There Are Twice As Many Sumatran Orangutans As We Thought

Sumatran orangutan. Serge Wich
Janet Fang 04/03/2016, 20:50

Orangutans live in two places: Borneo and Sumatra. In 2004, we thought there were only 6,600 Sumatran orangutans (Pongo abelii) left in the wild, but this week, researchers conducting nest surveys on the Indonesian island report a new astounding estimate: 14,613 Sumatran orangs in 2015. 

Unfortunately, this higher estimate doesn’t reflect an actual increase in the population – rather, it’s due to a more wide-ranging survey effort that found orangs in places that weren’t previously canvassed. And despite this apparent doubling, the critically endangered apes are still seriously threatened by forest loss and poaching, according to a new Science Advances study.

When an international team led by Serge Wich of Liverpool John Moores University conducted line-transect nest surveys that covered the entire range of Sumatran orangutans, they encountered 3,166 orangutan nests on 259 line transects with a total length of 305.8 kilometers (190 miles). Based on multi-model predictions of orangutan density over their current range of 17,797 square kilometers (6871.5 square miles), that’s equal to 14,613 individuals – about 8,000 more animals than previous estimates. 

Orangutan densities are highest in peat-swamp forests and lowland forests on mineral soils. They were found living higher than 1,500 meters (4,900 feet) above sea level – elevations previously considered outside of their range – and in areas west of the Toba Lake that weren’t surveyed before. Additionally, orangs were also found more widely distributed in logged forests. Their range used to be hugely underestimated at 6,946 square kilometers (2681.9 square miles). 


Forest loss in the Leuser Ecosystem, the main stronghold of the Sumatran orangutan. Serge Wich

The team then developed a series of models to investigate the impact of nine different land-cover change scenarios. And despite this doubling of population estimates, Sumatran orangutan numbers are expected to decline under all of the scenarios they modeled because of future deforestation. As many as 4,500 individuals could vanish by 2030.

Forest loss is occurring in both protected and unprotected forests in their range. Peat-swamp forests are being converted into oil palm plantations, while forests on mineral soils are being used for other agricultural uses like rubber and candlenut plantations. Meanwhile, poaching is mostly confined to an area south of the species’ range, but orangutans are also being captured or killed when they venture into agricultural land or areas that were recently deforested. 

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