The skull of a Tapanuli orangutan, caked in debris, stares out from a tomb of mud in North Sumatra, killed in catastrophic flooding that swept through Indonesia.

The late November floods have been an “extinction-level disturbance” for the world’s rarest great ape, scientists have said, causing catastrophic damage to its habitat and survival prospects.

Between 33 and 54 critically endangered Tapanuli orangutans (Pongo tapanuliensis) are estimated to have been killed in widespread floods and landslides after more than 1,000mm of rain fell over just four days in North Sumatra province. Fewer than 800 Tapanulis were left in the wild before the flooding, and the entire population dwells solely in this tract of forest already threatened by mining, palm oil plantations and a large hydropower project.

“It’s a total disaster,” said biological anthropologist Erik Meijaard, one of the first experts to describe the species. “The path to extinction is now a lot steeper.”

In a photo reviewed by the Guardian, a dead orangutan was hoisted out of a mud pit in the Central Tapanuli region.

A Tapanuli orangutan in the Batang Toru forests. Photograph: Nature Picture Library/Alamy

“One of the rescue team members who was in search of human casualties had shown me a body believed to be an orangutan that was found in the logs and mud,” said Panut Hadisiswoyo, founding director of the Orangutan Information Centre in Indonesia. “After seeing the photos, I am confident that the decayed body, reddish hair and the size of the skull was a Tapanuli orangutan.”

There are few modern precedents for such a sudden shock to a great ape population, Meijaard said, apart from the 2000s Ebola outbreaks that devastated western gorilla and chimpanzee populations in central Africa.

Biologists have said that even a loss of just 1% of the Tapanuli population each year would be sufficient to drive the Tapanulis to extinction, as the animals only reproduce every six to nine years.

Meijaard’s preliminary findings on the toll of the Sumatra floods, to be published this week, estimate a loss of between 6.2% and 10.5% of the Tapanuli orangutan population in just a few days – a critical demographic shock. The draft paper warns that it represents an “extinction-level disturbance” for the species.

Assessing the affected zone from satellite imagery, Meijaard and his colleagues found that nearly 4,000 hectares (9,900 acres) of previously intact forest were swept away by landslides and floods, and estimating another 2,500 hectares (6,200 acres) was probably affected though unobserved due to cloud cover.

Satellite imagery shows huge gashes in the mountainous landscape, some of which extend for more than a kilometre and are nearly 100 metres wide, Meijaard said. The tide of mud, trees and water toppling down hillsides would have carried away everything in its path, including other wildlife such as elephants.

An aerial photograph shows the extent of the devastation caused by the flooding in Indonesia. Photograph: AFP/Getty Images

David Gaveau, a remote-sensing expert and founder of the conservation startup the Tree Map, said he was flabbergasted by the before and after comparison of the region.

“I have never seen anything like this before during my 20 years of monitoring deforestation in Indonesia with satellites,” he said.

The devastation means remaining Tapanulis will be even more vulnerable, with sources of food and shelter now washed away.

A Tapanuli orangutan skeleton found in mud in North Sumatra – between 33 and 54 of the critically endangered apes are thought to have perished. Photograph: Decky Chandra

“The rainfall was intense. If you lose your fruit, you lose your flowers, there will be a significant reduction in habitat quality,” Meijaard said.

Scientists and environmental advocates have fought for years to protect the endangered apes from mining, hydropower projects and palm oil plantations. The Guardian recently reported that in the weeks before the flooding, PT Agincourt’s Martabe Gold Mine had begun operations to expand into a second open pit near Tapanuli orangutan habitat.

While human-caused climate breakdown is estimated to have increased rainfall intensity by between 28% and 160% in the affected region, according to a rapid analysis by World Weather Attribution, deforestation and associated land degradation has played a significant role, the government said.

In response to the floods, Indonesia’s environment ministry has halted all private-sector activity in the Batang Toru area for an undetermined period.

The orangutan experts urged an immediate halt to development that will damage the remaining Tapanuli habitat, and an immediate survey of the region.

They also back the expansion of protected areas and work to restore lowland forests.

Hadisiswoyo said the region had become eerily quiet after the landslides.

“This fragile and sensitive habitat in West Block must be fully protected by halting all habitat-damaging development,” he said.

AFP contributed to this report

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