A decade of proboscis monkey (Nasalis larvatus) population monitoring in Balikpapan Bay: Confronting predictions with empirical data.

Systematic and well-structured monitoring is essential for taxa with high extinction risk such as primates. Endangered proboscis monkeys Nasalis larvatus are endemic to Borneo, where they are found scattered across lowland habitats of the island, which are under strong anthropogenic pressure. A large population of proboscis monkeys in Balikpapan Bay, Indonesian Borneo, was predicted to decline due to the ongoing habitat loss and degradation, notably because of forest fires. We examined changes in the number and composition of groups of a part of this population from 2007 to 2017, which included a period of forest fires linked to the El Niño-Southern Oscillation events. We conducted a census from a boat; attempting to locate all proboscis monkey groups within the Balikpapan City administrative area in 2007, 2012, and 2017. During the most recent census, we observed a total number of 60 proboscis monkey groups in two subpopulations. The population density was 1.14 group per km of suitable habitat. Contrary to previously published predictions, we did not find evidence of a population decline. Contrary to predictions, the 2015 El Niño induced fires impacted mainly forests on ridges and slopes, thus affecting only a small part of the proboscis monkey habitat located close to rivers and mangrove swamps. However, the increasing population density of monkeys, coupled with ongoing habitat degradation and habitat loss in one of the subpopulations, suggests that proboscis monkey population in Balikpapan Bay may be approaching a limit of resilience to habitat changes. In case it proves infeasible to census all individuals in the whole population, we recommend using a group-level census, connected with systematic group counts to obtain a reasonably precise proboscis monkey population size estimate.