8
Implications of assumption violation in density estimates of antelope from dung-heap counts: a case study on grey duiker (Sylvicapra grimmia) in Zimbabwe Nicola Lunt 1, *, Andrew E. Bowkett 2 and Amy B. Plowman 2 1 Marwell Zimbabwe Trust, PO Box 3863, Bulawayo, Zimbabwe and 2 Paignton Zoo Environmental Park, Totnes Road, Paignton, Devon TQ4 7EU, U.K. Abstract Dung-heap counts were used to estimate density of grey duiker (Sylvicapra grimmia Linnaeus 1758) in the Matobo National Park, Zimbabwe. To test assumptions of this method, defecation rate and defecation site selection were investigated under captive and field conditions, and den- sities were compared with independent estimates derived from territory mapping. Many assumptions were violated: males defecated more frequently than females with mean dry mass per deposit greater in females, but total daily faecal production was similar between sexes. Spatial dis- tribution of faeces was clumped, and 52.8% of locations contained multiple deposits. Duikers exhibited habitat type preferences (i.e. low- to medium-density woodland) with herbaceous layer heights 40–100 cm and visibility >20 m. Calculated grey duiker density from dung-heap counts in cleared plots was 9.7 ± 1.3 animals km )2 , approximately double the territory-mapping estimate based on Minimum Convex Polygons (5.13 animals km )2 ) but similar to the 75% Fixed Kernel estimate (10.95 ani- mals km )2 ). Provided that sex ratios approach parity and sampled area is representative of all utilized habitats, violation of basic assumptions of the dung-heap count method has a minor effect on density estimate accuracy. Key words: density estimate, duiker, dung-heap counts, home range, Sylvicapra grimmia, territory mapping Re ´sume ´ On a utilise ´ les comptages de crottes pour estimer la densite ´ de ce ´phalophes de Grimm ((Sylvicapra grimmia Linnaeus 1758) dans le Parc National de Matobo, au Zimbabwe. Pour tester les hypothe `ses de cette me ´thode, le taux de de ´fe ´cation et le choix du site de de ´fe ´cation ont e ´te ´e ´tudie ´s, en captivite ´ et en liberte ´, et on les a compare ´s a ` des esti- mations inde ´pendantes de ´rive ´es de la cartographie du territoire. De nombreuses hypothe `ses ont e ´te ´ infirme ´es: les ma ˆles de ´fe `quent plus souvent que les femelles, avec une masse se `che moyenne plus grande pour chaque excre ´tion des femelles puisque la production fe ´cale totale quotidienne est semblable pour les deux sexes. La distribution spatiale des excre ´ments e ´tait groupe ´e, et 52,8% des endroits choisis contenaient de multiples de ´po ˆts. Les ce ´phalophes pre ´sent- aient des pre ´fe ´rences pour certains habitats, des fore ˆts peu ou moyennement denses dont le couvert herbace ´ se situait entre 40 et 100 cm de haut et dont la visibilite ´ atteignait au moins 20 me `tres. Le calcul de la densite ´ des ce ´phalo- phes d’apre `s le comptage des crottes dans des endroits de ´gage ´s donnait 9,7 ± 1,3 animaux km )2 , approximativ- ement le double de ce que donne l’estimation base ´e sur la cartographie du terrain selon la me ´thode du polygone convexe (5,13 animaux km )2 ), mais e ´tait semblable aux 75% de l’estimation a ` noyau de ´fini (10,95 anim- aux km )2 ). Pour autant que le sex-ratio soit proche de la parite ´ et que l’aire ou ` a eu lieu l’e ´chantillonnage soit repre ´sentative de tous les habitats fre ´quente ´s, cette viol- ation des hypothe `ses de de ´part de la me ´thode de comptage par les tas de crottes n’a qu’un effet mineur sur l’exactitude de l’estimation de la densite ´. Introduction Many standard methods for estimating population density of large mammals rely on direct observations (Rudran *Correspondence: E-mail: [email protected] 382 ȑ 2006 The Authors. Journal compilation ȑ 2006 Blackwell Publishing Ltd, Afr. J. Ecol., 45, 382–389

