(CNN) - The number of orangutans on the island of Borneo has more than halved in 16 years, as hunting of the critically endangered species continues and rampant deforestation destroys its habitat, scientists say.
An article published in Current Biology found the population had declined by more than 148,000 between 1999 and 2015, leaving an estimated 70,000 to 100,000 on the Southeast Asian island.
The study projected a further drop of 45,000 from deforestation alone by 2050 if nothing changes, in a grim forecast for the great ape's future.
Orangutans exist on only two islands -- Sumatra in Indonesia and Borneo, which is shared by Indonesia, Malaysia and Brunei.
Indonesia and Malaysia have for decades struggled to curb deforestation and illegal logging, losing swaths of tropical forest to palm oil and paper plantations, as well as mining, destroying crucial orangutan habitat.
The study found, however, that the steepest declines were not from land clearance, but in forests that were fully intact or only partly logged.
"We found that around 70 percent of the total loss occurs in primary forest areas or selectively logged forests, so the only explanation is that they are being hunted by people," said Serge Wich, a scientist in the team from the Liverpool John Moores University.
The researchers' findings were based on ground and aerial surveys of orangutan nests in 38 separate populations and satellite imagery.
Wich explained that hunters on Borneo who enter forests in search of pigs and other animals might kill an orangutan if they come across one.
Orangutans are also often forced to pass plantations to move between forested areas and are regularly killed by humans as they do. Some farmers consider them pests that threaten their crops while others shoot them out of fear.
Wich warned that with further declines forecast, orangutan populations would likely be confined to a few small protected national parks if action isn't taken.
"The first step is that the conservation community and the governments in Indonesia and Malaysia need to really recognize that hunting and killing is a major issue that is going to need a lot of attention very quickly," he said.
"We need to set up really smart awareness campaigns along with improved law enforcement to try to reduce this. And part of it is also working with palm oil and paper companies to ensure their management allows orangutans to move through plantations without being killed."
The International Union for Conservation of Nature lists the Bornean orangutan as critically endangered and found its population decreased by more than 60 percent between 1950 and 2010. It projects a 82 percent population drop between 1950 and 2025.
The Sumatran orangutan is also critically endangered, with only 13,846 individuals left, according to the IUCN. A third species, the critically endangered Tapanuli orangutan, was discovered last year in Sumatra with a population of less than 800.
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