Orangutan numbers on the Southeast Asian island of Borneo plummeted from 1999 to 2015, more as a result of human hunting than habitat loss, an international research team finds.
Over those 16 years, Borneo’s orangutan population declined by about 148,500 individuals. A majority of those losses occurred in the intact or selectively logged forests where most orangutans live, primatologist Maria Voigt of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and colleagues report February 15 in Current Biology.
“Orangutan killing is likely the number one threat to orangutans,” says study coauthor Serge Wich, a biologist and ecologist at Liverpool John Moores University in England. Humans hunt the forest-dwelling apes for food, or to prevent them from raiding crops, the investigators say. People also kill adult orangutans to steal their babies for the international pet trade.
Between 70,000 and roughly 100,000 orangutans currently live on Borneo, Wich says. That’s substantially higher than previous population estimates. The new figures are based on the most extensive survey to date, using ground and air monitoring of orangutans’ tree nests. Orangutans live only on Borneo and the island of Sumatra and are endangered in both places.
Still, smaller orangutan populations in deforested areas of Borneo — due to logging or conversion to farm land — experienced the severest rates of decline, up to a 75 percent drop in one region.
Satellite data indicate that Borneo’s forest area has already declined by about 30 percent from 1973 to 2010. In the next 35 years, Voigt’s team calculates that further habitat destruction alone will lead to the loss of around 45,000 more of these apes. “Add hunting to that and it’s a lethal mix,” Wich says. But small groups of Bornean orangutans living in protected zones and selectively logged areas will likely avoid extinction, the researchers say.
In Borneo, areas of high orangutan densities per square kilometer declined from 1999 to 2015.