Evaluating Orangutan Census Techniques Using Nest Decay Rates: Implications for Population Estimates

Abstract

An accurate estimate for orangutan nest decay time is a crucial factor in commonly used methods for estimating orangutan population size. Decay rates are known to vary, but the decay process and, thus, the temporal and spatial variation in decay time are poorly understood. We used established line-transect methodology to survey orangutan nests in a lowland forest in East Kalimantan, Indonesia, and monitored the decay of 663 nests over 20 months. Using Markov chain analysis we calculated a decay time of 602 days, which is significantly longer than times found in other studies. Based on this, we recalculated the orangutan density estimate for a site in East Kalimantan; the resulting density is much lower than previous estimates (previous estimates were 3-8 times higher than our recalculated density). Our data suggest that short-term studies where decay times are determined using matrix mathematics may produce unreliable decay times. Our findings have implications for other parts of the orangutan range where population estimates are based on potentially unreliable nest decay rate estimates, and we recommend that for various parts of the orangutan range census estimates be reexamined. Considering the high variation in decay rates there is a need to move away from using single-number decay time estimates and, preferably, to test methods that do not rely on nest decay times as alternatives for rapid assessments of orangutan habitat for conservation in Borneo.

Best practice guidelines for surveys and monitoring of great ape populations

“…Les comptages directs de grands singes sont rarement réalisables; la plupart du temps, on compte plutôt les indices de présence. (van Schaik et al 1995;Singleton 2000;Buij et al 2003;Ancrenaz et al 2004a,b;Johnson et al 2005;Marshall et al 2006Marshall et al , 2007Mathewson et al 2008). …”

“…La vitesse de dégradation des nids varie en fonction de l'espèce concernée, de l'espèce d'arbre de nidification et de paramètres abiotiques tels que les précipitations, l'altitude, la température, le type de sol et le pH (van Schaik et al 1995;Buij et al 2003;Ancrenaz et al 2004a;Walsh et White 2005;Marshall et al 2006;Mathewson et al 2008). …”

“…Il faut un important investissement en temps pour obtenir une estimation précise à partir du suivi de la dégradation d'un échantillon approprié de nids; plusieurs études utilisent donc une chaîne de Markov pour estimer la durée de vie des nids (van Schaik et al 1995;Russon et al 2001;Buij et al 2003;Morrogh-Bernard et al 2003;Johnson et al 2005;Mathewson et al 2008). Cette technique utilise une matrice mathématique basée sur les vitesses de transition entre des ëétats de dégra-dationí prédéfinis (ex.…”

Nest-building orangutans demonstrate engineering know-how to produce safe, comfortable beds

“…Previous research into orangutan nests can be broadly broken into two major avenues of investigation: one, focusing on nestbuilding behavior (3,(9)(10)(11)(12); and the other, concentrating on the use of nests to predict population numbers and distribution (3,10,15,16). Investigations into nest-building behavior have provided detailed, insightful, and descriptive knowledge of the basic nest-building process.…”

“…Such data have mostly comprised observations of nest building, such as the frequency and duration of builds, positioning of nests, and preferences within the ritual of nest building (3,(9)(10)(11)(12). The study of nests to understand populations of wild orangutans has led to knowledge of nest longevity and geographical distribution (3,10,15,16). However, to date there is still relatively little information about the structural patterns of construction, and almost nothing has been reported on the mechanics of the nest structure itself (3).…”