- Nearly two-fifths of adult tigers in Nepal’s Chitwan National Park could be killed over the next 20 years as a result of vehicle strikes on the roads near the park, a new study says.
- The projection is based on tiger movement data going back to the 1970s, and shows that the addition of a proposed railway line would result in an additional 30 tiger deaths.
- Chitwan is home to 133 tigers, and the large number of projected losses would be devastating to the population, which is already increasingly cut off from its range in neighboring India as a result of human-made obstacles, including roads.
- The study authors say this worst-case scenario should be a wake-up call to authorities to plan infrastructure projects with wildlife mobility as a key concern.
KATHMANDU — Roads that run close to an important national park in Nepal’s southern plains could result in an increased rate of roadkill that would cut the park’s population of adult tigers by nearly two-fifths over two decades, a new study shows.
The study forecasts that 46 Bengal tigers (Panthera tigris tigris) could be killed on the roads near Chitwan National Park, bringing the adult population down from 133 at present to 81 in 20 years. Another 30 tigers could be killed in the same period if a proposed railway line in the area is built, according to the study, which modeled its projections using tiger movement data collected in Chitwan since the 1970s.
“The mortality associated with increased traffic volumes and expansions of the roads and railway would have cascading negative consequences on population viability in the long term,” study lead author Neil Carter, from the University of Michigan, told Mongabay.
As the tiger population in Chitwan is relatively small compared to those in other habitats across the Indian subcontinent, this reduction of 39% could make them more vulnerable to diseases or inbreeding, Carter said. That would increase their risk of going locally extinction.
A century ago, an estimated 100,000 wild tigers roamed the grasslands across Asia. But by early 2000s, their number plummeted by 95%, largely due to poaching and habitat loss and fragmentation. During this time, three subspecies — the Java, Bali and Caspian tigers — went extinct.
In 2010, tiger range countries committed to doubling their population by 2022, the year of the tiger in the Chinese zodiac. Since then, the population of Bengal tigers has bounced back, with Nepal and India leading the way toward achieving the goal. On July 29, International Tiger Day, Nepal is expected to announce the achievement of the goal of doubling the population, at least in some of its protected areas.
“The most surprising result from the study is that even small increases in tiger mortality could have huge consequences for the population,” Carter said.
The study shows that even the death of one or two tigers, especially females, could trigger a massive decline in the overall population. Female tigers not only give birth to young ones, but also nurture and protect them until they are capable of living on their own, said study co-author Narendra Man Babu Pradhan, from the Nepal office of global conservation authority the IUCN.
This means that no matter how or when mitigation measures such as animal crossings and underpasses are built, one or two deaths could be disastrous for the population’s overall health, Carter said.
While looking at how roads and railways affect the number of prey herbivores, the authors of the study observed that there are several gray areas that need to be explored further. The researchers found that when populations of herbivore prey decreased near road infrastructure, the number of tigers in these areas also decreased.
“But there’s also examples in the literature where roads can actually attract some types of herbivores as they prefer vegetation along the road,” Carter said.
In those cases, the tigers would follow their prey to the road and risk being struck by a vehicle. This calls for further research into the behavior of prey and how roads impact them, Carter added.
The study comes a few months after another pointed that planned roads in tiger habitats in Nepal threaten to derail not just the conservation of the big cats, but also that of other apex predators. The study said predators such as sloth bears (Melursus ursinus) and dholes (Cuon alpinus) are also at risk from the network of roads in Nepal.
“The new study on tigers reinforces our finding that roads will have catastrophic effects on apex predators such as tigers,” said Bibek Raj Shrestha, co-author of the study on apex predators. Studies like these, despite their limitations, are important as they provide benchmarks for future work, he added.
Kanchan Thapa, a conservation biologist at WWF-Nepal, agreed, saying the study’s strength is that it makes use of data that Chitwan National Park pioneered and collected since the 1970s.
Carter said the model has several limitations, in that it doesn’t incorporate certain factors that may have an impact on the tiger population. For example, it doesn’t look at population losses caused by a reduction in genetic diversity due to their isolation from tiger populations in India, as a result of physical obstructions including roads. Similarly, it doesn’t account for the impact of deaths of male tigers on the overall population.
Pradhan said the model also doesn’t factor in long-term changes in management of protected areas that would be based on new science-generated knowledge.
Thapa told Mongabay there’s evidence that the availability of water plays an important role in determining the movement of tigers. This is also missing from the model. Scientists have observed that in water-scarce habitats, the number of wildlife struck by vehicles increases. Similarly, water bodies also attract wildlife, Thapa added.
However, these impacts could be balanced out by other factors, Carter said. For example, tigers could learn to cross roads safely, which would lower the chances of their being killed on the road. But that’s not certain.
With a small population to begin with, officials and planners need to take the precautionary principle of avoiding tiger habitats when building roads and railways, Carter said. “We don’t fully understand the impacts, so why let it happen?”
The study, therefore, highlights identifying “no go” zones where transport infrastructure should be prohibited, and the importance of recommending alternative placement of roads and railways, he said.
“There’s some good reason to not expand the road aiding increased traffic volume,” Carter said. “At a minimum, the volume and the speed of traffic should be restricted inside the forest, including the buffers zones.”
In the case of the proposed railway line, Carter said it should be bundled with the existing east-west highway, so that its effects are concentrated in one area. “The overpasses and underpasses could be shared by both the infrastructure and that could save resources as well,” he added.
Carter said the study shows the need for strong research programs in collaboration with transportation planners and authorities. “We need a lot more communication between the research activities and the planners so that they can share information in real time with each other,” he said. “Inviting researchers who are collecting information and data will help to inform those timely actions.”
Pradhan said the model presents a worst-case scenario for tiger habitats in Nepal and serves as a warning for conservation efforts that things need to change.
“We are hopeful that the changes will take effect and things will not deteriorate as projected by the model,” he said.
Quintana, I., Cifuentes, E. F., Dunnink, J. A., Ariza, M., Martínez-Medina, D., Fantacini, F. M., . . . Richard, F. (2022). Severe conservation risks of roads on apex predators. Scientific Reports, 12(1). doi:10.1038/s41598-022-05294-9
Carter, N. H., Pradhan, N., Hengaju, K., Sonawane, C., Sage, A. H., & Grimm, V. (2022). Forecasting effects of transport infrastructure on endangered tigers: A tool for conservation planning. PeerJ, 10. doi:10.7717/peerj.13472
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