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Explained: Building bridges in the forest, to help wildlife

Eco-ducts or eco-bridges aim to enhance wildlife connectivity that can be disrupted because of highways or logging.

Written by Shiny Varghese | New Delhi | Updated: December 9, 2020 10:23:14 am
Explained: Building bridges in the forest, to help wildlifeA macaque crosses a canopy bridge in a rainforest on Valparai plateau in Anamalai Hills, Tamil Nadu.

Ramnagar Forest Division in Nainital district, Uttarakhand, recently built its first eco-bridge for reptiles and smaller mammals. What are these, and why are they important?

Why eco-bridges matter

Eco-ducts or eco-bridges aim to enhance wildlife connectivity that can be disrupted because of highways or logging. These include canopy bridges (usually for monkeys, squirrels and other arboreal species); concrete underpasses or overpass tunnels or viaducts (usually for larger animals); and amphibian tunnels or culverts. Usually these bridges are overlaid with planting from the area to give it a contiguous look with the landscape.

A 2020 study by the Wildlife Institute of India (WII) noted that nearly 50,000 km of road projects have been identified for construction in India over the next five to six years, while many highways are being upgraded to four lanes. The National Tiger Conservation Authority, New Delhi, had identified three major sites that were cutting across animal corridors, including National Highway 37 through the Kaziranga-Karbi Anglong landscape in Assam, and State Highway 33 through the Nagarhole Tiger Reserve in Karnataka.

On the Kaladhungi-Nainital Highway, Chandra Sekhar Joshi, Ramnagar Divisional Forest Officer, supervised the building of the new 90-foot eco-bridge. “We found many roadkills on this route, especially of reptiles such as the monitor lizard. The bridge is an awareness-building mechanism for this very congested tourist route,” he said. “… The bridge was a way to see how we can preserve the ecosystem necessary for reptiles that feed on insects, for snakes that feed on reptiles, and for eagles that feed on snakes.”

What builders look at

Bilal Habib, Head, Animal Ecology and Conservation Biology, WII, said there are two important aspects of building eco-bridges – size and location. “When you see a roadkill, you imagine that the animal has died there and so you place a bridge there. However… this may not be the only indicator. Often you don’t see collisions because the road has already become a wall for animals in that area. For instance, when highways are upgraded from two lanes to four lanes, you stop seeing roadkills. It doesn’t mean it becomes a green highway… It’s important therefore to understand what the animal habitats are in the area, topography, disturbance types, road length and its curvature,” he said.

The span and distribution of eco-bridges should depend on animal movement patterns. “The bigger bridges will see sambar, spotted deer, nilgai, wild pig using them, while for tigers or leopards, if the bridge is 5m or 500 m, it doesn’t bother them. But some animals like the barking deer, which prefer closed habitats, need smaller bridges,” Habib said.

Explained: Building bridges in the forest, to help wildlife A Royal Bengal tiger uses one of several underpasses along National Highway 44.

The challenges

Senior scientist Divya Mudappa of Nature Conservation Foundation, working in Tamil Nadu’s Anamalai Hills, built canopy bridges for lion-tailed macaques and Nilgiri langurs. “In 2008, we built six bridges across a 3-km stretch where these arboreal animals could move freely without being run over. Our smallest bridge was about 10m and the longest was about 25m. These have been very successful with the monkeys taking to them pretty fast,” she said. 📣 Follow Express Explained on Telegram

Habib recounted his team’s observations on NH 44, which intersects Kanha-Pench and Pench-Navegaon-Nagzira corridors in various sections. With five animal underpasses and four minor bridges on the 6.6-km road within the forests, it’s one of India’s success stories. They captured nearly 18 species that used these underpasses, including tiger, leopard, and golden jackal.

“We have a 750m long bridge here, possibly the world’s largest underpass. Most species use this. However, we found that on our 50m bridge, sloth bears and female nilgais don’t travel… In this 750m bridge, the sloth bear took two years to cross, the wolf and the pangolin took less than a year, while the spotted deer and jungle cat barely took a month,” he said.

Explained: Building bridges in the forest, to help wildlife The new eco-bridge built for reptiles in Ramnagar, Uttarakhand

One of the largest underpasses – 1.4km – for animal conservation is India is being built along the Madhya Pradesh-Maharashtra border, he said. Other proposals include the Chennai-Bangalore National Highway, in the Hosur-Krishnagiri segment, near reserve forests for elephant crossings, and in the Tadoba-Andhari Tiger Reserve in Chandrapur, Maharashtra.

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