Wolves drive trophic cascade in Banff National Park

The presence of wolves cascades throughout terrestrial ecosystems, effecting populations of elk, trees, beavers and songbirds, according to a research team studying the recolonization of the canine predator in the Bow Valley of Banff National Park.

The research was published in the August issue of Ecology in an article, "Human Activity Mediates a Trophic Caused by Wolves," written by Mark Hebblewhite of the University of Alberta and colleagues.

Top-down effects of predators in ecosytems are termed "trophic cascades." While studies have demonstrated this phenomenon in aquatic environments, the Hebblewhite study is one of the first terrestrial, large-scale studies that exemplifies the strong role played by a top predator, according to the Ecological Society of America (ESA).

Experimental evidence of trophic cascades initiated by large vertebrate predators is rare in terrestrial ecosystems, noted Hebblewhite in the abstract of the article. He found that a serendipitous natural experiment provided an opportunity to test the trophic cascade hypothesis for wolves (Canis lupus) in Banff National Park.

The first wolf pack recolonized the Bow Valley of Banff National Park in 1986. The nearby town of Banff was 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 elk, and on other species that depend on the willow habitat.

High-human activity partially excluded wolves from one area of the Bow Valley (low-wolf area), whereas wolves made full use of an adjacent area (high-wolf area). Hebblewhite and team investigated the effects of differential wolf predation between these two areas on:

  • Elk population density
  • Aspen recruitment and browse intensity
  • Willow production, browsing intensity and net growth
  • Beaver density
  • Riparian songbird diversity, evenness and abundance

The team found:

  • Elk were an order of magnitude more numerous in the low-wolf area — 10 times as high as in the high-wolf area. Annual survival of adult female elk was 62% in the high-wolf area vs. 89% in the low-wolf area. Annual recruitment of calves was 15% in the high-wolf area vs. 27% without wolves.
  • Wolf exclusion decreased aspen recruitment, willow production and increased willow/aspen browsing intensity.
  • Beaver lodges declined, probably because beavers could no longer find sufficient trees with which to build dams.
  • A songbird, the American Redstart, strongly dependent on willow, also vanished from the wolf-excluded area.

These alternating patterns between high- and low-wolf areas support the wolf-caused trophic cascade hypothesis, according to Hebblewhite. Humans, by limiting habitat use by wolves, mediated these cascade effects.

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," said Hebblewhite.

Source: Ecological Society of America.

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