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Phenomenon: Wildlife Crossings

How did the bear cross the road? See a few ways engineers are helping wildlife go over, under, and around traffic.

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Wildlife Crossings

Who says commuting isn’t natural? Alaskan caribou cover 2,000 miles a year, while wolverines regularly patrol 250 square miles. But such travel is getting tougher because of new roads and development. The National Academy of Sciences estimates that, each year, 2 million wildlife-vehicle collisions result in hundreds of thousands of dead animals and 200 human fatalities. To reduce this toll, developers routinely build crossings to detour animals safely around, over, or under roads. Today, there are more than 700 crossings in North America that help species from grizzly bears to spotted turtles stay off the highway. Here’s how they work.

From the Top

Biologists often map a critical wildlife corridor by tracking the movements of alpha predators like cougars and wolves, which often have the largest ranges. Identify and address the needs of these species, the theory goes, and the rest of the food chain will benefit. The strategy works because a porcupine that lingers within a mile of its den can take advantage of the same crossing used by a gray wolf, which covers a territory as big as 1,000 square miles.

Over or under

Most animals prefer crossings that mimic their natural habitat. Grizzlies and elk favor large bridges with plenty of light and visibility, while black bears and cougars seek out cramped tunnels. Amphibians will use cool, shaded watery culverts, while rodents require cover to hide from predators. As a result, most crossings are built for a specific animal. A recent study by Patricia Cramer, an ecologist at Utah State University, found that 68 of 70 crossings were used by the intended animals–from wild turkeys to panthers.

Road Blocks

Fences are designed to keep animals away from highways, but they can also trap them on the wrong side. These specialized barriers do more than keep animals out–they keep them safe.


Keeping Track

Biologists use chalk pads and motion-activated cameras to monitor crossings. Data collected at Canada’s Banff National park showed that deer and elk used new routes immediately, but grizzlies took five or six years to acclimate. Researchers also collect DNA samples by stringing barbed wire around nearby posts and trees. When animals brush against the wire, they leave behind skin and fur that biologists can use to identify their gender, family history, and relationship to other animals using the site.

Animals Only

Backpackers should avoid wildlife crossings if there is a safe alternative. “At Banff,” says wildlife biologist Tom Clevenger, “we found that hikers and mountain bikers can have a negative effect on the success of crossings.” In popular hiking areas, animals either wait until darkness or ignore the crossing altogether. Although zero human presence around these areas is ideal, at Banff they developed a compromise: Park visitors can use underpasses, but overpasses are reserved exclusively for wildlife.

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