GUWAHATI Primatologists have suggested rerouting a 1.65-km-long railway track that has divided an eastern Assam sanctuary dedicated to the western hoolock gibbon (Hoolock hoolock) into two unequal parts.
Their report in Science, a journal, follows that of the Wildlife Institute of India (WII) on designing an artificial canopy bridge to facilitate the movement of the hoolock gibbons across the broad-gauge line within the Hollongapar Gibbon Sanctuary. The track is yet to be electrified.
The authors of the study are Rohit Ravindra Samita Jha and Gopi Govindan Veeraswami of the Dehradun-based WII, Dilip Chetry of Assam-based biodiversity conservation group Aaranyak, and Nandha Kumar of Assam’s Department of Environment and Forests.
Housing about 125 hoolock gibbons (India’s only ape) organised in more than two dozen groups, the sanctuary in the Jorhat district covers 21 sq. km. It also shelters six other primate species — the Assamese macaque, the Bengal slow loris, the capped langur, the northern pig-tailed macaque, the rhesus macaque, and the stump-tailed macaque.
The western hoolock gibbon inhabits the jungles with tall trees on the southern bank of the Brahmaputra (Assam)-Dibang (Arunachal Pradesh) river system. Like the other 19 gibbon species in the world, it is marked as endangered due to habitat loss and habitat fragmentation.
‘Sensitive to canopy gaps’
“The sanctuary has become a ‘forest island’, having lost connectivity with surrounding forest patches. Since gibbons are exclusively arboreal animals inhabiting the forest’s upper canopy, they are particularly sensitive to canopy gaps,” the WII’s technical report said on May 2023, advising an artificial canopy across the railway track in the Hollongapar protected area.
“Gibbon families on both sides of the railway track have thus been effectively isolated from each other, thereby compromising their population’s genetic variability and further endangering their already threatened survival in the sanctuary,” the report said.
An artificial canopy bridge is a conservation initiative for facilitating the movement of arboreal animals across life-threatening man-made structures or projects.
‘Bridge’ design sought
The Divisional Forest Officer of Jorhat (Territorial) Division of the State Forest Department approached the WII in 2022, seeking specific design inputs for canopy ‘bridges’ across the railway track in Hollongapar.
In 2015, the Northeast Frontier Railway constructed an iron canopy bridge but it was not found suitable for the gibbons to swing across the track. The Forest Department and Aaranyak joined hands four years later to grow a natural canopy bridge but regular pruning of the trees by the railways during track maintenance affected the movement of the apes.
The WII report underlined the distress caused by the railway track to the gibbons and the other primates, posing conservation complications. “Hence, a future doubling of the line (if planned) will increase the canopy gap to a large extent and render any conservation interventions (such as artificial canopy bridge installations) futile,” it warned.
The authors of the Science report, two of whom were part of the team that prepared the WII report, indicated that it would be better to realign the railway track (proposed to be doubled and electrified) outside the sanctuary than build a canopy bridge.
“Given the western hoolock gibbon’s conservation status, Hollongapar Gibbon Sanctuary’s small size, and the availability of surrounding non-forest land, the existing railway track should be rerouted to areas outside the sanctuary,” the authors said.
“Moving the track would be a critical step towards gibbon conservation and would also unambiguously demonstrate the government’s commitment to balancing India’s rapidly growing economy with its fragile ecology,” they said.
Other suggestions include reforestation on both sides of the existing track, enforcing train speed limits within the sanctuary and adjacent wildlife corridors, connecting the sanctuary’s isolated ‘forest island’ with neighbouring forests, and setting up sustainable eco-tourism accommodations.
