Rapid decline of Tapanuli orangutan spells trouble for its future

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The Tapanuli orangutan, one of the three surviving orangutan species, is the rarest great ape species on Earth. Maxime Aliaga/IUCN

A recently published study estimates that the Tapanuli orangutan only retains 2.5 per cent of the range it occupied 130 years ago. This new insight means that the species is in even more trouble than previously thought.

The Tapanuli orangutan, one of the three surviving orangutan species, is the rarest great ape species on Earth. Fewer than 800 individuals remain across three tentatively connected subpopulations in a small mountainous region on the island of Sumatra, Indonesia. To put that in perspective, that is far less than the circa 1,000 surviving mountain gorillas or the more than 2,000 giant pandas currently living in the wild.

The new study titled “The historical range and drivers of decline of the Tapanuli Orangutan”, which we published in the journal PLOS ONE, indicates that the distribution range of the Tapanuli orangutan declined from circa 40,796sq km in the 1890s to 1,023sq km in 2016. It means that the remaining habitat is currently only around 2.5 per cent of the habitat 130 years ago.

To estimate the historic range, we searched historical journal and newspaper archives and found 23 previously unpublished orangutan records, all outside the currently known distribution range. These historic records may have been overlooked because much of the historic literature about Indonesian wildlife is written in Dutch or German, which are languages not commonly used by conservation scientists working in Indonesia today. Important information for species conservation is thus often overlooked.

Although historic information is not often used in conservation biology, there is real value in studying such data. Much of what we know about species trends is what ecologists have measured over the past few decades. This is how we know, for example, about severe declines in European insect populations studied since the 1970s, or the decline and subsequent recovery of giant pandas in China. But what is often overlooked in conservation is the deeper historical context, and that is especially important in a species-rich country like Indonesia.

Knowing where a species occurred in historical times is important for understanding its ecology and thus informs conservation strategies. Avoiding the historical angle can lead to incorrect assumptions.

Some scientists, for example, have claimed that the Tapanuli orangutan is a species that is specifically adapted to living at high elevations, because it currently occurs at an average altitude of 834m above sea level. Our new study, however, indicates that the Tapanuli orangutan used to primarily occur in lowland forest areas, but that a combination of unsustainable hunting and forest fragmentation drove the species to extinction in historic times.

Our study also revealed the significant extent of historic forest conversion to agriculture. 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. For example, 52 per cent of North, South and Central Tapanuli – the districts where the Tapanuli orangutan now occurs – were already deforested in the 1930s.

The new insights from this historical analysis are important for developing improved conservation strategies. First, 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.

Second, the Tapanuli orangutan is not specifically adapted to highland conditions, and should occur in a full range of habitats such as peat-swamp and lowland-dryland forests for optimal likelihood of survival in the wild.

Given how rapidly the species has disappeared from its former range (a 97.5 per cent reduction in 130 years) 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.

Survival of the Tapanuli orangutan means that we have to prevent any further losses to the species and its habitat. 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. In addition, restoration of connectivity between the forest blocks where the species now occurs is required as well as potentially increasing its habitat, especially in lowland areas.

The study also points to a possible connection between the orangutan and the mythical orang pendek. One of us, Onrizal, remembers growing up in his home town of Sungai Dareh, east of Padang, where he often listened to stories of human-like creatures living in the forest. Elders told him that these orang pendek ceased to exist in the area in the 1970s. Now that we know that orangutan historically occurred in that area, we wonder whether the orang pendek stories referred to the dwindling orangutan populations of the region.

Stories of orang pendek abound in other parts of Sumatra and it may well be that all of these referred to historic orangutans. This would mean an even greater decline in range and an even greater urgency to step up conservation efforts.

There is still a lot to learn about the Tapanuli orangutan. Our study did not 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 a great many additional orangutan records from students, government and company surveys in these Indonesian reports that would help better understand the drivers of decline in the Tapanuli orangutan.

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 effectively counteract the threats that led to is historic demise.

Erik Meijaard is a professor at Borneo Futures, Brunei Darussalam. Onrizal is an associate professor at Department of Forestry, North Sumatra University (USU).