Implications of assumption violation in density estimates of antelope from dung-heap counts: a case study on grey duiker (Sylvicapra grimmia) in Zimbabwe

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Implications of assumption violation in density estimatesof antelope from dung-heap counts: a case study on greyduiker (Sylvicapra grimmia) in Zimbabwe

Nicola Lunt1,*, Andrew E. Bowkett2 and Amy B. Plowman2

1Marwell Zimbabwe Trust, PO Box 3863, Bulawayo, Zimbabwe and 2Paignton Zoo Environmental Park, Totnes Road, Paignton, Devon TQ4 7EU,

U.K.

Abstract

Dung-heap counts were used to estimate density of grey

duiker (Sylvicapra grimmia Linnaeus 1758) in the Matobo

National Park, Zimbabwe. To test assumptions of this

method, defecation rate and defecation site selection were

investigated under captive and field conditions, and den-

sities were compared with independent estimates derived

from territory mapping. Many assumptions were violated:

males defecated more frequently than females with mean

dry mass per deposit greater in females, but total daily

faecal production was similar between sexes. Spatial dis-

tribution of faeces was clumped, and 52.8% of locations

contained multiple deposits. Duikers exhibited habitat type

preferences (i.e. low- to medium-density woodland) with

herbaceous layer heights 40–100 cm and visibility

>20 m. Calculated grey duiker density from dung-heap

counts in cleared plots was 9.7 ± 1.3 animals km)2,

approximately double the territory-mapping estimate

based on Minimum Convex Polygons (5.13 animals km)2)

but similar to the 75% Fixed Kernel estimate (10.95 ani-

mals km)2). Provided that sex ratios approach parity and

sampled area is representative of all utilized habitats,

violation of basic assumptions of the dung-heap count

method has a minor effect on density estimate accuracy.

Key words: density estimate, duiker, dung-heap counts,

home range, Sylvicapra grimmia, territory mapping

Resume

On a utilise les comptages de crottes pour estimer la densite

de cephalophes de Grimm ((Sylvicapra grimmia Linnaeus

1758) dans le Parc National de Matobo, au Zimbabwe.

Pour tester les hypotheses de cette methode, le taux de

defecation et le choix du site de defecation ont ete etudies,

en captivite et en liberte, et on les a compares a des esti-

mations independantes derivees de la cartographie du

territoire. De nombreuses hypotheses ont ete infirmees: les

males defequent plus souvent que les femelles, avec une

masse seche moyenne plus grande pour chaque excretion

des femelles puisque la production fecale totale quotidienne

est semblable pour les deux sexes. La distribution spatiale

des excrements etait groupee, et 52,8% des endroits choisis

contenaient de multiples depots. Les cephalophes present-

aient des preferences pour certains habitats, des forets peu

ou moyennement denses dont le couvert herbace se situait

entre 40 et 100 cm de haut et dont la visibilite atteignait

au moins 20 metres. Le calcul de la densite des cephalo-

phes d’apres le comptage des crottes dans des endroits

degages donnait 9,7 ± 1,3 animaux km)2, approximativ-

ement le double de ce que donne l’estimation basee sur la

cartographie du terrain selon la methode du polygone

convexe (5,13 animaux km)2), mais etait semblable aux

75% de l’estimation a noyau defini (10,95 anim-

aux km)2). Pour autant que le sex-ratio soit proche de la

parite et que l’aire ou a eu lieu l’echantillonnage soit

representative de tous les habitats frequentes, cette viol-

ation des hypotheses de depart de la methode de comptage

par les tas de crottes n’a qu’un effet mineur sur l’exactitude

de l’estimation de la densite.

