August 1, 2005

Wolves’ Top-down Effect

Willow trees, riparian willow warblers and beaver dams once were bountiful in an area near the town of Banff, Alberta, Canada. But once wolves left this area, elk grew more plentiful, browsing heavily on young willows. Today, there is little trace of beavers, and sparrows have replaced the warblers in what is now a grassland meadow. These profound changes were driven by the absence of the wolf, a top predator. That's the compelling finding of the paper, "Human Activity mediates a Trophic Cascade caused by Wolves" that appears in the August issue of Ecology by Mark Hebblewhite (University of Alberta) and colleagues.

Top-down effects of predators are called trophic cascades. While studies have demonstrated this phenomenon in aquatic environments, the Hebblewhite et al study is one of the first terrestrial, large-scale studies that so clearly exemplifies the strong role played by a top predator.

In the mid-1980s, wolves naturally recolonized the Bow Valley of Banff National Park in Alberta, Canada. The nearby town of Banff has been steadily growing and prevented wolves from fully recovering in areas surrounding the town while wolves fully repopulated adjacent areas. Hebblewhite and his fellow researchers were able to examine the effects of wolf exclusion on elk--wolves' preferred prey--on plants such as willow, which are favored by the ungulates, and on other species that depend on the willow habitat.

Hebblewhite and his colleagues found that in the low-wolf area of Bow Valley elk populations were 10 times as high as in the high-wolf area.

"We also found that as elk populations climbed, active beaver lodges declined, probably because beavers could no longer find sufficient trees with which to build their dams," says Hebblewhite.

In addition, songbirds, such as the American Redstart, which is strongly dependent on willow, also vanished from the wolf-excluded area.

Although the presence of people in the Bow Valley area also kept away other large predators, such as grizzly bears, Hebblewhite and colleagues believe their large-scale natural experiment demonstrates a wolf-driven cascade effect. Only wolves were completely eliminated from the study area and subsequently recovered. Bears and other large carnivores were never completely extirpated. In addition, the researchers traced elk deaths in the high-wolf area to wolves, which also supports the major role wolves have in this trophic cascade.

"Our study findings strongly bolster the use of conservation and restoration strategies which are based on the key role of large predators," says Hebblewhite.

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Ecological Society of America