Using Dung to Estimate Gorilla Density: Modeling Dung Production Rate

Abstract

There is an urgent need for information on western gorilla population sizes and distribution to improve present and plan future conservation actions. Researchers traditionally have estimated gorilla densities on the basis of nest counts despite demonstrated variation in nest production and decay rates. The variation may lead to large biases in estimates of gorilla abundance. We investigated the use of an alternate index of gorilla abundance, via defecation data collected from habituated gorillas at Bai Hokou, Central African Republic. Our sample of 274/370 defecation events/dung piles produced a production rate of ca. 5 dung piles/d: comparable to previous estimates based on much smaller sample sizes. Heuristic models that failed to account for imperfect dung pile detection produced a lower defecation rate estimate than that of a maximum likelihood model that explicitly modeled detection probability. Generalized linear modeling (GLM) showed that dung pile production rate was strongly linked to rainfall, suggesting that failure to correct for seasonal variation in dung pile production rates could lead to substantial biases in gorilla abundance estimates. In our study, failing to distinguish between the number of defecation events and the number of dung piles produced would lead to a ca. 31% overestimate of true gorilla numbers. The use of dung as an index of gorilla abundance shows potential, but more fieldwork and modeling on seasonal variation in dung production rates is required.

Best practice guidelines for surveys and monitoring of great ape populations

“…Pour ce dernier cas, le comptage de crottes a un certain potentiel, selon une étude récente à Bai Hokou qui montre que les crottes et les nids sont trouvés à une fréquence comparable (Todd et al 2008). Les comptages de crottes peuvent être réalisés de manière similaire aux comptages des nids sur des transects (méthodes ci-dessus) (Plumptre 2000 van Schaik et al 2005;Ancrenaz et al 2004a;Johnson 2005 dégradation des crottes de gorille sont sujettes à une énorme variation spatio-temporelle (Kühl et al 2007).…”

Maximizing Nonhuman Primate Fecal Sampling in the Republic of Congo

“…Secondarily, the ''known'' scientific observations are rife with variability. Dung pile production rate has strong associations with rainfall and feeding behavior and striking differences between age classes, varying from 2-11 piles a day (Todd et al, 2008). Once deposited, climate, environment, and fecal consumers affect sample longevity in the field.…”

“…The large spatial scale and short duration of the fecal surveys was intended to reduce the likelihood of sampling individuals more than once. Although most researchers prefer to use nest detection over feces for primate population surveys, we analyzed feces to generalize our results to nonnesting species (Kü hl et al, 2008;Todd et al, 2008).…”

“…Stokes et al (2010) reported densities of 0-7 individuals/km 2 for similar-sized landscapes within northeastern RC, so we selected 13 gorilla densities ranging from 0.005 to 7.0 individuals/km 2 . Fecal production rate was set at five piles/day (Todd et al, 2008). The fecal assay requires a sample collection within 24 hours of deposition; therefore we simulated 1 day's worth of fecal production.…”

Is the Western Lowland Gorilla a Good Gardener? Evidence for Directed Dispersal in Southeast Gabon

“…This species disperses seeds in a spatially and temporally bimodal pattern with more than 50% of seeds deposited at nest sites (Todd et al, 2008), which are preferentially installed in open habitats (Tutin et al, 1991;Rogers et al, 1998;Voysey et al, 1999;Petre et al, 2015a). Seedling growth is enhanced in tree fall gaps, even for shade tolerant species (Howe et al, 1985;Popma and Bongers, 1988 and references therein; Schupp et al, 1989), and a beneficial effect of deposition at gorilla nest sites was observed for three tree species at Lopé, Gabon (Tutin et al, 1991;Rogers et al, 1998;Voysey et al, 1999).…”