Introduction

Many standard methods for estimating population density

of large mammals rely on direct observations (Rudran*Correspondence: E-mail: [email protected]

382 � 2006 The Authors. Journal compilation � 2006 Blackwell Publishing Ltd, Afr. J. Ecol., 45, 382–389

et al., 1996). However, such methods are often impractical

for shy, nocturnal or inconspicuous species such as duikers

(Bowland & Perrin, 1994; Jenkins et al., 2002; Rovero &

Marshall, 2004). As a result, a variety of alternative, and

often indirect sampling methods have been devised, such

as drive counts (Schmidt, 1983; Bowland, 1990), territory

mapping (Bowland & Perrin, 1994), camera trapping

(O’Brien, Kinnaird & Wibisono, 2003; Silveira, Jacomo &

Diniz-Filho, 2003) and faecal deposit counts (Schmidt,

1983; Plumptre & Harris, 1995; Marques et al., 2001).

Ungulate density estimation based on dung-heap counts

has the advantages of being relatively inexpensive, requires

only adequate tracking skills, and has been utilized suc-

cessfully in a number of environments and for a variety of

taxa, including secretive species (Schmidt, 1983; Bowland &

Perrin, 1994; Plumptre & Harris, 1995; Marques et al.,

2001; Jenkins et al., 2002; Ellis, 2003). Two methods are

commonly used – faecal standing crop (FSC) and faecal

accumulation rate (FAR) (Campbell, Swanson & Swales,

2004). The FAR method requires that sampled areas are

cleared of faecal deposits a known time before the area is

sampled, which negates the need to calculate dung decom-

position rates (required for FSC) if the period between

clearing and sampling is sufficiently short. The number of

dung piles encountered in the sampled area can then be

extrapolated to a wider area (e.g. a study site or management

area). The method relies on several assumptions that: (i)

defecation rates per individual per day are constant; (ii)

spatial deposition of faecal piles is random; (iii) the time

between clearing and sampling does not exceed faecal

decomposition rate; (iv) animal density in the management

area is homogeneous or that relevant ‘habitat’ types are

distinguished between and treated separately; and (v) the

probability of detecting faecal deposits is uniform among

habitats.

The grey duiker (Cephalophinae: Sylvicapra grimmia

Linnaeus 1758) is a savannah-dwelling species that is

tolerant of a wide variety of habitats (Kingdon, 1997).

Because of its relatively small size and secretive habits, its

numbers are clearly underestimated by standard antelope

counts, e.g. 0.3–1.7 animals km)2 for ground surveys and

0.01–0.15 animals km)2 for aerial surveys (summarized in

East, 1999). Reliable monitoring methods would be of value

to wildlife managers as grey duikers can be commercially

important as a source of game meat (Ferreira & Hoffman,

2001) and as forestry pests (Schmidt, 1983). This paper

investigates the utility of density estimates using dung-heap

counts and territory mapping in a heterogeneous

environment, documents grey duiker defecation behaviour

under field and captive conditions in Zimbabwe and com-

ments on the implications of the violation of basic

assumptions of the FAR method of density estimation.

Methods

Study sites

Ex situ observations were done at Dambari field station

(20�15¢09¢¢S; 28�46¢29¢¢E) near Bulawayo, Zimbabwe.

Study individuals were maintained as singletons, breeding

pairs or as small family groups, in fenced, semi-natural

enclosures, and fed a mixed artificial/natural diet (Bow-

man & Plowman, 2002; Plowman, 2002). In situ studies

were carried out in a c. 34-km2 study area in the Matobo

National Park (MNP) in southern Zimbabwe (20�20¢40¢S;

28�25¢45¢E). The Matobo area comprises an exposed

granite batholith dissected by deep drainage systems

carved down into NNW to NW trending joints and faults

(Lightfoot, 1981). A sub-rectangular system of jointing

and consequent erosion predominates, resulting in

numerous rocky outcrops and boulders (bornhardts and

‘castle kopjes’) interspersed with narrow valleys. Rainfall

averages 601.1 mm year)1 (60-year mean, Zimbabwean

Parks and Wildlife Management Authority, unpublished

data), with the majority falling between November and

March; however, the timing and quantity of rainfall are

erratic. Vegetation mirrors the heterogeneity of the phys-

ical environment, and ten broad communities, based on

woody plant composition, have been distinguished

(N. Lunt, unpublished data).

