe50/500rulestatesthatpopulationswithaninbreedingeffectivepopulationsize(Nef )under50areatimmediateriskofextinction.Thisisbecause,insuchsmallpopulations,inbreed-inganddemographicstochasticitycanquicklypushthepopulationintoanextinctionvor-tex.The“500”partoftherulemeansthatpopulationswithavarianceeffectivesize(Nev )oflessthan500areatlong-termriskofextinction.Inthesepopulations,geneticdriftmaybeastrongforce,leadingtoeventuallossofgeneticvariation(Franklin,1980).Aftervaria-tionis lost, thepopulationwillno longerbeable torespondtoenvironmentalchanges,andmaybereducedinsizeorgoextinctifanysuchchangesoccur.Evenwhenproperlyunderstood,Franklin’sruleisquitecontroversial.Someauthorsquestionitsgenerality,andotherssuggestthatthenumbersaretoosmall.Forexample,Lande(1995)suggestedthatanypopulationwithNev lessthan5,000willbesubjecttostronggeneticdrift,whichwilldepletethegeneticvariationinapopulationandcauselong-termextinctionrisk.Recentexperimentaltestsoftheruleincaptivehouseflypopulationsalsosuggestthatpopulationsmustbehigherthanimpliedbythe50/500ruleinordertosurviveandmaintaingeneticdiversity(ReedandBryant,2000).Asaconservationbiologist,youmaynotusethe50/500rule,butitisessentialthatyouunderstandwherethe50and500comefrom.

Homeworkforthisexercisetakesapproximately45minutes.

Case Studies and Data

European Adders

TheEuropeanadder (Vipera berus) is a small, venomous snake (figure12.2)distributedthroughout Europe (Arnold and Burton, 1978). This snake occupies a wide geographicrange,butwithin thatrangesnakesareoften found insmall, isolatedpopulationssepa-ratedfromeachotherbyhundredstothousandsofmeters.Undernaturalconditions,geneflowbetweenthesesubpopulationsishigh,becausethemalesnakesdispersewidelywhen



FIgure 12.2. EuropeanadderVipera berus.

searchingforfemalesduringthespring(Madsenetal.,1993).However,incertainregionshumanshavedisturbedadderhabitatthroughagricultureandurbanencroachment.Nowweseeextremelyisolatedsmallpopulationsofadders,surroundedbylargeareasofunsuit-ablehabitat(Madsenetal.,1996).ConsidertheisolatedpopulationofaddersatSmygehuk,onthesouthcoastofSweden.Theseaddersareseparatedfromthenearestpopulationby20kmoffarmland,whichisunsuitablehabitatforadders(Madsenetal.,1996).Usethedataintables12.2and12.3(Madsenetal.,1996)toanswerthefollowingquestions.

Questions to Work on Individually Outside of Class

1.Table12.2givesdataonthepopulationsizeoftheaddersforeachyearfrom1984to1990.

(a)Plotthetotalnumberofadultadders(y)overtime(x).(b)Usethesedatatocalculatetheinbreedingeffectivesizeandthevarianceeffectivesize

ofthispopulationofadders.(c)Explainwhytheinbreedingeffectivesizeandthevarianceeffectivesizeofthispopu-

lationdiffer.(d)Recalculatethevarianceeffectivesizeofthispopulationwiththenewinformation

onthesexratiosofsnakesinthepopulation(table12.3).(e)Dr.FernSkipehasarguedthatthereisnosuchthingasthe“real”effectivepopulation

size.Doyouagreewiththisstatement?Whyorwhynot?Usetheresultsaboveinyourargument.

(f)Basedonthesedataandyourcalculations,makearecommendationtotheSwedishgovernmentconcerningthispopulationofvipers.


132 • chapter 12

Table 12.2. TotalnumberofadultaddersatSmygehuk

Year Totalnumberofadults

1984 138

1985 40

1986 34

1987 42

1988 37

1989 41

1990 34

Table 12.3. Adultmaleandfemaleaddersineachyear

Year No.ofadultfemales No.ofadultmales Correctedpopulationsize

1984 98 40

1985 29 11

1986 24 10

1987 32 10

1988 27 10

1989 29 12

1990 27 7

Small-group/In-Class exercise

Pleasebringgraphpaperandacalculatortoclasstocompletethenextpartofthisexercise.First,readthefollowingbackgroundinformationonAfricanrhinoconservation.

