Guwahati: A single track of 1.65 route-km long railway line (currently broad-gauge but unelectrified as yet) has divided the Hollongapar Gibbon Sanctuary in the Jorhat district of Assam since 1887 into two unequal parts.
The Hollongapar Gibbon Sanctuary in Jorhat covers 21 sq. km and is a key habitat for the Western Hoolock Gibbon. It’s home to around 125 gibbons, organised into over two dozen family groups. It stands as India’s sole protected area named after a primate species.
Apart from the Western Hoolock gibbon, the sanctuary also harbours six other primate species – capped langur, stump-tailed macaque, northern pig-tailed macaque, Assamese macaque, rhesus macaque, and Bengal slow loris, thereby holding the distinction of harbouring the highest primate species diversity for any Indian protected area.
Of the 20 gibbon species – all threatened, according to the International Union for Conservation of Nature (IUCN) – the endangered Western Hoolock gibbon (Hoolock hoolock) is the only one found in India, inhabiting the forests on the southern bank of the Brahmaputra-Dibang river system.
Habitat loss and habitat fragmentation are two principal threats to most terrestrial biodiversity across ecosystems and geographies. Gibbons are a particularly vulnerable group of primates inhabiting the forests of South and Southeast Asia.
“The Sanctuary has become a ‘forest island’, having lost connectivity with surrounding forest patches. Since gibbons are exclusively arboreal animals inhabiting the forest upper canopy, they are particularly sensitive to canopy gaps. 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,” says a Wildlife Institute of India (WII) report on Artificial Canopy Bridge Design to facilitate Western Hoolock Gibbon crossing over the Mariani-Dibrugarh broad gauge single-track Railway Line in Hollongapar Gibbon Sanctuary, Assam.
Worldwide, and even in India, several conservation initiatives have attempted to bridge such canopy gaps in forests through artificial canopy bridge structures to facilitate arboreal species’ movements.
The Wildlife Institute of India, Dehradun, was approached by the Divisional Forest Officer, Jorhat (Territorial) Division of the Assam State Forest Department, to provide specific design inputs for the installation of such canopy bridges at the sanctuary.
The report offers design guidelines, considerations, and site-specific details for installing canopy bridges in Hollongapar Gibbon Sanctuary. It’s based on literature research, field data, and discussions with stakeholders like Assam State Forest Department, railway officials, and local conservationists.
In 2015, the North East Frontier Railway and Assam State Forest Department constructed an iron canopy bridge over a railway track. Despite good intentions, the bridge wasn’t used by arboreal mammals like gibbons due to design issues not suitable for their swinging movement. A natural canopy bridge was later grown through joint efforts of Aaranyak and the forest department. This natural bridge, established by 2019, saw use by gibbons and other animals. However, its viability is compromised as the railway regularly prunes trees along the track for maintenance, affecting the animals’ passage according to the report.
In a Committee meeting on July 8, 2022, the Chief Secretary of the Assam Government expressed concerns about the threat posed to Hoolock gibbons by the mentioned railway line. As a response, the Assam Forest Department decided to develop a canopy bridge design, facilitating the movement of gibbons and other arboreal mammals between forest fragments (compartments 1 & 2 of the Hollongapar Gibbon Sanctuary).
On October 17, 2022, the Additional Divisional Railway Manager (ADRM), Tinsukia Division, NEFR, also requested the Principal Chief Conservator of Forests (PCCF), Assam, to provide designs for canopy bridges. Subsequently, the Divisional Forest Officer (DFO), Jorhat (Territorial), approached the Wildlife Institute of India, Dehradun (WII), to discuss the most appropriate ACB design incorporating domain-specific knowledge and concerns raised by various agencies.
“As is clear, the present un-electrified single-track ~1.65 km railway line passing through the sanctuary has caused distress and posed significant conservation issues to arboreal animals. 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,” the report warns.
The authors of the report, in an article in Science Magazine, say the Indian government promotes sustainable development and emphasises coexistence with wildlife and habitats in its growth philosophy.
“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. 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,” the authors say.
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The report suggests maintaining the status quo, with potential electrification of the single track pending legal approval. Ecological impacts would be investigated and addressed through mitigation and compensation measures.
The report proposes various long-term measures to ensure the survival and well-being of Western Hoolock gibbons and other canopy-dwelling species in the sanctuary. These actions include reforesting both sides of the existing track, enforcing train speed limits within the sanctuary and wildlife corridors, connecting the isolated ‘forest island’ of Hollongapar Gibbon Sanctuary with neighbouring forests, relocating the railway line away from the sanctuary, and setting up sustainable eco-tourism accommodations.
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