How beavers have affected ecosystem at Voyageurs National Park

The ecologist received a two-year National Science Foundation grant for more than $143,000 to compile a book based on her previous NSF-funded research on how beavers have affected the ecosystem at Voyageurs National Park near International Falls, Minnesota.

"Beavers influence the environment at a rate far beyond what would be expected given their abundance," said Johnston, who has been doing beaver research since the 1980s and authored or co-authored 28 of the 37 articles in the compilation.

Beavers create patchiness because they cut down big trees and make dams that flood the landscape creating wet meadows and marshy vegetation, Johnston explained. However, historical and aerial photos from 1927 and 1940 showed solid forests, meaning little evidence of beaver activity.

From the 1940s through the 1980s, the beaver population in the nearly 218,000-acre park increased steadily, according to Johnston. By 1986, 13 percent of the landscape was impounded by beavers.

"We saw lots of ponds where before there were none," she said. In addition to duck and amphibians, moose and upland mammals use this habitat extensively. "Having beaver on the landscape creates a lot of biodiversity."

Since 1991, the number of beavers has begun to decrease, Johnston pointed out. However, thanks to National Park Service officials mapping the active beaver lodges, she can now relate the population data to changes in the landscape.

"It's unusual to have both those types of data for such a large area," she said. That will allow her to track what happens to the landscape when beaver numbers are reduced.

Both predation and depleted food supply may account for the beavers' decline.

"Aspen is the preferred food," she said, noting beavers don't hibernate and must rely on having a large supply of edible food in their underwater cache to survive the winter.

Beavers forage up to 110 yards from the pond edge, creating what Johnston calls a "bathtub ring of conifers" when most of the aspen and deciduous trees have been harvested. Venturing beyond that comfort zone makes them susceptible to predators, she pointed out. "Beavers are a preferred prey for wolves."