African Rhino Conservation

Background Therhinoceros is anexampleofa“charismaticmegavertebrate” thathasplayedacentralroleinpromotingworldwideconservationefforts.Thefiveextantspeciesofrhinoarethelastrepresentativesofalargegroupofspeciesthatreachedapeakindiver-sitybetween25and5millionyearsago(Estes,1991).TwoofthefiveextantspeciesoccurinAfrica:thewhiterhinoceros(Ceratotherium simum)andtheblackrhinoceros(Diceros bicornis).Despitetheirnames,bothspeciesareadullgraycolor,andcanbedistinguishedbytheshapeoftheirmouthparts(figure12.3).Theblackrhinohasahook-shapedtriangu-larupperlipthatallowsittoobtainitsfoodbybrowsingleguminousherbsandshrubs.Thewhiterhino,ontheotherhand,hasaverywide,squaremouth,andisspecializedingrazingareasofdensegrasses.All rhinoshavepooreyesightandrelativelysmallbrains,butex-



FIgure 12.3. Black(left)andwhite(right)rhinoceri.

tremelysensitivehearingandsmell(Estes,1991).BothAfricanspeciesofrhinoshowgeo-graphicvariation.Theblackrhinohasbeendividedintofoursubspecies,western(Diceros bicornis longipes),eastern(D. b. michaeli),southwestern(D. b. bicornis),andsouthcentral(D. b. minor).Thewesternsubspeciesistherarestandmostisolated,withonlyafewindi-vidualslivinginwesternAfrica.Thewhiterhinohasbeendividedintotwosubspecies,thenorthern(Ceratotherium simum cottoni)andsouthern(C. s. simum).Thesetwosubspeciesoccupyseparaterangesandaremoredistinct,bothmorphologicallyandgenetically,thanthesubspeciesofblackrhinos(EmslieandBrooks,1999).

Bothspeciesofrhinohaveundergonemajorreductionsintheirrangesinthepastsev-eralhundredyears.Earlycolonial explorers reported thatblack rhinoswerewidespreadin distribution and fairly common, while white rhinos were more restricted in range.AfterEuropeancolonization, southernwhite rhinoswereveryquickly reduced tonear-extinction,reachingalowofjust20individualsin1895.Sincethen,numbersofthesouth-ern white rhino have steadily increased; there are over 8,000 alive today. The northernwhiterhino,ontheotherhand,hasshownadramaticdecreaseinrecentyears,decliningfromover2,000in1960toonly25individualsin1998.Numbersofblackrhinoshavealsodeclinedsincecolonialtimes.Declineswereespeciallyseverebetween1970and1992,whenblack rhinosdeclined96%.The specieshas recently shown somepotential for recovery(Emslie andBrooks, 1999).Themain reason for thedeclineof all rhinos ishuntingbyhumans.Europeancolonistskilledhundredsofthousandsofrhinosduringthenineteenthcentury.Morerecently,rhinoshavebeenkilledbypoacherssupplyingmarketsinAsiaandtheMiddleEastwithrhinoceroshorns.InAsia,rhinohornsareusedintraditionalChinesemedicine,whosepractitionersbelievethatthehornslowerfevers,increasemalepotency,andcancureahostofdiseases.IntheMiddleEast,theyareusedashandlesforornamentaldaggerscalledjambiyas(EmslieandBrooks,1999).Althoughrhinohornshavebeenusedforthesepurposesforhundredsofyears,recentincreasesindemandputseriouspressureonwildrhinos,andpoachingforhornsisthemajorthreattoAfricanrhinopopulationstoday(EmslieandBrooks,1999).

Black Rhinoceros Diceros bicornis Blackrhinoswereformerlythemostwidespreadandabundantspeciesofrhino(Estes,1991),butarenowlistedintheIUCNRedBookascriticallyendangered.Directcountsofblackrhinoshaveshowndeclinesofover80%in


134 • chapter 12

7

6

5

4

3

2

1

0

Year

FIgure 12.4. Theblackrhinoceroshasbeeninseveredeclineinrecentyearsandisenteringapopu-lationbottleneck(datafromEmslieandBrooks,1999).

