Recent surveys of the population of endangered Bornean orangutans in Sabah, the Malaysian state in the north-east of Borneo, show mixed results. Populations have remained stable within well-managed forests, where there is little hunting, but declined in landscapes comprising extensive oil palm plantations, according to a new study in the open-access journal PLOS ONE by Donna Simon of the World Wide Fund for Nature - Malaysia, and colleagues. The study is the largest and most complete population survey of orangutans on Borneo, home to this endangered and endemic species.

Lowland forest is the most important habitat for orangutans in Sabah. Over the past 50 years, however, extensive logging and land clearance for agriculture caused habitat loss and fragmentation, which led to a drastic decline in their numbers, but the full extent of the effects on orangutan population have been difficult to estimate.

In the current study, the authors conducted aerial transects totaling nearly 5,500 kilometers across Sabah state, almost three times the length of a previous survey done in 2002-2003. Based on the number of nests, they calculated a population of 9,558 orangutans, including a previously unknown population of about 1,770 orangutans in many widely dispersed sub-populations.

The largest populations of orangutans, numbering about 5,500, were within forests that are either sustainably managed or unlogged in the central uplands of the State. In this area, the population has been stable since the 2002-2003 survey. In contrast, in fragmented forest areas surrounded by extensive areas of oil palm plantations, orangutan populations have declined by as much as 30% since the earlier study. These data are expected to be used by the government of Sabah to shape environmental policies to sustain these important Malaysian orangutan populations.

Simon adds: "A recent survey on orangutan populations in Sabah, North-east Borneo showed a mixed picture from different regions. However, overall the research shows that they have maintained the same numbers over the last 15 years and can remain so as long as proper conservation management measure continues to be put in place."

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Citation: Simon D, Davies G, Ancrenaz M (2019) Changes to Sabah's orangutan population in recent times: 2002-2017. PLoS ONE 14(7): e0218819. https:/ / doi. org/ 10. 1371/ journal. pone. 0218819

Funding: The survey work was funded for 6 years by WWF-UK with some support from WWF-Belgium. MA also acknowledges a contract with WWF Malaysia to support analysis and development of this paper. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing Interests: The authors have declared that no competing interests exist.