Monitoring population decline: can transect surveys detect the impact of the Ebola virus on apes?

Abstract

In 2004 the Ebola virus caused a drastic decline in western gorilla Gorilla gorilla abundance at Lokoué Bai, a clearing in Odzala National Park, Republic of Congo. This decline was detected by observations of gorillas visiting the clearing. We confirm that the sympatric chimpanzee Pan troglodytes population was also affected by the Ebola outbreak, and test whether the decline in the ape population would have been detected with linetransect surveys, the most commonly used wildlife monitoring methodology in Central Africa. We also evaluate the potential of transect surveys for describing the extent and pinpointing the timing of drastic population declines when this information is not known from other evidence. Both nest survey using the marked nest count method and standing stock survey of other signs of ape presence (dung, feeding remains, prints) were able to detect the decline. However, only nests and dung were reliable indices for estimating the magnitude of the decline and accurately pinpointing the timing. It was necessary to pool data across many survey replicates because of small samples sizes. Our results suggest that transects methods are able to detect drastic changes in ape abundance but that large sample sizes are necessary to achieve adequate statistical power. We therefore recommend that those intending to use transect methods as tools for monitoring large forest mammals evaluate in advance how much effort will be necessary to detect meaningful changes in animal abundance.

Assessment of in situ nest decay rate for chimpanzees (Pan troglodytes ellioti Matschie, 1914) in Mbam-Djerem National Park, Cameroon: implications for long-term monitoring

“…From our best fitted model (Model 2), there was no evidence of a significant direct correlation between the probability of nest decay and "topography". The latter is a potentially overlooked variable which may influence tree cover and thus the availability of plants used for nesting and should be considered in future studies (Devos et al 2008). Rainfall had an effect on the mean nest decay rate in the MDNP, increasing the probability of nest decay in the first month of the nest's existence.…”

Chimpanzee oil‐palm use in southern Cantanhez National Park, Guinea‐Bissau

“…Linear Transect Surveys are the most widely used methods to estimate mammal densities [Chiarello, 2000; Devos et al, 2008b; Drummer & McDonald, 1987; Plumptre, 2000; Plumptre & Reynolds, 1996; Wanyama et al, 2010]. These methods assume that there is a detection probability that depends exclusively on the perpendicular distance from the trail to the nest [Buckland et al, 2001].…”

High Population Density of Pan troglodytes verus in the Haut Niger National Park, Republic of Guinea: Implications for Local and Regional Conservation

“…This value takes into account the reuse of old nests by chimpanzees, a behavior that we documented in our study site (we regularly observed old degraded nests with fresh leaves at the top). The methodological limitations of the standing crop nest count method have been subject to numerous studies (Blom et al 2001;Devos et al 2008;Hashimoto 1995;Plumptre and Reynolds 1996;Tutin et al 1995;Walsh and White, 2005). The formula developed by Ghiglieri (1984) to estimate ape density is based on the density of individual nests and rests on the assumption that nests are placed randomly throughout the study area.…”