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.