Extreme rainfall further endangers the world's rarest great ape.

The critically endangered Tapanuli orangutan (Pongo tapanuliensis) was recognized as a species in 2017 and is the great ape species with the smallest wild population.1 Fewer than 800 individuals remain in three isolated populations in Batang Toru (West, East, and South Block) in Sumatra, Indonesia, all affected by habitat fragmentation and degradation.2,3,4 Studies indicate that sustained additional population losses exceeding 1% annually will result in extinction.5 In November 2025, an extreme, multi-day rainfall event triggered widespread landslides in the West Block of the Batang Toru ecosystem-the orangutan's largest remaining habitat area. Satellite imagery revealed 8,303 ± 1,760 ha of landslide scars across the West Block, accounting for an 11.7% loss of forest cover. Spatial analysis suggests that 11% of the population (∼58 individuals; range 18-120) resided within the landslide-affected areas. Landslide patterns indicate rapid and highly destructive events that left any orangutans caught in landslides with little chance of escape. The loss of these estimated 58 individuals represents a major shock to the viability of this Tapanuli orangutan population. Potential mortality caused by other effects, such as rainfall-induced canopy breakage and reduced food availability, has not been included and makes the estimate conservative. Given the species' slow reproduction and sensitivity to additional mortality, this single event constitutes a severe demographic shock with long-term implications for its viability. Our findings provide quantitative evidence that extreme rainfall events can directly threaten great ape survival, underscoring the urgent need for strengthened habitat protection and climate-responsive conservation planning.