Hunting and killing have driven a dramatic decline in the orangutan population on Borneo where nearly 150,000 animals have been lost from the island’s forests in 16 years, conservationists warn.

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While the steepest percentage losses occurred in regions where the forest has been cut down to make way for palm oil and acacia plantations, more animals were killed by hunters who ventured into the forest, or by farm workers when the apes encroached on agricultural land, a study found.

Researchers estimate that the number of orangutans left on Borneo now stands at between 70,000 and 100,000, meaning the population more than halved over the study period which ran from 1999 to 2015. Without fresh efforts to protect the animals, the numbers could fall at least another 45,000 in the next 35 years, the conservationists predict. The real decline could be worse, because the prediction is based only on habitat loss, and does not include killings.

Facebook Twitter Pinterest The steepest percentage losses occurred in areas where the forest has been cut down to make way for palm oil and acacia plantations. Photograph: Marc Ancrenaz

The bleak assessment of the state of the Bornean apes comes from an international team of conservationists who compiled one of the most comprehensive reports yet on the animals, which in 2016 were declared “critically endangered” by the International Union for Conservation of Nature (IUCN).

Conservationists predict orangutan numbers could fall at least another 45,000 in the next 35 years. Photograph: Marc Ancrenaz

“I expected to see a fairly steep decline, but I did not anticipate it would be this large,” said Serge Wich, a co-author on the report at Liverpool John Moores University. “When we did the analyses, we ran them again and again to figure out if we had made a mistake somewhere. You think the numbers can’t be that high, but unfortunately they are.”

The researchers studied 16 years of ground and helicopter surveys that recorded the numbers and locations of nests that orangutans built in the trees from branches and leaves. The nests have long been used to infer the sizes of orangutan populations because the animals themselves are so elusive.

Writing in the journal Current Biology, the team describe how the decline in nests from 1999 to 2015 points to the staggering loss of 148,500 orangutans in Borneo. The conservationists identified 64 separate groups of orangutans on the island, but only 38 are thought to comprise more than 100 individuals, the minimum that is considered viable for a group.

The forests of Borneo are being fragmented by new plantations and building projects and the associated loss of trees led to falls in local orangutan populations of up to 75%, the study found. In the dense forests, orangutan numbers fell by 50%. While that is a lower rate, it amounts to more animals because most of the apes live in these areas. “This is the biggest chunk of the loss,” said Maria Voigt at the Max Planck Institute for Evolutionary Anthropology in Leipzig. “More than 70% of the orangutans lost are in the forest.”

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Hunters in Borneo tend to enter the forest to find pigs and deer, but if they encounter a large orangutan, they can take the animals for food. Female orangutans are occasionally killed for their young, which are sold on as pets. Far more of the apes die when they venture on to plantations, and into people’s gardens, where they are shot or killed with machetes. Last week, authorities on Borneo found the body of a male orangutan bearing machete scars and wounds from 130 airgun pellets.

“We need to work with people to help them understand that orangutans are not dangerous and that it’s illegal to kill them,” Wich said. One approach that might work, he said, is to have Indonesian and Malaysian role models raise awareness of orangutans through social media.

“We know this decline has been largely due to hunting, and if we can turn that around, these orangutans could, over a long period, bounce back. When you lost the habitat, it’s gone forever, but the forests are still there. If we can stop the hunting and killing, we can reverse the trend.”

Emma Keller at WWF said: “Orangutans are disappearing at an alarming rate. Their forests homes have been lost and degraded, and hunting threatens the existence of this magnificent great ape. Immediate action is needed to reform industries that have pushed orangutans to the brink of extinction. Consumers can make a difference through only supporting brands and retailers that buy sustainable palm oil.”