Duiker density estimation – dung-heap counts

Fifteen 1 km by 2 m strip transects were demarcated in

2001. Because of the rugged terrain (with some sheer-

sided rocky outcrops) and habitat mosaic, transects were

not statistically representative of all vegetation types,

although all habitats were sampled and those that were

under-represented tended to be avoided by grey duikers

(N. Lunt, unpublished data).

Transects were assessed ten times between 2002 and

2005. Each transect was cleared of dung 9.6 ± 6.7 days

(n ¼ 154) before sampling with similar within-session

time lapses (mean 2.8 days, n ¼ 10). Transects were

walked at a maximum speed of 2 km h)1 and all faecal

deposits within the sampling band were recorded.

Density estimates of grey duikers 383

� 2006 The Authors. Journal compilation � 2006 Blackwell Publishing Ltd, Afr. J. Ecol., 45, 382–389

Field densities were calculated for each transect for each

sampling session using the formula (Boyce, 1995):

D ¼ X

ATR;

where D is the density (animals km)2), X is the number of

pellet groups encountered, A is the area sampled (km2), T is

the time available for deposition (days) and R is the daily

defecation rate.

Data were pooled across transects for each session, and

seasonal and annual trends were investigated. A daily

defecation rate of 6.45 piles was used, based on observa-

tion of captive animals (Table 1) (see below).

Duiker density estimation – territory mapping

Six adult animals (four males, two females) were captured

with nets and fitted with radio-collars (Wildlife Materials

Inc., Carbondale, IL, USA) in 2001 and 2002. Radio-

tracking of individuals was undertaken on foot approxi-

mately twice weekly over a 13-month period. Attempts

were made to distribute tracking evenly between 06:00 h

and 19:00 h. Animal positions were estimated by trian-

gulation (based on at least five fixes per position) using

Locate 2 (Nams, 1990–2000). Estimated locations with

95% error areas >5000 m2 were discarded.

Home range sizes were estimated using the fixed kernel

(FK) and minimum convex polygon (MCP) algorithms of

Arcview’s (Environmental Systems Research Institute,

Redlands, CA, USA) Animal Movement Analyst Extension

(AMAE) (Hooge & Eichenlaub, 1997) and applying an ad

hoc smoothing factor (Hooge & Eichenlaub, 1997).

Given that males maintain exclusive territories (Dunbar &

Dunbar, 1979), male home range estimates were assumed

to equate with territory size and only male home ranges

were used for territory mapping. Each territory was assumed

to have a mean of two occupants, given that male and female

home ranges overlap (Dunbar & Dunbar, 1979), and

assuming an equitable sex ratio (which was supported by

random sightings in MNP, sign test; n ¼ 55, P ¼ 0.178).

Females with offspring may offset single occupants. Thus

density was estimated using the following formula:

D ¼ 2=t;

where D is the density (animals km)2) and t is the mean

territory size of collared male animals.

Defecation rates

Daily defecation rates were obtained from breeding pairs or

small family groups (2001) or singletons (2004) (Table 1).

At 24-h intervals over 4–5 days, dung piles were counted

and divided by the number of enclosure occupants to

determine individual defecation rates. Juveniles that were

less than a month old were excluded. In 2004, individual

deposits were collected and oven-dried to a constant mass

at 100�C, and dry mass per deposit and total daily faecal

production were measured.

Distribution of defecation sites

Between May 2004 and April 2005, six routes – each 6- to

10-km long and covering a total distance of 44 km – were

established in MNP, and were walked monthly. To ensure

representative sampling of all vegetation communities, the

total distance walked within each community was com-

pared with the total proportional coverage of that commu-

nity. Positions of fresh (<2 weeks old) grey duiker dung piles

within 1 m either side of the path were recorded using a

handheld Garmin GPS III Plus unit (Garmin Corporation,

Olathe, KS, USA). If multiple deposits were encountered at a

site, the total number of piles (irrespective of age) was coun-

ted and the approximate spatial area covered was recorded.