thelast50years(EmslieandBrooks,1999).Thetotalpopulationsizeofblackrhinosin1970wasestimatedat65,000.Historically,blackrhinosoccupiedalargerangethroughoutAfrica.Today,blackrhinopopulationsarefragmented,andthespeciesisrapidlydeclininginnumbers(figure12.4).Thespecieshasbeendividedintofoursubspecies,eachoccupy-ing a separate geographic range: the western, eastern, southwestern, and south-centralblack rhinos. The ranges of each subspecies have different climates and habitats. Theycansometimesbedistinguishedbycharacterssuchasskintextureandhornlengthandshape.Theremayalsobegeneticandbehavioraldifferencesacrossthesesubspecies.Thewesternblackrhinocerosisthemostrange-restrictedandendangeredofallthesubspeciesofblackrhinos: theentiresubspecies is representedbyonlya fewscattered individualsinCameroon.Thissubspecies,separatedfromtherestoftheblackrhinosbyhundredsofmiles,mayrepresentageneticallydistinctlineage,butisthreatenedwithextinctionintheimmediatefuture.ThelargestpopulationofblackrhinosisinKenya,whilethemainpopulationofsouthwesternblackrhinosisinNamibia.Thesouth-centralblackrhinoisthemostcommonsubspecies,with largenumbers inSouthAfricaandZimbabwe,andsmallerpopulationsinsouthernTanzaniaandMozambique.Thegoalofyourgroupistoestablishaconservationplanforblackrhinosasawhole.Youneedtodecidehowmuchmoneyneedstobeallocatedtoeachofthefoursubspecies,andwhichpopulationswillhavethehighestconservationpriority.

White Rhinoceros Ceratotherium simum Whiterhinoshavealwaysbeenlesscom-monandmorelimitedindistributionthanblackrhinos(EmslieandBrooks,1999).Thisisprobablyafunctionoftheirspecializedgrazingdiet.Thisfeedingstrategymakesthewhiterhinounique,asitisquitedifferentfromthatofblackrhinosand,infact,fromallotherrhinosintheworld(Estes,1991).Thewhiterhinoisoneofthelargestpurelygrazingherbi-voresthathaseverlived.Southernwhiterhinosareseparatedfromnorthernwhiterhinosby2,000km,andnowhiterhinohaseverbeenrecordedintheinterveningarea.Thesetwo

Pop

ula

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n (i

n t

en t

ho

usa

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1969 1974 1979 1984 1989 1994



subspeciesaregeneticallydistinct,withmoregeneticvariationbetweenthesetwosubspe-ciesthanamongthefoursubspeciesofblackrhino(Smithetal.,1995).

Southern White Rhinoceros Ceratotherium simum simum Southernwhiterhinoswereonthebrinkofextinctionin1895,whenoverhuntinghadreducedthemtojust20individualsinonepopulationinSouthAfrica.Theirnumbershavesincerecoveredsub-stantially(figure12.5).Therecoveryofthesouthernwhiterhinoisoneofthemajorsuccessstoriesinmodernconservationbiology.Beforetheirdeclineintheninteenthcentury,theirrangeincludedmuchofsouthernAfrica.Afterinitialrecoveryofthesourcepopulation,manytranslocationswerecarriedout;thesehavesuccessfullyreintroducedrhinosintoar-easwheretheyhadbeenwipedout.Southernwhiterhinosarenowthemostnumeroussubspeciesofrhino;populationscanbefoundinSouthAfrica,Botswana,Namibia,Swazi-land,andZimbabwe.TheyhavealsobeenintroducedintoKenya,IvoryCoast,andZambia,alloutsidetheirnativerange.TherhinoshavefunctionedasamajorsourceoffundinginSouthAfrica,wherenationalparkshavesoldexcess rhinos toprivategameparks forasmuchasU.S.$25,000apiece.

9

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4

3

2

1

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Year

FIgure 12.5. Thesouthernwhiterhinoceroshasrecoveredfromnearextinctionattheturnofthelastcenturyandthecurrentgrowingpopulationisdescendedfromabottleneckpopulationofonly20animals(datafromEmslieandBrooks,1999).

Northern White Rhinoceros Ceratotherium simum cottoni Northernwhiterhinoshaveshownastrikingdeclineinrecentyears,andarenowperilouslyclosetoextinction.Table12.4givesdataonnorthernwhiterhinocerospopulationsbycountryfrom1960to1998(EmslieandBrooks,1999).Although the rangeofnorthernwhite rhinosonce in-cludedpartsofUganda,Chad,Sudan,theCentralAfricanRepublic,andtheDemocraticRepublicofCongo,theyarenowrestrictedtoasmallareainthenortheastoftheDemo-craticRepublicofCongo(EmslieandBrooks,1999).Thispopulationnumberedonly25individualsin1998,andDRChasbeensufferingfromtremendouspoliticalinstabilityoverthepast20years.Conservationbiologistshavenotlosthopeofpreservingthesubspecies,

Pop

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1890 1910 1930 1950 1970 1990


136 • chapter 12

Table 12.4. Northernwhiterhinosbycountry,1960–1998.

