black bear crossing

A black bear moves through one of the underpasses along Highway 93 between Evaro Hill and Polson in this camera-trap photo provided by the People's Way Partnership. For more images of wildlife crossing structures in use, see here

Courtesy of MDT, CSKT & WTI-MSU

Piles of research show that the wildlife crossing structures riddling U.S. Highway 93 north of Missoula work. Now what?

“What most people have done is count how many individuals of what species pass through,” said Marciel Huijser, of Montana State University’s Western Transportation Institute. “But that doesn’t tell us what that number means.”

Along with colleagues A.Z. Andis of Yale University and Len Broberg of the University of Montana, Huijser helped the Montana Department of Transportation analyze 39 wildlife underpasses and the one big overpass along 56 miles of roadway between Evaro Hill and Polson. They compiled much of it into a final report to the state at the end of 2016, showing that the crossings were reducing automobile collisions and attracting animals to use them.

But the scientists still wanted to know if grizzly bears use the crossings as well as deer and elk do. They wanted to know if wildlife use them deliberately, or just randomly wander through. And they hope to figure out how many crossings is enough, or too much.

“Now we’re outside the scope of that project, and we have so much more data,” Huijser said. “We’re finally free to publish the rest in scientific journals.”

Their latest paper, released on Oct. 26, shows that deer and elk do funnel toward the passages instead of running across the highway wherever they reach it, like humans seek out crosswalks instead of jaywalking. But it appears carnivores, especially bears, aren’t so good at traffic safety.

That’s important because roadways have big effects on wildlife populations. Speeding traffic scares (and often kills) them, so few animals like crossing a road. That can lead to poor nutrition if some herds are frightened from moving to better feeding grounds across a road. For isolated populations of grizzly bears, roads present significant risks to long-term survival.

“Highways are a barrier in landscape,” Huijser said. “Improving connectivity makes it less likely the populations on either side would disappear in a certain time period. That’s when we can have a great benefit.”

One of the biggest challenges for the consortium of state and federal agencies working on grizzly bear recovery involves linking the island communities of bears around Yellowstone National Park with those in the Northern Continental Divide Ecosystem (NCDE). Both groups have hundreds of grizzlies, but almost no evidence of interbreeding between the two regions. Interstate freeways 90 and 15 appear to be the main obstacles separating them.

Next week, the Interagency Grizzly Bear Committee’s working groups for Greater Yellowstone and NCDE bears hold their meetings in advance of the full body’s winter session in Missoula Dec. 12-13. The U.S. Fish and Wildlife Service has ordered removal of federal Endangered Species Act protection from the greater Yellowstone bears and handed over management of them to state wildlife agencies, although several private and Indian tribal organizations have challenged that move in court.

Interagency Grizzly Bear Committee members are working on a similar recovery status declaration for the NCDE bears.

The wildlife crossings along Highway 93 have been used at least 22,000 times a year, and reduced automobile-wildlife crashes by 50 percent or more near the structures. However, other highway improvements like curve-straightening and road-widening have allowed drivers to go faster, resulting in an increase in crashes overall.

So what level of improvement does the trick? Build too few structures, or the wrong kind of structure, or put them in the wrong places, and the benefit declines or vanishes.

“Or if one overpass per 20 miles gets a high degree of population viability, why build more?” Huijser said. “You’re better off investing those dollars somewhere else that you haven’t done mitigation.”


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