Humans are decimating Borneo’s orangutan population, new research shows

Around half of Borneo’s orangutan population was lost during a 16 year period from 1995 to 2015, according to research published today in Current Biology.

Many of the losses are associated with deforestation caused by logging, mining, and the demand for oil palm and paper, the researchers claim. However, more alarmingly, many orangutans have also disappeared from more intact forested areas because of “conflicts between orangutans and people”.

In order to establish how much the orangutan population changed over time, Maria Voigt of the Max Planck Institute for Evolutionary Anthropology, and Serge Wich, from Liverpool John Moores University in the UK – along with a team from more than 38 institutions – compiled orangutan field surveys carried out over a period of 16 years. They analysed this data to establish the numbers of orangutan nests seen throughout the island.

Based on this information, the researchers estimated a loss of 148,500 Bornean orangutans. Moreover, only 38 of the 64 distinct ‘metapopulations’ of orangutans were estimated to comprise more than 100 individuals, which is the accepted lower limit to avoid extinction.  

To establish the likely causes of the alarming decline, the researchers used estimated maps of land-cover change during the same period. After comparing these with the estimated orangutan population loss, the team established that land clearance caused the fastest rates of decline.

Image credit: Marc Ancrenaz

However, this wasn’t the only cause for the falling orangutan numbers. “Worryingly…the largest number of orangutans were lost from areas that remained forested during the study period,” explains Voigt. “This implies a large role of killing.”

When I asked Wich if there could be an explanation for the deaths other than killing, he told me the team considered a reduction in food availability and disease but found no evidence to support those hypotheses.

“Killing is mainly for food and during human-orangutan conflict, where people are afraid of orangutans,” he explains.

Based on anticipated losses of forest cover, the researchers predict that over 45,000 more orangutans will be lost over the next 35 years.

Working with logging companies and other industries to conserve the orangutan species, along with public education about the animal and its habitat. is now essential to the Bornean orangutan’s survival.

“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,” Wich says. “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.”