Borneo's orang-utan population has declined by more than half since 1999 due to palm oil production and logging, a study has found.

Between 70,000 and 100,000 of the endangered primates remain in the wild on the island, Current Biology journal found.

That means the number of orang-utans which have been killed or removed between 1999 and 2015 is much higher than previously thought.

However, the total population is also higher than past estimates, the study by the Max Planck Institute for Evolutionary Anthropology in Germany found.

The most comprehensive study of Bornean orang-utans since 1999 found the species are "highly likely" to become extinct if current trends continue.


The sharpest declines were found in areas where forests were cut down for timber or to make way for palm oil plantations.

Image: Swathes of forest on Borneo have been destroyed to produce palm oil

Palm oil is used in about 50% of supermarket products, from peanut butter to cleaning spray, and is the world's most popular vegetable oil.

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To maintain high yields producers continually clear the trees which grow in abundance in Borneo and Sumatra - the only places in the world where wild orang-utans live.

Other major factors to their species' decline include "conflict killing, poaching, and the collection of baby orang-utans for the pet trade," the authors said.

More individual orang-utans were hunted in forests than were lost when the primates were pushed out of their habitats.

"This was a really important finding, as it supports previous research that killing and hunting is a huge problem, and it might even be the biggest driver during this period," co-author Maria Voigt, of the Max Plank Institute for Evolutionary Anthropology, said.

Image: There are orangutan reserves on Borneo but people are still hunting them

Co-author Professor Serge Wich, of Liverpool John Moores University, said about 70% of the total number of orang-utans killed came from forested areas.

"Hunting is a widespread issue," he said.

"We had not realised that the issue was this large though."

Despite orang-utans being designated as critically endangered, many people in Borneo are still killing them for food.

Image: Orangutan's habitats are being lost to logging and palm oil production

Professor Wich said environmental awareness is increasing in Indonesia and Malaysia which he hopes will prevent further decline of the species.

"There are some promising collaborations between conservation and industry - be that oil palm industry or logging companies - where they try to maintain the orangutans in those areas.

"We need to be able to find a situation in which we protect orangutans in a matrix of different land uses."