Only scientifically classified as a species in 2017, tapanulis are incredibly rare, with fewer than 800 left in the wild, confined to a small range in part of Indonesia's Sumatra.
One dead suspected tapanuli orangutan has already been found in the region, conservationists told AFP.
"The loss of even a single orangutan is a devastating blow to the survival of the species," said Panut Hadisiswoyo, founder and chairman of the Orangutan Information Centre in Indonesia.
And analysis of satellite imagery combined with knowledge of the tapanuli's range suggests that flooding last month which killed nearly 1,000 people may also have devastated wildlife in the Batang Toru region.
The scientists focused on the so-called West Block, the most densely populated of three known tapanuli habitats, and home to an estimated 581 tapanulis before the disaster.
There, "we think that between six and 11 percent of orangutans were likely killed," said Erik Meijaard, a longtime orangutan conservationist.
"Any kind of adult mortality that exceeds one percent, you're driving the species to extinction, irrespective of how big the population is at the start," he told AFP.
But tapanulis have such a small population and range to begin with that they are especially vulnerable, he added.
Satellite imagery shows massive 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 like elephants.
David Gaveau, a remote sensing expert and founder of conservation start-up 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 told AFP.
- 'Fragile and sensitive' -
The devastation means remaining tapanulis will be even more vulnerable, with sources of food and shelter now washed away.
Over nine percent of the West Block habitat may have been destroyed, the group of scientists estimated.
In a draft paper shared with AFP and set to be published as a pre-print in coming days, they warned the flooding represents an "extinction-level disturbance" for tapanulis.
Environmentalists have long campaigned against industrial activity in Batang Toru, particularly a hydroelectric dam and gold mine.
The highland homes currently inhabited by tapanulis are not their preferred habitat, but it is where remaining orangutans have been pushed by development elsewhere.
Earlier this month, Indonesia's government said industrial plantations, hydropower and gold mining in the region had "contributed significantly to the pressure on the environment".
They announced they would suspend operating permits for all projects in the region pending a review.
The government, along with environmentalists, has said deforestation contributed to the scale of the flooding disaster.
A study published Thursday also said climate change-linked heavier rains and warmer seas that can turbocharge storms played a role.
The orangutan experts are urging an immediate halt to development that will damage remaining tapanuli habitat, and an immediate survey of the region.
They also back the expansion of protected areas and work to restore lowland forests.
Panut 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 told AFP.
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