World’s rarest great ape species ‘in more trouble than previously thought’

By Annette Gartland on January 7, 2021

Photo by Maxime Aliaga.

The world’s rarest and most threatened great ape species, the Tapanuli orangutan, is in greater danger of becoming extinct than conservationists feared, a group of researchers warn in a new report.

The researchers estimate that the Tapanuli orangutans retain only 2.5% of the range they occupied 130 years ago. “This new insight means that the species is in even more trouble than previously thought,” said the lead author, Dutch conservation scientist Erik Meijaard.

“Threats of unsustainable mortality and habitat fragmentation continue, suggesting further declines unless threats are stopped.”

The Tapanuli orangutans used to inhabit a much larger area and occurred across a much wider range of habitat types, and at lower elevations, than they do now, Meijaard et al. discovered.

Fragmentation of forest habitats, mostly for small-scale agriculture, along with unsustainable hunting, likely drove various populations living to the south, east, and west of the Tapanuli orangutan’s current range to extinction.

“Without concerted action, the remaining populations of Pongo tapanuliensis are doomed to become extinct within several orangutan generations,” Meijaard said.

The Tapanuli orangutan was only described in November 2017. Fewer than eight hundred of them remain and the International Union for Conservation of Nature (IUCN) has declared the species critically endangered.

The primate’s geographic range, in the mountainous Batang Toru forest on the Indonesian island of Sumatra, is only about 1,000 square kilometres.

In the new report, published in the journal Plos One, Meijaard et al. say their research indicates that the Tapanuli orangutan’s distribution range declined from about 40,796 km² in the 1890s to 1,023 km² in 2016.

The researchers calculated the previous distribution range by studying records in journal and newspaper archives. They also searched through natural history books, databases including the Biodiversity Heritage Library, and the online Leiden University Library colonial map repository.

Meijaard, who co-founded the conservation initiative and company Borneo Futures and is attached to universities in Britain and Australia, says some scientists have claimed that, because the Tapanuli orangutans currently live at an average altitude of 834 metres above sea level, the species is specifically adapted to living at high elevations.

However, the research conducted by Meijaard et al. indicates that the primates used to live primarily in lowland forest areas, but became extinct in those areas.

Serge Wich from the IUCN/SSC Primate Specialist Group’s Section on Great Apes (SGA) is one of the new paper’s co-authors. He said: “The Tapanuli orangutan only retains a tiny part of its former range, where it likely became extinct because of a combination of unsustainable hunting and habitat fragmentation, and both these threats still affect the remaining populations.”

The Tapanuli orangutans are not specifically adapted to highland conditions, Wich says. “They should occur in a full range of habitats such as peat swamp and lowland-dryland forests for optimal likelihood of survival in the wild,” he added.

Given how rapidly the Tapanuli orangutans have disappeared from their former range, and knowing that the main threats of unsustainable mortality rates and habitat fragmentation are still present, there is an urgent need to step up conservation efforts for the species, Meijaard et al. say.

“Survival of the Tapanuli orangutan means that we have to prevent any further losses to the species and its habitat,” Meijaard said.

“That means no killing, no harm, no translocations, and no forest destruction for mining, hydropower, roads, plantations, or smallholder agriculture, all of which fortunately is in line with Indonesia’s conservation laws. Effective implementation of these laws is now needed.”

The Tapanuli orangutans’ habitat consists of five blocks. There are two main blocks (east and west); an area in the Sibual-buali Nature Reserve, located next to the western block, where there is a third, smaller population; a block with an estimated 42 orangutans; and a “corridor” block where there are thought to be just six Tapanuli orangutans.

Wich, who is a professor in primate biology at Liverpool John Moores University in Britain, says that restoration of connectivity between the forest blocks is required as well as potentially increasing the Tapanuli orangutans’ habitat.

Onrizal Onrizal, a forestry researcher at North Sumatra University who is also a co-author of the new paper, says that knowing where a species occurred in historical times is important for understanding its ecology and informing conservation strategies. “Avoiding the historical angle can lead to incorrect assumptions,” he said.

“What is often overlooked in conservation is the deeper historical context, and that is especially important in a species-rich country like Indonesia.”

The discovery of the extent of historic forest conversion to agriculture in northern Sumatra surprised Onrizal and his fellow researchers.

“Using an Indonesian government map of forest cover in the 1950s showed that large parts of northern Sumatra had already been deforested for smallholder agriculture before the industrial-scale plantation developments that started in the 1970s,” Onrizal said.

“For example, 52% of north, south and central Tapanuli, the districts where the Tapanuli orangutan now occurs, were already deforested in the 1930s.”

Onrizal says there is still a lot to learn about the Tapanuli orangutans. “During this study we were not able to access any of the government or university reports written between the 1940s and 1990s as these are not yet electronically available, making it much harder to search them for relevant information.”

There are likely to be a great many additional orangutan records from students and government and company surveys that would help researchers better understand what has driven the decline in Tapanuli orangutan populations, Onrizal says.

“There is a lot we can still learn about this enigmatic species, but it is also clear that its extinction in the wild is imminent, unless we can prevent the drivers of decline that led to is historic demise.”

Threat from hydropower plant

Conservationists have expressed fears that a 510-megawatt hydropower plant that was being constructed in the Batang Toru forest would further imperil the Tapanuli orangutans, but there are reports that there will be a long-term delay in construction of the plant. (Full story.)

Construction for the project is currently halted because of concerns about the spread of SARS-CoV-2 and experts from the IUCN say this is the perfect time to follow its recommendation and conduct an independent, objective environmental assessment “and develop a conservation management plan for the Tapanuli orangutan”.

The Batang Toru ecosystem is home not just to the Tapanuli orangutans but also to many other rare and threatened animal and bird species. These include the Sumatran tiger (Panthera tigris sumatrae), the sun bear (Helarctos malayanus), the tapir (Tapirus indicus), and birds such as the Great Argus pheasant (Argusianus argus).

The ecosystem is also home to five other endangered and vulnerable primate species including siamangs (Symphalangus syndactylus) and agile gibbons (Hylobates agilis).

It is one of few areas in the world where three ape species coexist within the same geographical range.

Meijaard says that constructing roads through the Tapanuli orangutans’ habitat would bring in settlers and hunters.

Wich said: “If more than 1 percent of the population is lost annually this will spiral them to extinction.”

Three different orangutan species

Meijaard says he discovered the population of orangutans south of Lake Toba in 1997, but it took twenty years to compile the data that shows that the Tapanuli orangutan is a distinct species.

The scientists’ genome analyses revealed that the deepest split in the evolutionary history of extant orangutans – between the Batang Toru population and those living to the north of Lake Toba – occurred about three million years ago.

The Bornean and Sumatran orangutan species separated much later – about 674,000 years ago.

The Tapanuli orangutans have thicker, frizzier hair than their Bornean and Sumatran relatives, and, unlike the Bornean orangutans, the females grow beards.

The dominant males have a prominent moustache and flat flanges covered in downy hair, whereas the flanges of older males are more like those of Bornean males.

The Tapanuli orangutan is similar to the Sumatran orangutan (Pongo abelii) in its linear body build and has a more cinnamon-coloured pelage than that of its Bornean relative (Pongo pygmaeus).

Scientists say the Tapanuli orangutan is more closely related to the Bornean orangutan, whose home is on an island more than one thousand kilometres away, than it is to other orangutans in Sumatra, who live further north on that island.

The discovery of the Tapanuli orangutan, which is named after the area in which it lives, was the first time since 1929 that a new member of the great ape family had been identified.