The Outsized Impacts of Roadways on Plants and Animals and What We Can Do to Reconnect Nature
By Ben Goldfarb
The Kicking Horse Valley, a canyon tucked into Canada’s Yoho National Park, is a challenging place to be an elk. A herd of around two dozen migratory elk spend much of the year in the valley’s confines, grazing meadows and nibbling the shrubs that sprout in rocky avalanche chutes. Their quest for food is made more complicated by the Trans-Canada Highway, the massive asphalt artery that cleaves the valley in two. During especially busy periods, a car roars down the highway every five seconds — a wall of traffic that can impede elk from roaming the valley in search of food, and threatens to obliterate them when they step onto the asphalt.
Regularly braving this busy freeway, as the valley’s elk must to survive, would seem a certain death sentence. Yet the Kicking Horse herd is very much alive. Over years, the animals have learned to navigate the highway’s dangers with intelligence and savvy. “They must be making good decisions,” says Marie-Pier Poulin, an ecologist at the University of Wyoming who has studied the herd. But their long-term survival is far from assured. And their tenuous persistence tells us much about the risks and rewards that wildlife must negotiate on a planet spiderwebbed with 40 million road miles — and what we can do to help animals safely reach the habitats they need.
Road 1081 in Nan Province, Thailand.
Poulin launched her research in October 2019, when she and colleagues began the laborious process of tranquilizing fourteen female elk and affixing them with satellite collars. Over the nearly two years that followed, the animals collectively crossed the Trans-Canada Highway more than 3,000 times, yet not a single one was hit by a car — a remarkable testament to their skill as pedestrians. “When you go into the field, you can clearly see that they’re looking both ways before crossing, and waiting for good timing to cross the road as well,” Poulin says.
But while the valley’s elk have learned to live with cars, the Trans-Canada Highway still curtails their movements. Three thousand highway crossings might sound like a lot, but, given how close elk live to the road, Poulin says it’s actually not that many — certainly far fewer than you’d expect if the elk were able to wander back and forth across the Trans-Canada at will. The animals, Poulin and her colleagues wrote, “generally minimized their interaction” with the massive freeway that bisects their range.
A mule deer watches cars pass along a Colorado road. Photo by Ben Goldfarb
So why do they cross at all? In a word: food. As fresh vegetation sprouts around the valley in spring, elk pursue these tender and nutritious greens across the highway — particularly females, whose bodies need all the calories they can get as they prepare to gestate and eventually nurse their calves. In spring, Poulin found, elk “displayed riskier behaviours, such as selection for crossing at high traffic volumes.” Moreover, elk who crossed the highway tended to hang out in lusher patches of vegetation, suggesting the animals only braved traffic when the payoff was especially rewarding. Elk, like nearly all creatures, don’t want to cross roads — their need for nourishment simply leaves them no choice.
An Infrastructure Tsunami
The delicate tradeoff that the valley’s elk must make — between the risks of crossing roads and the rewards of finding food — will only become more complex in the decades ahead. More animals than ever live in the thrall of roads: As the ecologist William Laurance has noted, our world is experiencing an “infrastructure tsunami,” a massive explosion of highway projects that will produce 25 million kilometers of paved road-lanes by the middle of this century. Around ninety percent of this new construction is slated for developing countries in Asia, Africa, and Latin America — home, Laurance has noted, to “most of the world’s tropical and subtropical ecosystems that sustain unparalleled biodiversity.”
Wild female elephants with baby elephant traversing a paved road.
Like Yoho’s elk, tropical animals are already proving themselves capable of coping with our infrastructure — to a point. In Sri Lanka, where fishing cats often fall victim to roadkill, the felines have responded by commandeering sewer systems as subterranean travel corridors; in China, Asian elephants have adapted to high daytime traffic by walking highways at night. Yet animals can only paddle so much against the rising tsunami. From great apes to Asiatic cheetahs, many of the planet’s most iconic species are gravely imperiled by roads and other new infrastructure. In one 2022 study, for example, researchers estimated that existing roads stand to reduce the tiger population in Nepal’s Chitwan National Park by nearly 40 percent.
“When you have animals like tigers that are on the brink, there is no such thing as a non-disturbing form of infrastructure growth,” says Neil Carter, a conservation scientist at the University of Michigan and the study’s lead author. “Countries like Nepal have to build and expand to a certain extent, but you have to mitigate the risks to wildlife conservation.”
