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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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