Walk This Way


Increasingly popular animal crossings help prevent road kill.


By Alisa Opar


A grizzly bear at a wildlife crossing in Canada's Banff National Park. Credit: TClevenger/WTI

If you’ve ever driven down a highway, you’ve probably seen road kill. Whether it was a panther in Florida, a moose in Maine, or a deer in California, major roadways are deadly for wildlife. The collisions aren’t much fun for drivers, either (even those who live in West Virginia, where motorists who run down any animal can legally take it home for supper).

One increasingly popular way to keep animals off roads and prevent unnecessary road kill is to install wildlife crossings. These structures can run under highways as tunnels or over them as bridges. They are becoming more and more common along roadways: The number doubled in the last decade, and today there are more than 600 in North America. Experts say that as human development continues to encroach on wildlife habitat, crossings will become even more critical to protecting animals from motorists. Not only do they make roads safer for drivers and animals, linking habitat with safe passages can help stem the loss of genetic diversity.

“Roads fragment populations,” says John Lloyd, an ecologist at the Ecostudies Institute, a non-profit conservation organization. “When species are fragmented, it really increases risk that they’ll go extinct.”

Now, all but a handful of US states have wildlife crossings, and many states are looking to build more. Though the first structures were built in the 1970s, researchers have only begun to understand what really works in the last decade. They now know, for instance, to build several structures along stretches of road that link habitat, and that human usage of crossings deters animal use.

They’ve also discovered that certain species require certain structure design, says  Tony Clevenger, who has been studying the 24 wildlife crossings in Canada’s Banff National Park since 1996. The passageways are all within a 28-mile stretch of the Trans-Canada Highway, which was widened from a two-lane to four-lane highway a quarter century ago. 

“Grizzlies, wolves, elk, and deer tend to prefer open structures with good visibility,” he says. “Cougars and black bears generally the exact opposite: Darker, tighter spaces to go through; structures that provide a lot of cover and protection.”

It takes time for animals to locate newly built structures after they’re built and feel secure using them. “Our data shows it takes five to six years for carnivore species to start using on a regular basis; elk and deer probably three to four years,” says Clevenger.

But eventually, the animals do catch on: In Banff, researchers have recorded ten large mammal species using the crossings more than 95,000 times since monitoring began in 1996.

Determining the number of distinct animal crossings is difficult, though. “It’d be easier if animals could just write down their name, where they’re from, and who they’re related to every time they crossed,” says Clevenger. “We know we’ve had 461 grizzly bear passes since 1996; is it one bear, or 461, or somewhere in between?”

To find out, Clevenger and other scientists are tracking animals through their DNA. They snag hair on sticky string that runs along the crossing. From the sample, they can identify the individual animal, determine its sex, and figure out how it’s related to other animals.

Now in the second year of a three-year study, they’re starting to see some preliminary results. For instance, they’ve identified 12 different grizzly bears using the crossings (6 males, 6 females) during the first field season.

“There is an urgent need to provide an objective assessment of how these structures are contributing to populations,” says Clevenger. “There are still transportation agencies skeptical of the value of these structures and reluctant to build unless hard data show they work.”

In fact, finding the money and the support to create new overpasses can be difficult. In the US, federal highway funds usually pay for the structures. “It’s a pot of money many groups compete for,” says Patricia Cramer, a Utah State University ecologist who helps maintain a website on mitigating the effects of roads on wildlife. In some areas, citizens are pushing for wildlife crossings, but not all people support them. A proposal to build a $455,000 animal path over Interstate 405 near Los Angeles has some residents fuming mad—they say transportation dollars shouldn’t go to wildlife projects. But Paul Edelman, a biologist with the Santa Monica Mountains Conservancy, calls the structure “a bargain.”

“Without it, if you look into the future, because of new development and increased traffic levels, you can pretty much write off deer, bobcats, and grey foxes east of the Santa Monica Mountains,” he says. “But if it goes in as planned, animals don’t need to cross every week. If you just get a few genetically distinct animals to cross that are able to breed and add to the population on the other side of the freeway, then it’s working.”

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