Bridges to Bear Love

From the Jasper Fitzhugh, March 5, 2014.

Grizzly sow and cubs on overpass in Banff National Park. Credit: Banff Wildlife Crossings Project

Grizzly sow and cubs on overpass in Banff National Park. Credit: Banff Wildlife Crossings Project

As the days stretch longer this March, the bears of the Canadian Rockies will begin their slow emergence from winter dens in search of the first green shoots of spring. As April turns to May, they will begin using scent to locate mates—their sense of smell guiding them through vast home ranges that stretch hundreds of kilometres.

Finding a mate is hard enough, but in Banff National Park, bear habitat is bisected by the Trans Canada Highway, a stretch of pavement that has historically been viewed as a barrier to wildlife movement since its completion in 1962. To address this concern and to prevent collisions between cars and animals, two wildlife overpasses and 23 underpasses were built in the 1980s and the 1990s. Several more have been built since.

Although both grizzlies and black bears have been observed using these crossing structures, biologists have questioned whether or not the crossings facilitate enough mating opportunities to preserve good genetic exchange across the highway.

The answer to that question came last week in a study led by Dr. Michael Sawaya, a NSERC Visiting Fellow with Parks Canada. His research shows the first evidence that the crossings promote gene flow between bear populations on both sides of the highway.

Adequate gene flow is important in preventing inbreeding, which can result in genetic abnormalities, reduced fertility, and less resistance to diseases.

Based on a genetic analysis of bear hair, Sawaya found that black bears in the study appear to have unencumbered genetic exchange, meaning their gene structure is similar to a population of bears whose habitat has not been fragmented.

“They cross the Trans Canada Highway very frequently,” he said, adding that this is likely because black bears are “less wary, and more tolerant of humans and human infrastructure.”

Grizzly bears, on the other hand, appear to have a reduced genetic exchange.

“Grizzly bears tend to exhibit what’s called behavioural avoidance of the road,” said Sawaya, explaining that noise and manmade structures can deter them. They are more wary, and have been slower to adopt the crossings as safe routes, although their acceptance of them is increasing over time. Still, he said even this reduced exchange in grizzly bears allows adequate gene flow to prevent genetic isolation.

“One of the most powerful parts of this study is that we showed even when bear species behave differently, we’re still seeing more than adequate genetic interchange across the highway,” he said.

Sawaya has seen first hand how important the crossings can be for bear species. On one memorable day of fieldwork, he ran into five different grizzly bears while setting up a hair trap in a buffalo berry patch. “It really indicated to me how important little bits of habitat can be.”

Sawaya said grizzly bears may eventually catch up to black bears in terms of genetic connectivity.

“It takes time to build up the genetic differentiation between populations, and it takes time to see the dissolution of it.”

He noted that the crossings haven’t completely connected populations of grizzlies north and south of the highway, but suggested that if sampled again in five to 10 years, results could show the differentiation is gone.

“What we’re seeing now is the restoration of gene flow.”