Around half of all orangutans living on the Southeast Asian island of Borneo — nearly 150,000 in all — vanished during a recent 16-year period. The causes included logging, land clearance for agriculture and mining that destroyed their habitats, according to a study in Current Biology released today.

However, many orangutans also disappeared from more intact, forested areas, the researchers say, suggesting that hunting and other direct conflicts between orangutans and humans remain a major threat to the species.

“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 a lead researcher for the study, Maria Voigt of the Max Planck Institute for Evolutionary Anthropology in Germany.

“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.”

In February, Indonesian police arrested two rubber plantation workers in Borneo, accusing them of shooting an orangutan multiple times, decapitating it and then throwing its body into a river. The men claimed they were acting in self-defense, according to local media reports.

Orangutans, which inhabit both the Indonesian and Malaysian sides of Borneo, as well as the Indonesian region of Sumatra, are an endangered species, with some populations critically endangered.

Their prime habitat in Borneo is lowland rain forests, which have been ravaged for decades by illegal logging operations and slash-and-burn land clearing for palm oil plantations and other agricultural activities.

To estimate changes in the size of Borneo’s orangutan population over time, researchers representing 38 international institutions compiled field surveys conducted between 1999 and 2015. They extrapolated the overall size of the island’s population from the number of orangutan nests found throughout the species’ range in Borneo.

The team observed 36,555 nests and estimated a loss of 148,500 orangutans during that period. The data also suggested that only 38 of the 64 identified groups of orangutans now include more than 100 individuals, which the researchers say is the lower limit to be considered a viable grouping.

That would leave the surviving number at around 148,000, according to the report. However, the World Wildlife Fund estimates that the remaining population of Borneo orangutans is much smaller, at around 105,000.

To identify the likely causes of orangutan population losses, the researchers relied on maps of estimated land cover change on Borneo during the 16-year period.

The comparison of orangutan and habitat losses suggested that land clearance caused the most dramatic rates of decline. However, a much larger number of orangutans were lost in areas where there was less logging and deforestation. While the rates of decline were less in those areas, the additional forest cover means far more orangutans are found in them, the researchers said.

By 2015, they reported, about half of the orangutans estimated to live in Borneo in 1999 were found in areas in which human resource use has caused significant changes to the surrounding environment. Based on predicted future losses of forest cover and the assumption that orangutans ultimately cannot survive outside forest areas, the researchers predicted that more than 45,000 additional orangutans will die during the next 35 years.

“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,” said Serge Wich, a professor at Liverpool John Moores University in Britain and a member of the research team.

“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,” he said.

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