Onrizal, S.Hut, M.Si., Ph.D., a Faculty of Forestry, Universitas Sumatera Utara lecturer, was born and raised in Tapanuli, North Sumatra. The Tapanuli region is home to a particular species of orangutan, the Pongo tapanuliensis, which Onrizal still remembers was mentioned in local folklore as the ‘short man’ of the forest, who disappeared from the forest in the 1970s. The story remains relevant to the current reality about the orangutans in Tapanuli: these creatures have long vanished from the region’s forest.
Working together with Erik Meijaard, an assistant professor on the conversation from the University of Kent, UK, Onrizal published their study titled “The historical range and drivers of decline of the Tapanuli orangutan,” published in a Q1 Scopus-indexed journal PLOS ONE. The team scoured historical records for references to the Tapanuli orangutan (Pongo tapanuliensis). This species is the newest orangutan and is listed as the most endangered great ape in the world. The Tapanuli orangutan faces a much greater risk of extinction than previously thought.
Today, the Tapanuli orangutan is estimated to occupy only 2.5% of their historical habitat, and the threat of extinction is linked to habitat loss and hunting. This threat persists today and is exacerbated by infrastructure development and forest conversion in the last habitat of the Tapanuli orangutan in North Sumatra. According to the researchers, with habitat shrinking and hunting, the extinction of the Tapanuli orangutan is inevitable.
As stated in the study, less than 800 individual Tapanuli orangutans live in the Batang Toru forest, North Sumatra. The remaining habitat is estimated to cover only 2.5% of the range in which they lived about 130 years ago when researchers discovered them. That number shrank from nearly 41,000 km2 in the 1890s, to just 1,000 km2 in 2016.
To arrive at these figures, the researchers referred to colonial-era literature, such as newspapers, journals, books, and museum records, from the early 1800s to 2019 by using location-specific keywords such as “Sumatra”, “Batang Toeroe,” and “Tapanoeli,” with Dutch spelling then cross-referenced them with terms that specifically refer to orangutans used commonly in historical literature.
The researchers found that the Tapanuli orangutan historically inhabited a much wider area at a lower altitude than the Batang Toru mountain forest they occupy today. Much of this historical habitat was lost in the 1950s to smallholder agriculture before industrial-scale plantation development in Sumatra emerged in the 1970s. The combination of historical fragmentation of forest habitat and unsustainable hunting is likely to push them from the lowland forest areas, where they used to live, to the upland forests of the Batang Toru ecosystem.
The researchers think that it is likely that Pongo tapanuliensis was hunted to extinction in an increasingly fragmented part of its former home range. They survive in the remote and rocky Batang Toru Mountains which may have protected orangutans from hunting. These findings show that the Tapanuli orangutan is not a species specifically adapted to live in the highlands as some scientists claim.
Map of Sumatra Island showing the current distribution of Pongo tapanuliensis and Pongo abelii, as well as the main areas mentioned in the study. This finding raises concerns over the survival of the Tapanuli orangutan. It is estimated that fewer than 800 of these orangutans live in the Batang Toru forest, divided among three connected subpopulations.
The Tapanuli orangutan species face conflicting threats of hunting and killing, as well as habitat loss due to agriculture and plantations. New threats have also emerged, namely infrastructure development and the construction of hydropower roads, which have caused fragmentation of the remaining habitat of the Tapanuli orangutan.
Conservation experts say the most severe threat currently comes from the Batang Toru hydroelectric power plant. The construction of this hydropower plant is considered to endanger the connectivity between orangutan subpopulations in the west, east, and south. This fragmentation would cut the diversity of the orangutan gene pool dramatically, leading to inbreeding, disease, and, eventually, each subpopulation to extinction.
The researchers calculated that more than one percent of adult orangutans disappear from the wild per year, either being killed, translocated, or captured. With that data, extinction is inevitable, regardless of the initial population size.
Without further rescue efforts, the researchers estimated that this species could become extinct in the next few generations. The ongoing fragmentation of the Batang Toru forest only exacerbates this risk. Onrizal called on all stakeholders, including governments, civil society organizations, scientists, donors, local community representatives, and industry, to develop concrete action plans as soon as possible for the survival of this species.
