A new study estimates that the orangutan population dropped by nearly 150,000 on Indonesia's Borneo island between 1999 and 2015.
Estimates now place the number of orangutans at 50,000-100,000.
Logging and other natural resource extraction are responsible for killing many of the great apes by destroying their habitat, according to the report, which is published in the scientific journal Current Biology.
"The decline in population density was most severe in areas that were deforested or transformed for industrial agriculture, as orangutans struggle to live outside forest areas," said Maria Voigt of Germany's Max Planck Institute for Evolutionary Anthropology, which was one of 38 institutions working on the study.
"Worryingly, however, the largest number of orangutans were lost from areas that remained forested during the study period. This implies a large role of killing," Voigt said in a press release accompanying the study.
The long-term study compared field surveys from 1999 to 2015 to come to its estimates.
The researchers estimate that over 45,000 more orangutans will be lost on the island over the next 35 years due to a loss of habitat.
Classified as a "critically endangered" species, orangutans number around 111,000 in the wild on the islands of Sumatra and Borneo, according to the World Wildlife Fund conservation group.
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