Photo courtesy of the Borneo Orangutan Survival Foundation.

The number of orangutans on the Southeast Asian island of Borneo plummeted between 1999 and 2015, according to a new report.

The population decreased by more than 140,000 over the period, scientists have deduced, and the causes range from land clearance for industrialised plantations to hunting.

The most severe population declines occurred in areas in which habitat had been removed, the researchers concluded. However, most orangutans were lost from forests, which, the scientists say, suggests that hunting is a major cause of the decrease in numbers.

The researchers say habitat degradation and loss is happening in response to the local and global demand for natural resources, including timber and agricultural products.

In their report, which was published in Current Biology, the researchers point to what they call “the unsustainable use of natural resources”.

They say their modelling indicates that, between 1999 and 2015, half of the Bornean orangutan population was affected by logging, deforestation, or industrialised plantations. They estimate that there were 148,500 more orangutans in Borneo in 1999 than in 2015.

“Only 38 out of 64 remaining metapopulations have more than 100 individuals, the assumed threshold for viability of Bornean orangutan populations,” the new report states.

“Although land clearance caused the most dramatic rates of decline, it accounted for only a small proportion of the total loss.”

A much larger number of orangutans were lost in selectively logged and primary forests, where rates of decline were less precipitous, but where far more orangutans are found, the researchers say.

“This suggests that further drivers, independent of land-use change, contribute to orangutan loss. This finding is consistent with studies reporting hunting as a major cause in orangutan decline.”

The scientists say their findings are alarming. “To prevent further decline and continued local extinctions of orangutans, 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,” they said.

They predict that, if action is not taken to stop the decline, the total future loss for all Bornean orangutan metapopulations by 2050 will be at least 45,300.

“This projected future decline,” the researchers said, “is only based on the direct consequence of habitat loss.

“It does not consider the effects of orangutan killing for food and in conflict and is therefore most likely an underestimate.”

The scientists gathered 16 years of data, collected by researchers on the ground and in surveys in which helicopters were used to identify orangutan nests. They then combined this data with satellite images that indicated how the landscape had changed. They also looked at climate and human population density.

When they began their study, the researchers found a yearly average of 22.5 orangutan nests per every kilometre travelled, but, by 2015, they found only 10.1 nests when travelling the same distance.

“Practical solutions to prevent future orangutan decline can only be realised by addressing its complex causes in a holistic manner across political and societal sectors, such as in land-use planning, resource exploitation, infrastructure development, and education, and by increasing long-term sustainability,” the scientists said.

When the researchers compared regions in Borneo, they found that, in both Kalimantan and Sabah, rates of orangutan decline were highest in areas deforested or converted to plantations (a loss of between 63 and 75 percent).

The loss of orangutans in primary and selectively logged forests between 1999 and 2015 accounted for 67 percent of the total loss in Kalimantan, 72 percent in Sabah, and 83 percent of the total loss in Sarawak.

Oil palm plantation in the PT Bumi Sawit Sejahtera concession in Ketapang, West Kalimantan, in Indonesian Borneo. (December 3, 2015; © Ulet Ifansasti / Greenpeace.)

The team of researchers who produced the latest report were led by primatologist Maria Voigt from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and Serge Wich, from the International Union for Conservation of Nature/Species Survival Commission (IUCN/SSC) Primate Specialist Group’s section on great apes. Wich is also a professor of primate biology at Liverpool John Moores University in England.

The team included Marc Ancrenaz, who co-founded the French NGO HUTAN and the Borneo Futures initiative, and runs the Kinabatangan Orangutan Conservation Programme in Sabah, and the other co-founder of Borneo Futures, conservation scientist Erik Meijaard.

In an article in The Conversation, Voigt, Wich, and Meijaard point to the fact that orangutans are increasingly killed when their forest habitat is cut down and they are pushed into people’s gardens and into plantations. People who encounter the primates become scared or angry and resort to killing them, the scientists write.

“Orangutans are very slow breeders,” they added. “Previous research has indicated that a population will probably go extinct even if only one reproductive female per 100 adults is removed per year. But killing rates have been identified as being as much as three to four times higher than this, which would explain the immense losses seen within Borneo’s forests.”

There have been many horrific cases of orangutans being found injured or dead on oil palm plantations.

In one case in December 2014, an orangutan died from her injuries after being discovered on an oil palm plantation in Central Kalimantan, Indonesia, with more than 40 shotgun pellets in her body.

Female orangutans are also killed so that their babies can be captured and sold as pets.

In 2016, the International Union for Conservation of Nature (IUCN) changed the classification of the Bornean orangutan (Pongo pygmaeus) from endangered to critically endangered, citing the main causes of its population decline as habitat loss and fragmentation, primarily for logging and oil palm plantations, along with illegal hunting and fires.

There are estimated to be between 55,000 and 62,000 Bornean orangutans living in the wild, split into three distinct subspecies.

The Sumatran orangutan (Pongo abelii) is also listed as critically endangered. There are now only about 14,600 left in the wild.

The IUCN says the population trend is a decrease for both species.

A report published in July last year by the International Institute for Environmental Development (IIED) says the development of large-scale oil palm plantations is responsible for significant loss and degradation of orangutan habitat in Borneo.

The IIED report says that, as a result of habitat loss and hunting, it is expected that, by 2025, the orangutan population in Borneo will decline by 86 per cent as compared to its size in 1973.

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