At each location, herbaceous layer height was recorded, and

at 1 m above the ground, the approximate visibility distance

was estimated and vegetation density was subjectively

Table 1 Daily defecation rates for grey

duiker at Dambari Field Station. Diets were

comparable between experimental ses-

sions, but animal groupings varied (see

text)

Session

n

Stat

Piles per day

Male Female Male Female Combined

2001 7 11 Median – – 6.0

Mean – – 6.6 ± 0.33

Range – – 1.5–13.6

2004 4 4 Median 8.1 4.6 6.4

Mean 8.0 ± 0.74 4.6 ± 0.24 6.3 ± 0.49

Range 4–14 3–6 3–14

384 Nicola Lunt et al.

� 2006 The Authors. Journal compilation � 2006 Blackwell Publishing Ltd, Afr. J. Ecol., 45, 382–389

assigned to a seven-point ordinal scale. Faecal deposition site

preferences (vegetation community and the three habitat

variables) were tested using chi-squared tests.

The dispersion pattern of faeces was investigated using

the blocked quadrat variance method (Krebs, 2002).

Routes and faecal deposits were mapped using Cartalinx v.

1.2 (Hagan & Eastman, 2002). Each route was subdivided

into a series of contiguous 200 · 200 m quadrats, and

the number of faecal piles recorded in each quadrat over

12 months was counted. A quadrat was only counted as

being sampled if the average route taken through it was in

excess of 100 m in length. Data were analysed separately

for each route using the Contiguous Quadrat method

(Two-term local quadrat variance: TTLQV) module of

Ecological Methodology (Krebs, 2002), and allowing a

maximum block size of ten. Although this exceeded 10% of

total n, repetition of transects was assumed to reduce type

II error (Campbell et al., 1998).

Results

Density estimates – dung-heap counts

Averaging across all transects, density estimates per session

ranged from 1.5 to 18.5 animals km)2 (Table 2). Density

estimates among transects within and among sampling

sessions were not significantly divergent (ANOVA, F143 ¼0.737; P ¼ 0.392), but variation among transects was

often great (Table 2). Density estimates were comparable

between the dry (May to October) and wet (November to

April) seasons (t-test, t ¼ )1.64; P ¼ 0.152). The overall

density estimate for the study site was 9.68 animals km)2.

Density estimates – territory mapping

Density estimates based on the mean FK home range size of

the four males ranged from 4.3 to 24.7 animals km)2

depending on which probability area was taken to repre-

sent a defended territory. Density based on MCPs was very

similar to that for the 95% FK estimate (Table 3). Neigh-

bouring males had apparently exclusive ranges (Fig. 1),

whilst female/male ranges overlapped substantially (Fig. 2).

Defecation rates

Based on 2001 experiments, mean daily defecation rates

were relatively consistent (Table 1). However, when ani-

mals were sampled individually in 2004, it was evident

that males defecated more frequently than females (Krus-

kal–Wallis test; H1 ¼ 12.43; P < 0.001) and there was a

large individual variation, especially amongst males

(Table 1). The total daily mass of faeces produced by both

sexes was comparable (males 349.8 g, females 311.1 g;

z ¼ )1.243, P ¼ 0.214), but the mean mass per deposit

was higher for females than males (males 48.2 g, females

65.5 g; z ¼ 2.042, P ¼ 0.041). Both sexes exhibited

preferences for one or two defecation sites within their

enclosures, usually, close to boundary fences.

Table 2 Density estimates (mean ± SE) derived from line transects

between 2002 and 2005

Session Mean ± SE Range

February 2002 7.80 ± 3.67 0–34.45

April 2002 10.34 ± 3.40 0–41.34

September 2002a 18.46 ± 7.75 0–44.30

November 2002a 14.77 ± 5.51 0–55.37

April 2004 8.13 ± 3.66 0–44.30

August 2004 11.00 ± 4.71 0–66.04

October 2004 12.22 ± 4.97 0–63.04

January 2005 1.50 ± 0.91 0–11.04

April 2005 4.68 ± 2.90 0–31.89

July 2005 8.40 ± 3.11 0–38.66

Overall 9.68 ± 1.33 1.50–18.46

aThree transects not sampled.

