How many Sumatran orangutans are there? More than we thought but fewer than there were.

For a creature that lives in remote, dense forest, getting a handle on the population size is exceedingly difficult, even when the animal is as large as an orangutan.

According to the last big survey of Sumatran orangutans (Pongo abelii), published in 2004, there were just 6600 members of this species left on the Indonesian island of Sumatra. But a new, more expansive survey more than doubles this figure to 14,613. This might sound like good news for the Sumatran orangutan, but it isn’t. It’s just that the 2004 count was way off the mark.

There were several shortcomings of the previous study, says Serge Wich, a conservation biologist at Liverpool John Moores University and the lead author of both the old and the new analysis. First, it was assumed that there were no orangutans living above 900m. Second, the last survey ignored a couple of areas now known to harbour populations. Third, it failed to consider large areas of logged forest. But there are orangs in all these places.

Indonesia's Sumatran orangutan population hit by deforestation – in pictures Read more

“We were much more inclusive of areas,” says Wich. So rather than being confined to an area of around 7000 km2, the Sumatran orangutan range is probably closer to 18,000 km2. If the previous survey had covered this kind of area, the population estimate in 2004 would certainly have been far larger than it is now.

“Over the past ten years, we have lost a large amount of forest,” says Wich. Take the Tripa peat swamp forest in the northeast of Sumatra, for instance, where palm oil plantations have replaced much of the primary forest. “There were, in 1990, probably 3000 orangutans there and there are maybe now only 200 left,” says Wich. “That’s just one area. That’s a very steep decrease in 25 years.”

Facebook Twitter Pinterest On fire. The Tripa peat swamps in Aceh Province in 2012. Perhaps only 200 orangutans remain in this region Photograph: Carlos Quiles/AFP/Getty Images

Poaching is also a problem, particularly when orangutans wander onto agricultural land or plantations. “There are about four million people living around the Leuser Ecosystem, this orangutan stronghold in the north,” says Wich.

“Probably the majority of these people don’t know that the orangutan is critically endangered. They do probably know that it’s illegal to kill them but they probably also know that it’s extremely rare to get prosecuted for killing and catching an orangutan,” says Wich. It’s been illegal to kill orangutans since 1931. “Since then but there’s only a handful of cases and all these in very recent years.”

The paper, published today in Science Advances, makes two specific recommendations. First, agricultural development must not stray into areas where deforestation is prohibited according to Indonesian laws and regulations. Particularly, important for orangutans are the peat swamp areas where such expansion is prohibited and large numbers of orangutans occur. Second, there needs to be better enforcement of existing laws prohibiting the poaching and trading of organutans.

“If all the Indonesian laws and regulations were followed, the vast majority of orangutans would be fine.”

Wich, S.A. et al. (2016) Land-cover changes predict steep declines for the Sumatran orangutan (Pongo abelii) Science Advances 2: e1500789