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And it’s not just animals that are affected by roads — plants, too, are plagued. Deforestation is fundamentally a consequence of roads, which permit loggers, legal or otherwise, to bring in heavy machinery and haul out logs. In the Amazon, researchers have found, the vast majority of deforestation occurs within 3.5 miles of roads. So too in the Congo, where roads and forest destruction have exploded in tandem since 2003.
As roads transmogrify landscapes, plant communities change dramatically, too. Invasive species follow road corridors, sometimes literally hitching a ride in truck tires — like Phytophthora lateralis, a fungus that assaults the roots of Port Orford cedar trees in the Pacific Northwest. In Mexico, flowering trees in fragmented habitats are visited less often by pollinating insects, and thus suffer higher rates of inbreeding and reduced genetic diversity. So-called “edge effects” also become increasingly dangerous. When roads gash the Amazon, permitting heat, light, and wind to rush into the rainforest, trees drop leaves to conserve water which in turn fuel groundfires that open more gaps that kill more trees that become kindling for more fires, a fatal feedback loop.
Habitat fragmentation has negative consequences for plants like jocote (Spondias purpura), a dioecious tree native to the central tropical regions of the Americas.
For all these reasons and more, as new highways arise, it’s vital that road developers skirt critical habitats: Once fractured, an ecosystem is not easily healed. In those cases when a new road is truly unavoidable, however, wildlife crossings — passages that permit animals to safely navigate roadways — can help blunt traffic’s harms. It’s true that crossings don’t do much for plants, but animals certainly benefit. In Brazil, a land bridge permits golden lion tamarins to traipse over a highway that fragments Atlantic forest; in Singapore, an overpass reconnects populations of civets and pangolins. Today African elephants trudge through Kenyan underpasses as tall as houses, red crabs scramble up steel bridges on Christmas Island, and elevated roadways in India allow tigers and leopards to slink beneath traffic undisturbed. These structures take advantage of animals’ innate adaptability, giving them the opportunity to exploit resources on either side of the highways that shatter their ranges.
Providing Safe Passage
Often, wildlife crossings work for many species at once: Banff National Park’s famous overpasses, for example, accommodate grizzly bears, elk, moose, and a host of other critters. Sometimes, though, biologists must tailor passages to specific wild users. Consider Australia’s squirrel glider, which sails through forest canopies on wing-like membranes that stretch between front and hind feet. Although most of the glider’s forests were lopped down generations ago, tattered remnants of old-growth habitat still hug the Hume Freeway in the state of Victoria. In 2007, Victoria’s transportation agency outfitted the freeway with road-spanning rope bridges and “glider poles,” wooden columns in the median that act as stepping stones for gliders as they coast between forest patches. The agency briefly shut down traffic to install the structures, a move that did not endear gliders to the public. “If it didn’t work I think we would’ve been hung, drawn, and quartered,” recalls Kylie Soanes, the ecologist tasked with monitoring the poles and bridges.
BR 163 highway in Para, Brazil.
Eighteen anxious months followed before a bold glider tiptoed across one of the rope bridges — followed by another, and another. Soon gliders were weaving back and forth nightly, smearing their scent glands along the bridges to claim them as their territory. Most importantly, Soanes’s research showed that the animals were breeding on either side of the freeway, and that the bridges reunited once-isolated populations. “Now these structures are being installed as a matter of course every time a new road is built through glider habitat,” Soanes says. If we give animals the ability to cross our roads safely, they take advantage.
That’s a lesson that officials are now trying to apply to Yoho’s Kicking Horse Valley. Engineers are considering plans to potentially expand the Trans-Canada Highway from two lanes to four; as part of the project, they may also install roadside fences that will guide elk to underpasses at either end of the seven-kilometer valley. According to Poulin, an additional underpass nearer the valley’s center, though expensive, would allow elk to more seamlessly feed on either side of the Trans-Canada, at once mitigating the risk of road-crossing and giving the animals access to the reward of fresh forage. Our roads have fractured the lives of elk, squirrel gliders, and every creature in between; now it’s our obligation to reconnect the ecosystems and wild movements we’ve sundered.
Ben Goldfarb is an award-winning journalist and author of Crossings: How Road Ecology Is Shaping the Future of Our Planet and Eager: The Surprising, Secret Life of Beavers and Why They Matter. In 2018, he participated in the National Tropical Botanical Garden’s Environmental Journalism Program.