The Bornean orangutan is a critically endangered great ape known for its arboreal lifestyle, distinctive orange-red hair and close genetic link to mankind (about 97 percent shared DNA). It's currently the world’s most populous orangutan species, but thanks to deforestation and hunting, the creatures are in trouble.

Between 1999 and 2015, the popular tree-swingers’ numbers dropped by nearly 150,000. Now, NPR’s Merrit Kennedy reports, the island of Borneo is home to just 70,000 to 100,000 orangutans. Of the 64 orangutan communities on Borneo, only 38 are made up of more than 100 animals, or the minimum number needed to ensure long-term viability.

These findings, which were published in Current Biology on Thursday, draw on field surveys conducted by an international team of conservationists. The researchers estimated orangutan numbers by tracking the animals’ nests and comparing these figures to historical data on land-use.

According to The Guardian’s Ian Sample, orangutan populations in areas affected by deforestation fell by up to 75 percent. In heavily forested areas, numbers fell by a rate closer to 50 percent.

Serge Wich, one of the study’s co-authors, tells Sample, “I expected to see a fairly steep decline, but I did not anticipate it would be this large. 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.”

Although the numbers seem to suggest that deforestation is the main driver of population loss, another study co-author, Maria Voigt of the Max Planck Institute for Evolutionary Anthropology, explains that forests are more densely populated, which means they actually experience the largest drop in total number of creatures.

“This is the biggest chunk of the loss,” she says. “More than 70 percent of the orangutans lost are in the forest.”

According to the researchers, orangutan killing is the main culprit behind this population decline. In Borneo, acts of violence against orangutans are becoming increasingly common–and concerning. National Geographic’s Sarah Gibbens writes that just this month, a male orangutan died after being shot with an air gun at least 130 times, and in January, a decapitated orangutan was found adrift in a river. Hunters also kill orangutans to use as food, adds Sample. Overall, however, the majority of killings stem from the apes’ perceived trespassing on locals’ plantations or gardens.

Although these killings occur on a relatively small scale, they have an enormous impact on the orangutan population–partly because the ape’s reproductive cycle lasts six to eight years. They produce only one baby during this period. Voigt notes that continued killings may “already be enough to wipe the population out in the long term.”

Based on current predictions of forest cover change, the researchers estimate that Borneo will lose another 45,300 orangutans over the next 35 years. Factoring in the effect of continued killings, this figure is likely an underestimate.

According to Wich, the population decline “has been largely due to hunting, [but] 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.”