Orangutan distribution, density, abundance and impacts of disturbance

Knowledge of the density and distribution of the endangered orangutan is essential both for their conservation and to identify ecological factors that limit their population size. The authors of this chapter collated orangutan density estimates from 110 locations throughout their range, standardized these for differences in survey design and categorized each location using broad descriptions of the habitats present and the degree of logging disturbance. They found that orangutan density is higher in Sumatra than Borneo, as expected, but found no difference in density between subspecies in Borneo, which suggests that Pongo pygmaeus morio has evolved adaptations to cope with food scarcity in eastern Borneo. The authors of this chapter found support for the hypothesis that orangutan densities are higher in sites with less extreme periods of food shortage, by demonstrating that sites with a mosaic of different habitat types support significantly higher densities than those with only a single habitat type present. Orangutan density declines significantly with altitude in Borneo but the authors of this chapter did not find this relationship for Sumatra, although this may be due to other underlying factors. Heavy logging disturbance has a severe negative effect on density but light logging carried out by well-managed selective logging operations appears to have no effect on orangutan density. The chapter discusses the implications of these findings for orangutan conservation.