Kokrajhar canopy bridges help endangered Golden Langurs avoid fatal road crossings

Guwahati, Dec 30: Installation of artificial canopy bridges on at critical golden langur (Trachypithecus geei) crossing points along the State Highway-14 in Kokrajhar has significantly reduced road accident-induced fatalities of the endangered primate.

The bridge installation followed a study in the Chakrashila-Amguri-Buxamara-Nayekgaon forest complex which had recorded 18 langur-vehicle collisions in the Nayekgaon-Choibari section of SH-14, leading to seven deaths, five major injuries, and six minor injuries.

“To mitigate these risks, 15 artificial canopy bridges of four designs were installed along the SH-14. During the monitoring, 112 instances of road and canopy bridge crossings by eight golden langur groups were recorded. Langurs used canopy bridges significantly more than the road. Among canopy bridges, pipe and ladder bridges were most effective...,” a report on the canopy bridges published in the December issue of the Journal of Wildlife Science states.

Linear infrastructure like roads and power lines often fragment the forest habitats used by golden langurs in Assam. Artificial gaps in the forest canopy force these arboreal primates to descend to the ground, resulting in road-kill and other forms of anthropogenic mortality.

“Some power lines in the study area were insulated, providing additional pathways for their movement across the road. The initiative also integrates community outreach education to promote the conservation of golden langurs, providing incentives for restoring corridors through plantations, and maintaining them to mitigate conflicts.

These interventions can restore fragmented habitat and, thus, corridor connectivity, reduce mortality risk, and are expected to enhance their persistence in the fragmented landscape,” primatologist Dr Jihosuo Biswas, the lead author of the report, told The Assam Tribune.

The other team members were Joydeep Shil, Kanmaina Ray, Mehtab Uddin Ahmed, Dharma Kanta Ray, Amulya Boro, Puja Muchahary, Benjamin Dorsey, and Honnavalli N Kumara.

While all the bridges were placed at a height of nine to ten metres from the ground, 13 camera traps were deployed across all artificial canopy bridge (ACB) sites alternatively. All the camera traps were active simultaneously during the survey period without any gap.

“We identified 18 critical crossover points by the golden langurs on SH-14 on a 5.2 km stretch from Nayekgaon to Choibari. These crossover points were used by eight groups of golden langurs, of which seven were mixed groups, while one was an all-male band. Before installing the canopy bridges, golden langurs crossed the road by walking on the ground on 71 per cent of the occasions, while they altered their route to use the existing natural canopy on 29 per cent of the occasions,” Dr Biswas said.

An obligate canopy-dwelling primate endemic to the Indo-Bhutan border, the golden langur is found primarily in four districts of western Assam and six districts of south-central Bhutan, making it one of the most ‘range-restricted’ primate species in South Asia. Across its range, the species faces habitat loss and population decline, with over half of its natural habitat lost in recent decades.

“This decline is especially critical in the southern part of their range – particularly in Kokrajhar and Bongaigaon districts – where deforestation and the conversion of forests into agricultural fields and human settlements have fragmented once-continuous forests into smaller, isolated patches, jeopardizing the long-term viability of these populations,” the report observes.

Stressing that ACBs substantially improved golden langur crossing by reducing their collisions with vehicles on the SH-14 stretch between Nayekgaon to Choibari, the report says that ACBs are increasingly used as a practical mitigation tool to restore connectivity, reduce mortality, and support conservation.