Extreme Rainfall Event in Sumatra Caused Critical Habitat Loss and Lethal Impacts to the Critically Endangered Tapanuli Orangutan

In late November 2025, over 1,000 mm of precipitation fell within four days across North Sumatra, triggering widespread landslides and flooding in the West Block of the Batang Toru ecosystem, the core habitat of the critically endangered Tapanuli orangutan (Pongo tapanuliensis), estimated to contain 581 individuals (95%CI [180–1,201]) prior to the event [1]. Using pre- and post-event Sentinel-2 and PlanetScope satellite imagery, we quantified the extent of forest loss. Initial analysis indicates that 3,964 hectares of previously intact forest were swept away by landslides and floods. An additional 2,487 ha of scars is extrapolated from areas that are cloud-covered in the Sentinel-2 image. These scar estimates were combined with a 1km² density surface from a 2016 survey to estimate the number of orangutans potentially affected. Our results indicate that 33-54 individuals may have been impacted, with a substantial proportion likely killed by landslides, treefall, or flooding. Given that annual mortality exceeding 1% is sufficient to drive this species to extinction, our preliminary estimate of 6.2-10.5% mortality suggests a critical demographic shock. We call upon the Indonesian government and the international community to take immediate action and provide support to ensure the survival of the Tapanuli orangutan. The government is urged to enact emergency protections, halt habitat-damaging development, and expand protected areas to restore critical lowland forest, with essential financial and technical support from the global community.