- Hoolock gibbon habitat has declined in the past few decades, but enough suitable patches exist today to guarantee the long-term survival of the genus if properly conserved.
- Particular populations are at greater risk of local extinction and should be translocated, including scattered western hoolock populations in Bangladesh.
- Researchers have also identified strongholds where a relatively high number of hoolock gibbons have been estimated, and which are currently highly threatened, to be prioritized for conservation.
- Hoolock gibbons are particularly vulnerable to forest fragmentation and degradation due to certain behavioral traits, which makes protecting large patches of habitat much more effective than conserving many small and fragmented areas.
Swinging lithely from branch to branch high up in the forest canopy, a family of hoolock gibbons hoots vigorously to guard its territory against intruders. A type of small ape endemic to South and Southeast Asia, hoolock gibbons spend most of their lives up in trees, rarely touching the forest floor. This makes large stretches of uninterrupted forest crucial to their survival — yet such suitable patches are getting scarcer, a new study has found.
Researchers, whose study was published July 16 in Global Ecology and Conservation, came up with estimates for the area of hoolock gibbon habitat that disappeared between 2000 and 2018, and the area remaining today, across the four countries of Myanmar, India, Bangladesh and China where the apes occur.
Analyzing all three species of hoolock gibbons — the Gaoligong hoolock (Hoolock tianxing) and the western hoolock (Hoolock hoolock), both endangered, and the eastern hoolock (Hoolock leuconedys), which is vulnerable — they found that enough suitable patches exist today to guarantee the long-term survival of each species.
Despite this rosy conclusion, the report also highlighted particular populations at greater risk of local extinction. Scattered populations of western hoolock gibbons living in degraded and fragmented forests in Bangladesh are most vulnerable and should be translocated to ensure their survival, the researchers said.
“Gibbons are not like other primates. Gibbons travel from tree to tree. They cannot walk on the ground,” first author Ngwe Lwin, a Ph.D. candidate at King Mongkut’s University of Technology Thonburi, Thailand, and acting country director for Myanmar at U.K.-based NGO Fauna & Flora International, told Mongabay. When their habitats are degraded by human activity, they “cannot just climb down and cross a road” to seek out a new patch of undisturbed forest, he said. They are trapped.
This makes hoolock gibbons particularly vulnerable to forest degradation and fragmentation. As humans continue carving roads through forests and clearing trees for timber and agricultural land, populations are set to become more isolated and restricted to island habitats in a sea of degradation — with consequences.
“When groups are isolated, genetic diversity also goes down. There is more inbreeding and they become less healthy. They get diseases more easily,” Lwin said.
Beyond inbreeding, more fights over food and territory will likely erupt. “Gibbons are territorial animals. If their area of forest gets degraded, they will try to move to an adjacent area. But if the forest is quite small, and the area is already occupied by another group, they will fight,” Lwin said. “Even if they [successfully] share the area, there will likely be a scarcity of food. One day, the group will just disappear.”
Based on these behavioral traits, protecting larger patches of forest is much more effective than conserving many small and fragmented areas, Lwin said. “The larger the area, the better it is for the gibbons’ long-term survival.”
To protect remaining hoolock gibbon populations, the study identified 27 “stronghold areas,” or large patches of suitable habitat at least 250 square kilometers (97 square miles) in size, for conservation. Today, these strongholds amount to 165,679 km2 (63,969 mi2) — an area slightly larger than Bangladesh — of forest across the four countries, with 22% currently highly threatened due to hunting and forest loss, 23.5% at a medium threat level, and 55% at a low threat level.
Between 2000 and 2018, 7,396 km2 (2,856 mi2) — an area five times the size of the Indian capital Delhi — of suitable patches were lost, the researchers wrote. Western hoolock habitat accounted for a disproportionately large 58% of this figure, due to expanding agricultural land in India and Myanmar. Faced with dual threats of hunting and habitat loss, populations have declined more than 90% in the past 40 years, with an estimated 3,000 western hoolock gibbons left in the wild.
Apart from a translocation program for scattered western hoolock populations in Bangladesh, the researchers proposed that conservationists focus their efforts on identified strongholds where a relatively high number of groups have been estimated, and which currently face high threat levels with limited legal protections. They also called for transboundary conservation programs to be initiated between the four countries.
“I hope our paper will be used for conservation planning for hoolock gibbons,” Lwin said. “These gibbons are indicator species. If they are there, the forest is good and healthy. And if we can protect the forest for gibbons, we can protect the other species too.”
Banner image of female hoolock gibbon by Programme HURO via Wikimedia Commons (CC-BY-3.0).
Lwin, N., Sukumal, N. & Savini, T. (2021). Modelling the conservation status of the threatened hoolock gibbon (genus Hoolock) over its range. Global Ecology and Conservation, 29. doi:10.1016/j.gecco.2021.e01726
Pachuau, S.V., Qureshi, Q., Habib, B., & Nijman, V. (2013). Habitat use and documentation of a historic decline of western hoolock gibbon (Hoolock hoolock) in Dampa Tiger Reserve, Mizoram, India. Primate Conservation, 2013(27), 85-90. doi:10.1896/052.027.0108
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