More than 100,000 orangutans on the Asian island of Borneo have already died from human interference in the 21st century, according to new estimates.

In a study for the journal Current Biology, researchers point to the extraction of natural resources on the island, such as logging and agriculture, as main culprits behind the orangutan losses, which cut deeply into the animal’s population there between 1999 and 2015.

Hunting played a role as well, according to the scientists, based upon the fact that the orangutans are also missing from places that are still forested. Those places were where the apes took the biggest hit in terms of volume.

“This implies a large role of killing,” researcher Maria Voigt said in a statement from journal publisher Cell Press.

However, the most extreme effect on the population occurred in locations that experienced habitat loss because of the way it dramatically reduced their density.

The researchers used data from field surveys and remote sensing to come up with their estimates and compare them to the distribution of human activity.

That data provide more information about the struggle of the Bornean orangutan, a critically endangered primate whose numbers are still decreasing.

Losing more than 100,000 total members on Borneo has also affected the orangutans on a more local level. A large chunk of their 64 population areas have fewer than 100 primates, “the assumed threshold for viability of Bornean orangutan populations,” the study says.

It calls out the “unsustainable exploitation of natural resources” as a primary driver of the decline.

“In essence, natural resources are being exploited at unsustainably high rates across tropical ecosystems, including Borneo,” according to the paper. “As a consequence, more than 100,000 Bornean orangutans vanished between 1999 and 2015. The major causes are habitat degradation and loss in response to local to global demand for natural resources, including timber and agricultural products, but very likely also direct killing.”

The researchers say the data are “alarming” and call for strong conservation efforts to save the endangered animal.

According to Cell Press, the declining population trend suggests that humans can expect to lose another 45,000 orangutans from Borneo over the next few decades if nothing changes.

“Orangutans are flexible and can survive to some extent in a mosaic of forests, plantations and logged forest, but only when they are not killed,” researcher Serge Wich said in the statement. “So, in addition to protection of forests, we need to focus on addressing the underlying causes of orangutan killing. The latter requires public awareness and education, more effective law enforcement, and also more studies as to why people kill orangutans in the first place.”