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 six days across North Sumatra, triggering widespread landslides and flooding in the West Block of the Batang Toru ecosystem, the main habitat area of the Critically Endangered Tapanuli orangutan (Pongo tapanuliensis), estimated to contain 581 individuals (95%CI [180–1,201]) prior to the event (Wich et al. 2019) though more recent assessments suggest further declines (Wich et al. 2023). Based on interpretation of pre- and post-event Sentinel-2 and PlanetScope imagery, we quantified the extent of forest loss. Using a validated, unbiased area estimation, we determined that 8,303 ± 1,760 ha of previously intact forest vanished in landslides and floods. 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 approximately 58 individuals (range: 18–120, about 10% of the West Block population), have likely perished in landslides, tree falls, or flooding. Population viability models indicate that sustained additional annual mortality exceeding 1% (e.g., from hunting) drives extinction in suboptimal habitats; our preliminary estimate of an ~11% one-time mortality from this event, compounded by extensive habitat loss, thus represents a critical demographic shock equivalent to multiple years of such losses.We call upon the Indonesian government, the international community, and local land users 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 including restoration of critical lowland forest, with essential financial and technical support from the global community.