Wolf Magic

An aspen sapling may not look like much, but it’s a subtle sign that, 15 years after the return of wolves to Yellowstone National Park, one of the nation’s most controversial reintroduction efforts is having quiet but profound effects, scientists conclude in a new study.

“Yellowstone increasingly looks like a different place,” said William Ripple of Oregon State University (OSU), a lead author of the study, published in Biological Conservation. As wolves have thinned elk populations, reducing grazing and browsing, trees and shrubs have begun recovering along some streams, providing improved habitat for beaver and fish. “These are still the early stages of recovery, and some of this may still take decades,” Ripple says. “But trees and shrubs are starting to come back and beaver numbers are increasing. The signs are very encouraging.”

The study outlines an ecosystem renaissance that has taken place since wolves were restored to Yellowstone after being extirpated in the 1920s. Along four streams in the Lamar River basin, for instance, 100 percent of the tallest young aspen sprouts were being browsed in 1998, compared to less than 20 percent last year. Heavy browsing by elk on this favorite food had caused new aspen tree recruitment to essentially grind to a halt in the mid-to-late 1900s, when wolves were absent, but new trees are now growing again in places.

Among other observations:

• Since their reintroduction in 1995-96, the wolf population generally increased until 2003, forcing changes in both elk numbers and behavior due to what researchers call the “ecology of fear.” The northern range elk populations decreased from more than 15,000 individuals in the early 1990s to about 6,000 last year, and remaining elk now have different patterns of movement, vigilance, and other traits.

• By 2006, some aspen trees had grown tall enough that they were no longer susceptible to browsing by elk, and cottonwood and willow were also beginning to return in places.

• Improved willow growth is providing habitat that allows for a greater diversity and abundance of songbirds such as the common yellowthroat, warbling vireo and song sparrow.

• The number of beaver colonies in the same area increased from one in 1996 to 12 in 2009, with positive impacts on fish habitat.

• The coyote population decreased with the increase in wolf numbers, potentially allowing more small mammals that provide food for other avian and mammalian predators, such as red foxes, ravens and bald eagles. “The wolves have made a major difference in Yellowstone,” said OSU’s Robert Beschta, a co-author of the study. But “whether similar recovery of plant communities can be expected in other areas, especially on public lands outside national parks, is less clear,” he says. “It may be necessary for wolves not only to be present,” but also in sufficient numbers – and to stay out of harm’s way from humans. – David Malakoff & press materials | January 3, 2012

Source: Ripple, W., & Beschta, R. (2011) Trophic cascades in Yellowstone: The first 15years after wolf reintroduction. Biological Conservation. DOI: 10.1016/j.biocon.2011.11.005

Image Oregon State University