Flash floods that struck the Batang Toru forest in North Sumatra in late November 2025 have dealt a severe blow not only to local communities, but also to the critically endangered Tapanuli orangutan.
A study published on December 15 estimates that between 33 and 54 orangutans were killed by the floods and landslides representing up to 10.5 percent of the population in the West Batang Toru Block, the species’ core habitat. With fewer than 800 individuals left worldwide, researchers warn the loss could push the species closer to extinction.
“This is a demographic shock of extinction-level significance,” said Erik Meijaard of Borneo Futures, who led the study. “For a species this small, such losses are critical.”
The disaster followed days of extreme rainfall linked to Tropical Cyclone Senyar, which affected northern Sumatra between November 23 and 28. In Batang Toru alone, rainfall reached 564 millimeters in less than a week, including two days exceeding 200 millimeters, an event researchers described as exceptionally rare.
The heavy rains triggered widespread landslides on Batang Toru’s steep slopes, worsened by long-term deforestation. Satellite data showed nearly 4,000 hectares of forest were destroyed, with further damage likely hidden by cloud cover.
Researchers say the damage is particularly alarming because it hit lowland river valleys, areas with the highest orangutan density. Displaced animals have been forced into higher elevations or agricultural land, increasing the risk of conflict with humans.
At least one orangutan carcass has been found following the floods, and researchers believe many more may have died unseen beneath landslides.
Tapanuli orangutans reproduce slowly, with birth intervals of six to nine years. Scientists warn that even small annual population losses can drive the species toward extinction.
The research team is urging the Indonesian government to impose a permanent moratorium on habitat-destroying activities, expand protected areas, and designate the Batang Toru ecosystem as a National Strategic Area to strengthen legal protections.
The Tapanuli orangutan was only recognized as a distinct species in 2017 and is already the world’s rarest great ape. Its survival is further threatened by climate change. A separate study found that human-induced warming has increased extreme rainfall in the region by up to 50 percent, with projections pointing to more frequent severe events.
“Without immediate intervention,” Meijaard warned, “the Tapanuli orangutan could become the first great ape species to go extinct in the modern era.”