Table 3 Home range estimates for radio-

collared male grey duiker and territory

mapping population density estimatesDuiker

Number of

locations Time (days) FK 50% FK 75% FK 95% MCP

Male 1 24 90 0.02 0.05 0.14 0.10

Male 2 44 185 0.07 0.15 0.35 0.26

Male 3 19 64 0.01 0.03 0.17 0.12

Male 4 70 352 0.23 0.50 1.22 1.08

Mean 39.25 172.75 0.08 0.18 0.47 0.39

Density (km)2) 24.69 10.95 4.27 5.13

FK, Fixed kernel # % probability area; MCP, minimum convex polygon.

Density estimates of grey duikers 385

� 2006 The Authors. Journal compilation � 2006 Blackwell Publishing Ltd, Afr. J. Ecol., 45, 382–389

Distribution of defecation sites

Over one annual cycle, 424 defecation sites were recorded

along set routes and 1209 individual deposits were

counted. Some sites were used habitually over several

months. At more than half of the locations, multiple

faecal deposits were observed, with a mean of 2.9 (range

1–41) piles at each site. Multiple deposits covered an

estimated mean area of 12 m2 (0.64–225 m2). More

faecal piles and defecation sites were encountered in

the dry season than in the wet season (faecal deposits;

z ¼ 2.467; P ¼ 0.0007: defecation sites; z ¼ 3.508; P ¼0.0002).

The proportion of sites that contained multiple faecal

piles was comparable between the dry and wet seasons,

both along strip transects (z ¼ 0.620; P ¼ 0.535) and

walked routes (z ¼ )0.746; P ¼ 0.240). Similarly, along

strip transects, short (<7 days) and long (>7 days) inter-

vals between clearing and walking transects did not affect

the number of plots that contained multiple deposits (z ¼0.235; P ¼ 0.814).

Grey duikers exhibited marked defecation site prefer-

ences for low- to medium-wooded grassland and dambos,

but tended to avoid kopjes and bornhardts (v2 ¼ 66.2;

d.f. ¼ 6; P ¼ 2.49 · 10)12). Preference was shown for

herbaceous layer heights between 40 and 100 cm (v2 ¼123.644; d.f. ¼ 6, P < 0.001) and visibility distances

greater than 20 m (v2 ¼ 19.707; d.f. ¼ 9, P < 0.025).

Selection for bush density approached significance (v2 ¼1.299; d.f. ¼ 1, P ¼ 0.058).

Spatial distribution of faecal piles was found to be loosely

clumped along four of six routes (routes a, b, d and e; peak

block variances at between 3 and 7; minimum number of

quadrats for any route ¼ 23), but clumping was not well

defined for transects c and f, although variance peaks were

noticed at block sizes 3 and 4. None of the graphic plots

indicated uniform or random distributions of faecal

deposits (Fig. 3).

Discussion

Comparison of methods

In this study, overall density estimates from dung-heap

counts were similar to those derived from territory map-

ping, particularly the FK 75% probability area. Although

arbitrarily selected, this home range area may represent a

more realistic territory estimate than the FK 95% prob-

ability area or MCP estimates, as males are unlikely to be

able to defend their entire home range. The congruence in

estimates based on dung-heap counts and territory map-

ping suggests that both are of equal accuracy, despite

violations of assumptions (dung-heap counts) and the

small sample size (territory mapping).

Territory mapping

The assumption that males occupy discrete territories that

overlap with female home ranges cannot be verified by the

small sample size in this study. However, two sets of

neighbouring radio-collared duikers suggest that these

assumptions are valid (Figs 1 and 2). The method also

assumes an even sex ratio, which was supported by ran-

dom sightings in MNP (sign test; n ¼ 55, P ¼ 0.178).

