How Beavers Build Biodiversity

It’s not postcard pretty to human eyes. But it’s habitat to wildlife.

Even species as small and relatively uncharismatic as beavers produce dramatic changes in the environment, to the benefit of many species and the detriment of others.  This press release caught my eye partly because of the debate over how reintroduction of wolves has changed Yellowstone National Park.  It’s also of interest because the British, who seem t0 suffer from a profound fear of their native wildlife (wolves, bears, badgers), are currently debating reintroduction of beavers (with much “we shall fight in the fields and in the streets” rhetoric):

Felling trees, building dams and creating ponds — beavers alter the landscape in ways that are beneficial to other organisms, according to ecologist Carol Johnston of South Dakota State University.

“Beavers influence the environment at a rate far beyond what would be expected given their abundance,” said Johnston, who is now completing a National Science Foundation grant to study how beavers have affected the ecosystem at Voyageurs National Park, near International Falls, Minnesota. She’s been doing beaver research there since the 1980s.

Beavers create patchiness because they cut down big trees and make dams that flood the landscape, creating wet meadows and marshy vegetation, Johnston explained. Historical and aerial photos from 1927 and 1940 showed solid forests, meaning little evidence of beaver activity.  But from the 1940s through the 1980s, the beaver population in the nearly 218,000-acre park increased steadily. By 1986, 13 percent of the landscape was impounded by beavers.

“We saw lots of ponds where before there were none,” she said. Ducks, amphibians,

moose, and upland mammals use this habitat extensively. “Having beaver on the landscape creates a lot of biodiversity.”

Beaver numbers have been decreasing since 1991, probably due to depleted food supply and increased predation.

“Aspen is the preferred food,” Johnston said. Beavers forage up to 110 yards from the pond edge, leaving behind 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.”