Hunting of Sumatran orang-utans and its importance in determining distribution and density

To conserve species it is essential to understand which factors determine their distribution and density. Here we focus on the critically endangered Sumatran orang-utan and examine factors that influence the distribution and density in the Batang Toru area, the southernmost area where wild orang-utans occur on Sumatra. We contrast a scenario in which orang-utan distribution is mainly determined by ecological, and topographic variables with a model that includes hunting and human impact. We show that orang-utan distribution and density are best explained by hunting pressure and elevation. These results indicate that an assessment of anthropogenic factors that might influence density such as hunting needs to be included in surveys that aim to predict orang-utan distribution and density. As anthropogenic impact becomes higher with increasing human population density and increased forest access in most areas where orang-utans occur the consequence is that orang-utan conservation will have to be achieved in an environment modified by humans. In such areas the potential for a range of conflicts such as hunting that lead to human-caused mortality for orang-utans will remain a constant threat and need to be mitigated.

► Effective conservation requires information on factors determining distribution and density.
► We determine factors that determine orang-utan distribution and density in a study area on Sumatra.
► We contrast scenarios with and without hunting pressure and human impact.
► We show that elevation and hunting pressure explain density variation best.
► Thus orang-utan conservation efforts should focus on mitigating hunting as well.