Fig 1 Minimum convex polygon home ranges for male grey duiker

in Matobo National Park (d, male 1; , male 3)

Fig 2 Minimum convex polygon home ranges for a male and female

grey duiker in Matobo National Park ( , male 4; h, female 1)

386 Nicola Lunt et al.

� 2006 The Authors. Journal compilation � 2006 Blackwell Publishing Ltd, Afr. J. Ecol., 45, 382–389

Density will be miscalculated if home range estimates

are inaccurate. Radio-collared duiker showed great

variation in home range size in MNP. The relative cov-

erage of unsuitable habitat (e.g. kopjes) within the local

area of each study subject is a likely cause, but this has

not yet been quantified. Sample size is also a major

source of potential inaccuracy; for two animals, this was

below the minimum of 30 location points recommended

by Seaman et al. (1999). In fact, inspection of MCP

bootstrap curves (100 replicates per interval) revealed

that home range area did not approach an asymptote

until around 40 location points (data not shown).

Inclusion of these two animals could lead to an overes-

timate of density, although the dung-heap count estimate

would still fall within the FK 50–95% derived density

range.

Dung-heap counts

Several assumptions of the dung-heap count method were

violated and are discussed below.

Defecation rate. Density estimates using faecal pile counts

rely heavily on accurate defecation rate data, but several

factors are believed to affect defecation rate in antelope,

including diet (Bowland & Perrin, 1994), age (Bowland &

Perrin, 1994) and sex (Brashares & Arcese, 1999; this

study). The effect of age is probably confounded with body

size and requires further investigation.

Diet has an effect on throughput rate and the quantity of

faecal matter produced, because of differential digestibility

of foodstuffs (Conklin-Brittain & Dierenfeld, 1996). Grey

duikers are predominantly browsers in savannah ecosys-

tems, but are opportunistic frugivores (Wilson, 1966). It

could be expected therefore that there would be seasonal

variation in defecation rates that would be related to

browse quality and the type of food ingested. Many studies

obtain defecation rate data from captive animals (e.g.

Bowland & Perrin, 1994; Plumptre & Harris, 1995), which

tend to be fed high-quality diets with limited fibre content

(Bowland & Perrin, 1994; Plumptre & Harris, 1995;

Plowman, 2002). However, preliminary experiments

indicated that defecation rates on two diets – (i) game

cubes (National Foods, Bulawayo, Zimbabwe) and browse

and (ii) browse only – were not significantly different in

grey duiker (n ¼ 2; z ¼ 1.64, P ¼ 0.099), steenbok

(Raphicerus campestris Thunberg 1811) (n ¼ 4; z ¼ 1.08,

P ¼ 0.278) or klipspringer (Oreotragus oreotragus Zimm-

erman 1783) (n ¼ 5; z ¼ 0.473, P ¼ 0.636) (N. Lunt,

unpublished data).

In this study, it was found that males deposited signifi-

cantly more and smaller faecal piles than females. As there

is only a marginal sexual dimorphism in grey duiker body

mass (Kingdon, 1997), and the total mass of deposit pro-

duced daily was similar for both sexes, more frequent

defecation events by males may be linked to territorial

marking, if faecal piles are used as visual and olfactory

signposts to announce occupancy. Brashares & Arcese

0

10

20

30

40

1 2 3 4 5 6 7 8 9 10

Block size

Var

ian

ce

0100200300400500600

1 2 3 4 5 6 7 8 9 10Block size

Var

ian

ce

050

100150200250

1 2 3 4 5 6 7 8 9 10

Block size

Var

ian

ce

0

10

20

30

40

1 2 3 4 5 6 7 8 9 10

Block size

Var

ian

ce

0100200300400500600

1 2 3 4 5 6 7 8 9 10Block size

Var

ian

ce

0102030405060

1 2 3 4 5 6 7 8 9 10Block size

Var

ian

ce

Fig 3 Plots of variance versus block size

(TTLQV method), for the six walked routes

(A–F)

Density estimates of grey duikers 387

� 2006 The Authors. Journal compilation � 2006 Blackwell Publishing Ltd, Afr. J. Ecol., 45, 382–389

(1999) found a similar pattern in male oribi (Ourebia ourebi

Zimmerman 1783), with territorial males with large har-

ems producing more and smaller faecal deposits than

females, juveniles and subordinate males.

The effects of sexual differences in defecation rate will be

inconsequential for density estimates if the sex ratio of the

sampled population approaches parity. As found in other

locales (e.g. Dunbar & Dunbar, 1979; Wilson & Clarke,

1962), sex ratios derived from random sightings in MNP

were not divergent from 1 : 1 (see above). In cases of

unequal (but known) sex ratios, a weighted mean should

be used.

