The critically endangered Tapanuli orangutans found on the Indonesian island of Sumatra are a step closer to extinction, scientists found, after landslides last year that were fueled by climate change.
More than 50 of the rare animals were estimated to have died in the landslides, out of a population of around 800.
The findings, published in the journal Current Biology on Wednesday, add a heavy ecological toll to a storm that caused mass human devastation, killing more than 1,000 people and displacing hundreds of thousands.
The catastrophic flooding and landslides resulted from Cyclone Senyar’s heavy rains in November. While precise estimates are difficult because of limited data, researchers had previously found that human-induced climate change caused an increase of 10 percent to 50 percent in the storm’s regional rainfall intensity.
“This will continue to get worse and worse as we continue to burn fossil fuels,” said Friederike Otto, an author on both studies and a professor of climate science at Imperial College London.
The analysis offers the latest example of how climate change is pummeling wildlife populations already depleted by decades or centuries of habitat loss and hunting. It underscores the urgent need for conservation action, the researchers emphasized.
While some amount of species extinction is natural, humans have sharply increased the rate, leading scientists to warn of a mounting biodiversity crisis that could threaten the ecosystems that provide water, air and food.
All three species of orangutan are critically endangered. The Tapanuli orangutan, which was recognized as a species in 2017, is the rarest. Fewer than 800 individuals live in isolated populations in or near the tropical rainforest of Batang Toru on Sumatra.
They have long faced threats from agricultural expansion, hunting, conflicts with humans and industrial activities including a hydroelectric project and gold mine.
After the storm, scientists turned to satellite images to try to understand the scale of the damage. In an area of key Tapanuli habitat, they saw strange patches that looked like flooding along steep slopes.
“Surely, that’s a data mistake,” Erik Meijaard, the study’s lead author, recalled thinking at first. When new images came in, he and his colleagues realized that they were looking at swaths of newly bare earth. “Entire hill slopes just came down,” said Dr. Meijaard, who is managing director at Borneo Futures, a consulting firm based in Brunei that focuses on conservation outcomes.
Analyzing the imagery, the team found that landslide scars covered roughly 20,500 acres in an area known as the West Block, where the largest of the Tapanuli populations lives. That amounts to an approximately 12 percent loss of forest cover.
While degraded areas are more vulnerable to erosion and landslides, the study focused on an area of mostly pristine rainforest, so the damage underscored the intensity of the deluge.
To estimate the number of orangutans affected, the researchers applied species density data, concluding that around 58 of the animals had been in swaths of forest that were wiped away. Landslide patterns indicate that the collapse was rapid and extremely destructive, offering little chance to escape. While it’s possible that some fled, the orangutans probably died by burial, trauma or drowning, the scientists said. Because they reproduce slowly, with only one birth every six to nine years, any losses are significant.
Access to some areas is still difficult, but the team is beginning surveys on the ground. Their findings are coming to light as local residents continue to face urgent needs.
“People are generally traumatized,” said Panut Hadisiswoyo, one of the study’s authors and the founder of the Orangutan Information Centre, a group based in northern Sumatra.
The intensity of the storm’s devastation is unlike anything the oldest people in the area have ever experienced or even heard about from their grandparents or great-grandparents, he said.
Joshua Roering, a geomorphologist at the University of Oregon who specializes in landslides and was not involved in the study, called it compelling.
“The extent of landsliding in the study area is truly profound,” he said. As climate change moves more water through the atmosphere and dumps huge amounts on landscapes, he added, scientists are learning about how rapidly the soil becomes destabilized.
Expanding and reconnecting habitat is essential to helping Tapanuli orangutans become more resilient to future climate-fueled storms, the authors said. If the species occupied more habitat, its risk from any particular weather disaster would diminish. More resources should be devoted to reducing conflict between people and orangutans, which sometimes raid fruit crops, they said.
“We can avoid extinction,” Dr. Meijaard said. “But whether it will realistically get done is a different question.”
