Eco-viaducts are built to facilitate movements of wildlife, but do they work?

A black giant squirrel utilising a wildlife crossing beneath a viaduct.

Eco-viaducts are built to facilitate movements of wildlife, but do they work as such? With that question in mind, biologist Dr Gopalasamy Reuben Clements embarked on a research with the Wildlife and National Parks Department.

Between 2011 and 2013, he monitored animal movements through camera traps at 10 viaducts each at the Aring-Tasik Kenyir road in Terengganu and at the Gerik-Kupang road which traverses the Bintang Hijau Range in Perak and Kedah. (Only three of the viaducts were specifically built for animal crossings; the rest are normal viaducts which can also function as such because of the passageway underneath.)

Gopalasamy found the animal crossings being used by almost half the mammal species recorded in nearby forests.

“However, this does not mean that the viaducts are effective crossing structures because some species were recorded just once under the viaduct during our entire study. Also, the same number of species may be crossing the road without the viaduct,” says the associate professor at Kenyir Research Institute in Universiti Malaysia Terengganu.

Biologist Gopalasamy Reuben Clements says more research is needed to determine the actual effectiveness of wildlife crossings.
Biologist Gopalasamy Reuben Clements says more research is needed to determine the actual effectiveness of wildlife crossings.

For a viaduct to be considered effective in mitigating the impact of a road on a particular species, the number of detections in the forest would be similar to those under the viaduct. Based on camera trap data on six mammal species (three herbivores, a carnivore and two omnivores), Gopalasamy found the viaducts to be effective crossing structures for only two herbivore species.

“There are several reasons for this. The road itself may have had an effect on the frequency of crossing. So inappropriate viaduct design or location is not always to blame. For a viaduct to be considered as ‘working’, the use of trails by animals leading to it would be higher than those leading to a normal road. However, we did not find any differences in trail use in one of the viaduct areas.”

One tiger was photographed in forests on either side of the road, but not at the wildlife crossing – which meant it did not cross under the viaduct. The time it takes certain species to adapt to a crossing structure or natural fluctuations in wildlife populations can also cause low crossing rates.

“We do not know enough about the animals’ movement patterns to conclude if they have used the viaducts, nor were we able to know whether large numbers of animals have used the viaducts because we cannot identify individuals of species that do not have distinguishable markings.

Banded leaf monkeys photographed under a viaduct – Gopalasamy Reuben Clements

“Ultimately, we have a long way to go to really know whether viaducts in general are working or effective. This sort of research requires much more funds and time,” says Gopalasamy, who is co-founder of research group Rimba.

His study did not monitor road kills at roads with and without wildlife crossings (which is one way to determine if the eco-viaducts work) because that would entail him driving up and down the highway every day.

Gopalasamy further cautions that we cannot just build viaducts and let them be; they must be managed through regular maintenance and monitoring. His cameras under viaducts have photographed hunters and people camping overnight.

“If there is no management plan to regulate this activity, particularly though regular patrols, then the viaducts will never fulfil their potential as wildlife crossing structures.”

As to whether we should continue building eco-viaducts what with their high costs and uncertain effectiveness, he says that would depend on many factors.

“If there is heavy traffic preventing wildlife from crossing, and if forests on either side of the road are still large and contiguous enough to support healthy populations of animals, I’d say, go for it. Most importantly, there needs to be a wildlife assessment first to decide on the cost-effectiveness of building the viaducts.”

In his research, Gopalasamy has found that roads are not ideal in many parts of Malaysia because of landscapes with high environmental values.

“The environmental costs of road expansion are massive. Not only do roads fragment important animal habitats, they contribute to forest conversion, illegal hunting and wildlife trade,” he says. As demand for new roads and connectivity remains incessant, he suggests that road planners and scientists work together to determine where it is best to site new roads and minimise any ecological damage.

Though wildlife crossing structures have proven to lessen the adverse impact of roads, the extent of their effectiveness remains unclear. Therefore, the first choice would always be to not build a road through wildlife habitat. Wildlife crossings are but one tool, and cannot be the ultimate panacea to foil the ill-effects of roads on wildlife. More importantly, eco-viaducts should not be employed just to appease conservationists and justify roads that inch into the wilderness. 

Related stories:

Bridging a forest: Animal crossings that reduce the perils of the roads

The best and worst places to build roads

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