Lost habitats slash Borneo orangutan population by 100,000 in 16 years

Julie Mollins
16 February 2018

BONN, Germany (Landscapes News) — On average, 17 orangutans disappeared on the Southeast Asian island of Borneo every day over a period of 16 years between 1999 and 2015, due to human encroachment on their environment.

The population of Bornean orangutans (Pongo pygmaeus) was reduced by 100,000 as a result of land cleared for logging, mining, paper pulp or oil palm plantations, according to a report in Current Biology released on Thursday. The total estimated loss of Bornean orangutans during the period amounted to 148,500.

Based on projected future loss of forests and the inability for orangutans to survive over the long term without them, researchers predict that more than 45,000 additional orangutans will disappear over the next 35 years.

The orangutans have also disappeared from intact forests, indicating that hunting and conflicts with humans have exacerbated the threat to their livelihoods.

“The decline in population density was most severe in areas that were deforested or transformed for industrial agriculture, as orangutans struggle to live outside forest areas,” said Maria Voigt a researcher at Germany’s Max Planck Institute for Evolutionary Anthropology. “Worryingly, however, the largest number of orangutans were lost from areas that remained forested during the study period. This implies a large role of killing.”

Orangutans, already listed as critically endangered by the World Wildlife Fund and the International Union for Conservation of Nature, will only survive as a species if they are protected through the development of protective partnerships with logging companies and other extractive industries operating in forests, the researchers said.


“Orangutans are flexible and can survive to some extent in a mosaic of forests, plantations and logged forest, but only when they are not killed,” said Serge Wich, a professor at Britain’s Liverpool John Moores University.

“In addition to protection of forests, we need to focus on addressing the underlying causes of orangutan killing,” he said. “The latter requires public awareness and education, more effective law enforcement, and also more studies as to why people kill orangutans in the first place.”

Borneo is shared by Indonesia, Malaysia and Brunei. In Indonesian Kalimantan, an average of 2,256 orangutans were hunted or killed by humans each year, according to the report.

David Gaveau, a landscape ecologist and coordinator of the Landscapes Laboratory at the Center for International Forestry Research (CIFOR), one of 38 international organizations involved in the study, said the research paper is the first to provide an estimate of orangutan individuals lost in recent times.

“We need stronger penalties for orangutan killings,” Gaveau said, referring to a disturbing news report that said two suspected orangutan killers face only a five year jail sentence under Indonesian law if convicted.

“In most cases sentences are not even applied,” he added.

By 2015, about half of orangutans estimated to live on Borneo in 1999 were found in areas where resource use has since caused substantial environmental changes.


To gather data on orangutans, researchers compiled field surveys undertaken between 1999 and 2015 to determine changes in the size of the population. They based their loss estimates on 36,555 orangutan nests found in Borneo.

Of 64 identified groups of orangutans referred to as meta-populations because they are spatially separated by rivers, roads or areas without forests, only 38 were found to include more than 100 individuals, the accepted lower limit considered viable subpopulations.

Researchers used maps developed through remote sensing technology of estimated land-cover change to understand the cause of population loss. Their findings, based on comparing orangutan and habitat losses, indicate that land clearance led to the biggest decline.

On the other hand, the research paper states that many more orangutans were lost in selectively logged and primary forests because, although the rates of decline were less dramatic in those areas, far more orangutans are found there.

The projected future decline in numbers of orangutans is mostly likely an underestimate, the researchers said, because it is based only on habitat loss and does not consider the potential losses that could result from hunting and killing for food.

“Many individuals currently occur in fragmented, small populations that are assumed not to be viable and will most likely disappear in the near future,” the paper states.

Although studies have suggested that orangutans can live in well-managed oil palm or paper pulp plantations that abut forested fragments of land, it is unclear if they can survive in the long term.

“Humanity must act now: biodiversity conservation needs to permeate into all political and societal sectors and must become a guiding principle in the public discourse and in political decision-making processes,” the researchers conclude.


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