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Once, there were hundreds of thousands of orangutans roaming the island of Borneo. Over the decades – as chainsaws and tractors have cleared millions of hectares of pristine rainforest – the number of these great apes has declined.

new study shows just how alarming that decline is. In 16 years alone, researchers found that half the population of this critically endangered species has disappeared, leaving an estimated 70,000 on the island.

“We estimate that 148,500 orangutans disappeared from Borneo between 1999 to 2015,” says Maria Voigt, a scientist at the German Centre for Integrative Biodiversity Research and lead author of the study.

“One of the reasons orangutans are so vulnerable is because of the fact that they reproduce very slowly, giving birth only once every six to eight years and usually to just one baby. The loss of a relatively low number of orangutans can have a major impact on the species’ survival.”

Voigt is among 41 researchers who contributed to the study, which used field survey data collected on the ground and by helicopter to identify orangutan nests. A complex modeling approach combining knowledge about orangutan habitats and threats was then used to estimate the number of apes left.

The study also looked at the size of the orangutan populations.

“Orangutans have a better chance of survival if they live in populations larger than 100,” says Voigt. “Out of the 64 populations that experts identified in Borneo, only 38 were large enough to have a good chance of survival.”

   A female Bornean orangutan with her offspring. HUTAN/KOPC Photo

Out of the 64 populations that experts identified in Borneo, only 38 were large enough to have a good chance of survival

Maria Voigt, scientist at the German Centre for Integrative Biodiversity Research


Loss of orangutans stems from loss of habitat, which is a direct result of increasing global demand for natural resources.

To understand more about habitat loss, the team relied on a new interactive map of Borneo developed by the Center for International Forestry Research (CIFOR) that calculates landscape changes by showing the impact of fires, industrial plantations and logging over the course of the past 40 years. Going one step further, the map identifies which companies are operating in a specific area and how fast the forest there is being converted.

The study found that during the 16-year period, half of the island’s orangutan population was affected by logging, deforestation or plantations.

“In a previous study we calculated that Borneo has lost 6.3 million hectares of old-growth forest – 15% of its forest cover – since 2000,” says CIFOR scientist David Gaveau, one of the paper’s co-authors.

Orangutan population density was lower in deforested areas – but, surprisingly, many orangutans disappeared from forested areas. Because there is no current support for alternative explanations, such as deaths caused by infections or diseases, Voigt says that this suggests the orangutans are being killed. When their natural habitat has been reduced, orangutans often become targets when they wander into villages searching for food.

“Conflict killing in these communities, as well as hunting, are major causes in the decrease in orangutan numbers,” says co-author Erik Meijaard, Director of the Borneo Futures research initiative and Chair of the International Union for Conservation of Nature’s oil palm task force.

“After speaking with these hunters, we found that most are generally hunting wild pig and deer for food. But if they have no luck hunting one day and then come across an orangutan, they may decide to kill it.”

Killing reproductive female orangutans is also a major threat to the survival of the species, such as when mothers are killed and their babies sold into the illegal pet trade. And although the killing of an orangutan is illegal in Indonesia and Malaysia, the practice does still exist – especially in remote areas.

“Indonesian and Malaysian law should not just say that these apes are ‘not things’, but rather ‘legal persons’ that have a right to live,” says Gaveau. “The penalty for killing orangutans should be more severe, and enforced.”

   A female orangutan on the tracks. Serge Wich


 Although the overall results of the study are worrying, some positive aspects have emerged. The number of orangutans is relatively stable in parts of Malaysian Borneo and in large conservation sites on the Indonesian side of the island.

Researchers also discovered that orangutans are a bit more adaptable to habitat change than previously thought, able to cover long distances by walking as well as swinging through forest trees.

“We also found that although orangutans rely on forest foods like fruit, leaves and insects, they can adapt their diet to include oil palm and acacia,” says Voigt.

“As long as they aren’t hunted, the research indicates that orangutans can possibly survive in fragmented landscapes as long as enough forest remains and is protected.”

She adds, though, that long-term proof is still lacking, and the findings shouldn’t be used to downplay the problems of degradation.

Conflict killing in these communities, as well as hunting, are major causes in the decrease in orangutan numbers

Erik Meijaard, Director of the Borneo Futures initiative

There are efforts to turn the down-ticking numbers around. The governments in both Malaysian and Indonesian Borneo plan to certify their palm oil as sustainable by 2525 – and this means instituting zero-killing policies.

Borneo has also long been a hub for orangutan preservation activities, focusing on rescue and rehabilitation – but the researchers say that this is not enough.

“It’s a drop in the ocean,” says Meijaard. “If you really want to save the species, you need a big picture strategy, and one that addresses the underlying causes.”

Meijaard says that these causes aren’t black-and-white, and over time have been pinned to everything from hunters and villagers to oil palm, climate change, logging and the illegal pet trade.

“If you want to save the orangutan you need a more transparent process – a plan with clear milestones and objectives,” says Meijaard. “It must involve stronger government commitment, community awareness, and private sector involvement.”

“We need to raise public awareness of the plight of the orangutan, and studies like this one can help do that,” says Gaveau.

For more information on this topic, please contact Maria Voigt at
This research was supported by the Max Planck Society and the Robert Bosch Foundation.
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Topic(s) :   Wildlife