Spatial distribution of faeces. Marques et al. (2001) com-

mented that faecal deposits tend to be aggregated. In this

study, both in situ and ex situ data indicated nonrandom

spatial distribution of grey duiker faecal deposits. If mul-

tiple deposits are the norm, the probability of encountering

a deposit is reduced, which would result in an underesti-

mate of population density. Conversely, if a deposition site

was encountered, it is likely that it would contain multiple

deposits. However, applying a simple correction factor to

dung-heap counts would be complicated, as cohabiting

animals in captivity utilized the same defecation sites

(N. Lunt, personal observation), which suggests that ani-

mals with overlapping home ranges may deposit faeces in

close proximity to each other.

Despite this serious assumption violation, increasing the

total area sampled and ensuring that the coverage of the

study area is representative can reduce error. This was

effectively demonstrated in this study. The estimates that

varied greatly from expected densities (from territory

mapping) were derived from sessions in when fewer tran-

sects were sampled (Table 2). Those transects that were

omitted comprised primarily marginal duiker habitat.

Thus, estimates were biased towards prime habitat, with

the result that overall densities were overestimated. With

the additional sampling effort, standard errors were

reduced and estimates approached the overall mean esti-

mate for the study area.

Faecal decomposition rate. Faecal decomposition can be

affected by climate, rainfall, humidity, season, microcli-

mate, insect activity and leaf fall (Edwards, 1991; Bowland

& Perrin, 1994; Plumptre & Harris, 1995; Marques et al.,

2001). It is advantageous therefore to clear plots or tran-

sects prior to sampling. The lapse between clearing and

sampling must not exceed faecal decomposition time, but

should not be so short that defecation in the area is pre-

cluded. In this study, dry season estimates tended to be

higher than wet season estimates, despite short clear-

sample intervals of approximately 4.5–14 days. The lowest

estimate (Table 2) was obtained at the height of the wet

season when insect activity was at its peak. At this time,

coprophagous insects were observed to completely remove

a pile of donkey dung within 36 h of deposition (N. Lunt,

personal observation). It is therefore recommended that

transects are sampled in the dry season when humidity

and insect activity are low.

Conclusions

All methods of determining population sizes of small ante-

lope have their limitations (Bowland & Perrin, 1994; Ellis,

2003; Lannoy et al., 2003). Because of their preference for

thick habitat, direct counts are rarely possible for grey

duiker, so obtaining sufficient data to use distance sampling

analyses is difficult. Camera trapping is also a promising

method for estimating density, especially for nocturnal or

very secretive species (e.g. O’Brien et al., 2003), but its

accuracy relative to other estimation methods has rarely

been rigorously tested (Silveira et al., 2003).

Although several of the dung-heap count method’s

assumptions were violated, it was evident in this case that

wide coverage by transects in the study area and ade-

quately sampling all habitat types limited the extent of

estimation error. However, researchers should be cautious

when implementing or interpreting the results of the dung-

heap count method, particularly if it is not supported by

additional methods.

Acknowledgements

The authors would like to thank Verity Bowman and the

Marwell Zimbabwe Trust staff for practical and logistical

support during the study, and Leonard Mpofu, Nkululeko

Sigola and Maxwell Ndlovu for assistance in the field.

Chipangali Wildlife Orphanage allowed NL to carry out

defecation experiments on steenbok and klipspringer, and

the Zimbabwean Parks and Wildlife Management

Authority granted permission to carry out research in the

Matobo National Park. Paignton Zoo Environmental Park

and the Disney Wildlife Conservation Foundation provided

funding for the project. Dr M. Jacquier, Dr F. Rovero and

an anonymous reviewer provided useful comments on the

manuscript.

388 Nicola Lunt et al.

� 2006 The Authors. Journal compilation � 2006 Blackwell Publishing Ltd, Afr. J. Ecol., 45, 382–389

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Density estimates of grey duikers 389

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