Climate change and biodiversity loss collide: landslides threaten world's rarest great ape

The study shows that extreme weather events may increasingly affect species whose populations are already vulnerable due to habitat loss and fragmentation. It also highlights how the global crises of climate change and biodiversity loss are increasingly intertwined “Extreme weather events and and biodiversity loss are often treated as separate crises, but in reality they are deeply connected,” says Professor Douglas Sheil of Wageningen University & Research, who contributed to the international study.

The Tapanuli orangutan is the world's rarest great ape. First recognised as a distinct species in 2017, it survives only in the Batang Toru ecosystem in North Sumatra, Indonesia. Fewer than 800 individuals remain, divided across three isolated forest blocks.

Scientists have long warned that even small additional losses could push the species towards extinction. The new study suggests that a single weather event may have removed around 7% of the global population.

In November 2025, Cyclone Senyar brought several days of exceptionally heavy rainfall to North Sumatra. The storm triggered widespread landslides throughout the West Block of the Batang Toru ecosystem, home to the largest remaining population of Tapanuli orangutans.

Using satellite imagery and spatial population models, researchers estimated that approximately 8,300 hectares of forest were affected by landslides, representing nearly 12% of the forest cover in the area. The authors estimate that around 58 orangutans were present in the affected areas when the disaster struck. 

“This level of loss is substantial for a species with such a small total population,” says Professor Erik Meijaard, lead author of the study and Chief Scientist at Borneo Futures. “When combined with ongoing pressures such as habitat degradation and human-wildlife conflict, it further increases the urgency of implementing and adequately resourcing a coordinated species action plan.”

While habitat loss and fragmentation remain major drivers of biodiversity decline worldwide, the study highlights an additional and growing risk: extreme weather events intensified by climate change.

The researchers examined the role of climate change in Cyclone Senyar and estimated that human-induced climate change increased the intensity of the rainfall associated with the storm by between 9 and 50 percent. This increase in rainfall intensity likely contributed to the severity of the landslides, demonstrating how climate change can amplify the impacts of extreme weather on already vulnerable ecosystems.

The findings demonstrate how climate change can amplify existing conservation challenges. Species with small and fragmented populations are particularly vulnerable because sudden losses can have disproportionate impacts on their long-term survival.

To assess the impact of the storm, the research team mapped more than 50,000 landslide scars using satellite imagery and overlaid these data with estimates of orangutan density.

The analysis revealed extensive damage across the West Block of Batang Toru. According to landslide experts involved in the study, the landslides occurred suddenly and moved at high speed, leaving wildlife caught in their path with little chance of escape. 

The estimated mortality figures are likely conservative. The estimates did not include potential additional deaths caused by forest damage, food shortages, injuries or other indirect consequences of the storm.

According to the researchers, the findings underline the urgent need to protect and restore the remaining forests of Batang Toru. Healthy and connected ecosystems are generally more resilient to disturbance and provide species with greater opportunities to survive and recover from extreme events.

The study also offers a broader warning. As climate change increases the likelihood and intensity of extreme rainfall in many regions, vulnerable wildlife populations may face growing risks that conservation strategies have only recently begun to consider. For the Tapanuli orangutan, one of the world's most endangered great apes, those risks are already a reality.

“We cannot prevent every extreme weather event,” says Sheil. “But we can reduce the vulnerability of species like the Tapanuli orangutan by protecting and restoring their habitat. In a changing climate, that becomes more important than ever.”