Positive news about Sumatran orangutans is rare. The species is critically endangered because of forest loss and poaching, and therefore, determining the impact of future land-use change on this species is important. To date, the total Sumatran orangutan population has been estimated at 6600 individuals. On the basis of new transect surveys, we estimate a population of 14,613 in 2015. This higher estimate is due to three factors. First, orangutans were found at higher elevations, elevations previously considered outside of their range and, consequently, not surveyed previously. Second, orangutans were found more widely distributed in logged forests. Third, orangutans were found in areas west of the Toba Lake that were not previously surveyed. This increase in numbers is therefore due to a more wide-ranging survey effort and is not indicative of an increase in the orangutan population in Sumatra. There are evidently more Sumatran orangutans remaining in the wild than we thought, but the species remains under serious threat. Current scenarios for future forest loss predict that as many as 4500 individuals could vanish by 2030. Despite the positive finding that the population is double the size previously estimated, our results indicate that future deforestation will continue to be the cause of rapid declines in orangutan numbers. Hence, we urge that all developmental planning involving forest loss be accompanied by appropriate environmental impact assessments conforming with the current national and provincial legislations, and, through these, implement specific measures to reduce or, better, avoid negative impacts on forests where orangutans occur.

Land-cover changes predict steep declines for the Sumatran orangutan ( <i>Pongo abelii</i> )

School of Natural Sciences and Psychology, Liverpool John Moores University, James Parsons Building, Byrom Street, L33AF Liverpool, UK.

Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Sciencepark 904, Amsterdam 1098, Netherlands.

Sumatran Orangutan Conservation Programme, Jalan Wahid Hasyim 51/74, Medan 20154, Indonesia.

Department of Anthropology, Southern Illinois University, 1000 Faner Drive, Carbondale, IL 62901, USA.

Fakultas Biologi, Universitas Nasional, Jalan Sawo Manila, Pasar Minggu, Jakarta Selatan 12520, Indonesia.

Leuser Conservation Forum, Jalan Geuchik Raja No. 89 A, Banda Aceh 23233, Indonesia.

Yayasan Orangutan Sumatera Lestari–Orangutan Information Centre, Jalan Bunga Sedap Malam 18C No. 10, Medan 20131, Indonesia.

Center for International Forestry Research, P.O. Box 0113 BOCBD, Bogor 16000, Indonesia.

Department of Primatology, Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, 04103 Leipzig, Germany.

German Centre for Integrative Biodiversity Research (iDiv), Halle-Jena-Leipzig, Deutscher Platz 6, 04103 Leipzig, Germany.

Land-cover changes predict steep declines for the Sumatran orangutan ( Pongo abelii )