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.

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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.

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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.