Shrinking forest a threat to Sumatra's gibbons

Syofiardi Bachyul Jb, The Jakarta Post/Padang

A researcher has warned that the gibbon population of Kerinci Seblat National Park are on the brink of extinction due to illegal logging and land clearance.

Gibbons are arboreal apes indigenous to Southeast Asia and are highly endangered, with only 13 species known to exist in the wild. They are characterized by their acrobatic swinging through treetops, suspended by their hands, and are known for their territorial vocalization, or song, which are emitted from a throat sac that can inflate to the size of their heads.

The government must take affirmative action to prevent the protected animals from becoming extinct, said Achmad Yanuar, a doctorate student from the Wildlife Research Group of the University of Cambridge's anatomy department.

Yanuar has just completed a two-year research project on the destruction of the socio-ecological habitats of the large black gibbon of Sumatra, also known as the Sumatran siamang (Symphalangus syndactylus), and the lowland agile gibbon (Hylobates agilis unko) as part of his Ph.D. program.

During his research, Yanuar discovered that the virgin forest in the park presently covers about 1.4 million hectares (ha).

An estimated 90,000 Sumatran siamang and 150,000 lowland agile gibbon are thought to live within this territory, which were calculated by multiplying population density per square kilometer with the area of the national park.

The Sumatran siamang prefer living in highland altitudes of 800 meters to 1,200 m above sea level. Mountainous forests, typically classified as 1,500 m to 2,300 m above sea level, are more densely populated by the Sumatran siamang, as lowland agile gibbon are seldom found at such heights.

The lowland agile gibbon, on the other hand, live in areas of about 500 m to 800 m above sea level.

"In the lowland forests on the western part of the park, the population density of the Sumatran siamang is higher than the lowland agile gibbon, whereas on the eastern part, there are more lowland agile gibbon compared to the Sumatran siamang," Yanuar said.

At least 40 percent of the total park area has an altitude of less than 1,000 meters above sea level, and comprise the forest habitat of the Sumatran siamang and lowland agile gibbon.

"Of the 500 ha forested area on the edge of the park where I conducted my research, 60 percent has vanished due to illegal logging and land clearance. Only about 30 percent of the primary forest remains," Yanuar said.

The shrinking forest contributes to the plight of the lowland agile gibbon and Sumatran siamang populations, whose survival depends on their habitat. As gibbons live in treetops, the destruction of forest areas means they will be forced to travel on the ground, making them easy prey to predators and poachers.

"They probably roam along the ground as they are not able to live in the trees, and that's the moment when they are caught by people to be kept as pets or sold," Yanuar said.

In addition, gibbons are one of the few monogamous primate species and live in nuclear families consisting of a mated couple and their young offspring, marking their territory by a song unique to each family group. As the forest continues to be destroyed, families will be confined to smaller and smaller territories, while new couples will find it difficult to establish their own home territories, as any available tree tops would be occupied by other gibbon families.

Yanuar's research, funded by the Gibbon Foundation, shows that more than 40 locations within the national park -- including Bangko in Jambi province and South Solok in West Sumatra -- are facing serious problems with illegal logging and land clearance, particularly for oil palm plantations, which had become an industry trend.

To save the gibbons, their habitats must be conserved, he said, particularly as lowland forests of up to 800 m above sea level were most susceptible to destructive human activity -- the very areas that make up the gibbons' natural habitat.

"Therefore, such forests must be protected, especially those within the park," he said, underlining that the government needed to draw up an action plan to combat illegal logging and land clearance.