KATHMANDU — It starts off like the premise for a joke: “Why did the tigers cross the road?”
But for researchers and conservationists in Nepal, the spread of roads throughout the big cats’ habitat is no laughing matter.
A new study underscores just how serious the problem is, showing that road traffic impedes tiger movements within their habitat. But it also shows that the animals can adapt quickly when traffic volume eases, pointing to measures that can be taken to mitigate road impacts not just on tigers, but on wildlife in general.
For their study, the researchers focused on Nepal’s East-West Highway, which runs through both Bardiya National Park and Parsa National Park, each of which is home to growing tiger populations (125 in Bardiya as of 2022, and 41 in Parsa). They fitted a female tiger in Bardiya with a GPS collar, and did the same with a male tiger in Parsa, then observed how the highway and its traffic affected the animals’ movements, space use and habitat selection.
Study co-author Babu Ram Lamichhane, from the National Trust for Nature Conservation, a semi-governmental body, said the study was especially important as the government prepares to expand the two-lane East-West Highway to four lanes.
The researchers began their observations in early 2021, before a second COVID-19 lockdown was imposed in Nepal. The lockdown, and its conclusion, ultimately allowed the researchers to compare any differences in the tigers’ movements in the periods before, during, and after the associated traffic restrictions.
“We found significant differences between the way the two tigers responded to changes in the traffic volume,” lead author Neil Carter, from the University of Michigan in the U.S., told Mongabay. This, according to the researchers, reflects differences in highway traffic patterns and regulations as well as ecological conditions in the two parks.
Using the GPS data from the tigers’ collars, the researchers compared the big cats’ speed as they crossed the road against when they were moving about away from the road. They then used statistical tools to estimate the tigers’ space use and how far they traveled each day during these three distinct periods. Finally, they ran different models accounting for factors such as distance from the road, time of day, distance to the nearest river, canopy height, and distance to built-up areas. They did this to determine the effect of the highway and the lockdown policy on each tiger’s behavior, ability to move through the landscape, cross the road, and select habitats.
While traffic in and around Bardiya National Park is heavily regulated, with authorities enforcing strict speed limits, this isn’t the case in Parsa, where heavy trucks frequently pass through on their way to and from neighboring India.
The male tiger from Parsa looked disturbed before the lockdown and felt immediately at ease during the lockdown period, Carter said. According to the study, the female tiger in Bardiya crossed the road frequently during all three periods: before, during, and after the lockdown.
In contrast, the male tiger never crossed the road during the day in the period before the lockdown, a strong indication that it wanted to avoid human disturbance. However, when the lockdown was enforced, the animal was found to cross the road more frequently at night, and even during the day.
The authors found that both the tigers were 2–3 times more likely to cross the highway when the lockdown was in place compared to the pre-lockdown days.
Also, during the month following the shutdown, the space used by the male tiger from Parsa increased by three folds from 160–550 km2 (62-212 mi2).
“With greater use of areas near roads and increased likelihood of crossing the highway, the male tiger greatly expanded his space use area to the west side of the highway immediately following the onset of the lockdown and before the start of the monsoon,” the study says.
However, both animals moved more quickly when near the highway regardless of the state of the lockdown. The energy used while doing so could come at the expense of other metabolic functions and behaviors, and could have long-term lasting effects on individual fitness, the authors note.
Although the study only looked at two tigers, the findings are crucial for the tiger population in Nepal, Carter said. Various factors such as density of prey tigers in the two national parks may have played some role in generating the different outcomes, but Carter said he’s certain that roads have a big role to play in it.
He said it’s high time officials in Nepal take at least some measures, like the ones in place in Bardiya, to control the movement of vehicle traffic in Parsa and other tiger habitats.
“Based on the precautionary principle — to take action before the harm occurs — we urge policy makers to mitigate the impacts and human disturbances to tigers caused by the expansion of the highway from two to four lanes before it is too late,” the authors wrote.
But in the long term, just enforcing speed limits may not be the answer, Carter added. “We need to build infrastructure such as overpasses and underpasses for tigers so that they can move about without coming into contact with humans,” he said.
In Bardiya, there have been cases where tigers were attracted to people using the road and attacked them, he said, adding this could also happen in Parsa.
The study comes amid growing concerns that roads running close to important tiger habitats in Nepal’s southern plains could result in an increased rate of roadkill that would significantly dent the population of the vulnerable species.
The government of Nepal recently introduced guidelines to build wildlife-friendly infrastructure, including overpasses and underpasses on roads linked to important wildlife habitat. However, there’s been little implementation since then.
Ecologist Bibek Raj Shrestha, who was not involved in the study, said it highlights the telling impact that roads have on tigers. “Tigers’ highway avoidance behavior might alter how they hunt or mate, impacting their survival and fitness at the population level,” he said. “With the stakes so high, new roads should be avoided in the wilderness and the existing ones should adopt mitigation measures for tiger conservation.”
The authors of the study note that more research is needed on the impact of roads on tigers, and that future studies need to incorporate a bigger sample size. “We also need to better understand interactions between tigers and their prey, and try out different mitigation measures to identify the ones that work,” Carter said.
Lamichhane said the government needs to facilitate more GPS studies to understand the behavior and ecology of tigers and why they cross or don’t cross roads.
A century ago, an estimated 100,000 wild tigers roamed Asia. But by the early 2000s, their number had plummeted by 95%, largely due to poaching and habitat loss and fragmentation.
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. Nepal, where tigers are found mostly in Bardiya and the Chitwan-Parsa complex, announced last year that it had nearly tripled the population of tigers in its territory.
This article by Abhaya Raj Joshi was first published by Mongabay.com on 27 February 2023. Lead Image: The new study underscores just how serious the problem of road traffic impeding tiger movements within their habitat is. Image by Rohit Varma via Flickr (CC BY-SA 2.0).
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