1960 1971 1976 1981 1983 1984 1991 1995 1998

CentralAfrican Few Few Few Few Few 0? — — —Republic

Chad Few Few ? ? 0? 0? — — —

DemocraticRepublic 1,150 250 490 <50 13–20 15 30 31 25oftheCongo

Sudan 1,000 400 ? <300 <50 0? 0? 0? 0?

Uganda 80 Few Few Few 2–4 0? — — —

Total 2,230 650 500+ <350 <70 15 30 31 25

however,andcitetherecoveryofthesouthernwhiterhino,whichhasgrownfromatotalpopulationofaround20individualsin1895toover8000today.ThenorthernwhiterhinoscurrentlysurvivingintheDemocraticRepublicofCongorepresentthelastsurvivorsofauniquelineageofrhinos;theirextinctionwouldbeagreatandirreversibletragedy.

Your Job: Help Create Species Survival Plans for African Rhinos (Questions 2–6)

SpeciesSurvivalPlans(SSPs)coordinatethemanagementofrareandendangeredspeciestomaintainhealthybreedingpopulations,retaingeneticvariation,andminimize“inbreeding.”SSPsoftenhavetheconflictinggoalsofpreservingspeciesinacaptiveenvironmentwhileatthesametimeminimizingevolutionarychangeinthespeciesandminimizinglossofgeneticdiversityfrominbreedingordrift(Templeton,1991).Thesecanbesignificantforcesaffect-ingwild(insitu)andcaptivepopulationsthatareenteringoremergingfrompopulationbot-tlenecks.YourjobwillbetouserealdatatohelptheIUCNRhino-RescueTeamgenerateanSSPforthetwospeciesofAfricanrhinos.Afteryouanswerquestions2–5,theentireclasswillmeetasacommitteeofthewholetoallocatefundsfortheconservationofAfricanrhinos.

2.Your instructorwill assignyou tooneof the three rhino speciesor subspeciesonwhichwehavedata.First,youneedtoassessthecurrentgeneticsituationforblackrhinos,orsouthernornorthernwhiterhinos.Thisassessmentshouldincludetheinbreedingandvarianceeffectivesizesforthewildpopulations.Youshouldbeabletoprojectaccumula-tionof inbreeding inwildpopulations if theyaremaintainedatcurrent levels.(Assumeequalsexratiosandagenerationtimeof8years.Forblackrhinosandsouthernwhiterhi-nosyouwillneedtoestimatecensussizesfromfigures12.4and12.5)Yourinstructorwillcopytable12.5ontheboardandyoucanshareyourresultswiththeothergroups.

3.Foryourspeciesorsubspecies,discussthelong-rangeplanformaintainingthegenetichealthofthepopulation.AddresstherecommendationsandthetheoreticalframeworkofFranklin’s50/500ruleinyourplan.

4.Youshouldalsodiscussthesituationforyourspeciesinthewildanddecidewhetheryou want to use the wild population to supplement the captive zoo population or viceversa.

5.Finally,yourplanmustincludeprioritiesforbothspeciesandfordifferentpopula-tionswithineachspecies.Therealityisthattherearelimitedfundsavailableforrhinocon-servation,andyoumustgenerateguidelinesaboutwhereresourcesshouldbespent.



Table 12.5. PopulationcensusandeffectivesizesofAfricanrhinos.

Censussize, Inbreedingeffective Varianceeffective1997 size(Nef) size(Nev)

BlackrhinocerosDiceros bicornis

N =2,600

SouthernwhiterhinocerosCeratotherium simum simum

N =8,440

NorthernwhiterhinocerosCeratotherium simum cottoni

N =23

6.Eachgroupwillhaveafewminutestodescribethesituationfortheirrhinosandpro-poseanallocationofthe$500,000whichAfricanRhinoRescuehasraised.Onceeachgrouphasmadetheirbriefpresentationyouwillhaveachancetoconvinceeachother(andyourinstructor)totakeyourrecommendation.

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12 Conservation of Small Populations: Effective Population Sizes