ASSAM — The capped langur (Trachypithecus pileatus) a leaf-eating primate listed as ‘Vulnerable’ by the IUCN, is rapidly disappearing from its habitats in Assam Upper Brahmaputra Valley according to a new study. Researchers attribute the decline primarily to severe forest fragmentation and food scarcity caused by decades of deforestation.
The study published in the Journal of Asia-Pacific Biodiversity, surveyed 40 tropical rainforest fragments and found the langurs in only 11 sites. Critically, the species has vanished from at least seven forest patches where they were previously recorded a decade ago.
Food Tree Diversity is Key to Survival
The research identified food tree density as the single strongest factor influencing the langurs’ presence, even more so than the size of the forest fragment.
As the only colobine (leaf-eating) primate in the region, the capped langur relies on a steady supply of young leaves, fruits, and seeds. In smaller, degraded fragments, the limited food source acts as a major ecological constraint on their survival.
A significant overlooked threat is the removal of saplings of food and roosting trees by local communities for firewood and non-timber forest products (NTFP). This prevents young trees from maturing, reducing both food availability and the essential canopy connectivity needed for the arboreal langurs.
In severely degraded forests some langur troops have been forced to forage, rest, and establish home ranges in human-dominated areas like tea estates and villages, increasing their vulnerability to predators and human-related disturbances.
Evidence of Decline and ‘Extinction Debt’
The capped langur, which is the second most commonly seen primate in Assam after the rhesus macaque, has seen a 30% decline in its global population over the past two decades.
The year-long survey found the largest troop (15 langurs) in the Hollongapar Gibbon Sanctuary, but noted that protected areas like the Borajan-Bherjan-Padumoni Wildlife Sanctuary, which previously hosted the species, reported no sightings.
In Borajan and Chala Reserve Forests, researchers observed small, non-viable groups, including a troop reduced to just three adult females and another with only two females. Without males, these groups face likely local extinction.
Experts warn of an “extinction debt,” where the consequences of historical habitat loss (dating back nearly a century due to agricultural expansion and tea gardens) are now becoming evident as generations of langurs struggle to survive in isolated pockets.
The Way Forward
Experts stress that the key to conservation lies in habitat restoration through collaboration between the Forest Department and NGOs. Recommended actions include:
• Protecting and Replanting native food and roosting trees (such as Gmelina arborea and Albizia lebbeck) to restore both food sources and canopy cover.
• Providing Alternatives to local communities, such as subsidized LPG or biogas, to reduce the pressure from cutting saplings for firewood.
• Improving the ‘Matrix,’ or the quality of surrounding areas like tea gardens and orchards, to help langurs survive, provided these areas do not become more resource-rich than the actual forest fragments.