Wolves, Elk, & Aspen: Predators May Benefit Trees in Yellowstone

Over the last century, quaking aspen (Populus tremuloides) has declined by more than half in Yellowstone National Park’s northern range. A big part of the problem is that elk like the new growth so much that given the chance, they overbrowse it. The solution may be the return of the wolf: new research provides the first quantitative evidence that wolves may help protect aspen from elk in Yellowstone.

“Wolves may have an indirect effect on aspen regeneration by altering elk movements, browsing patterns, and foraging behavior,” say William Ripple and Eric Larsen of Oregon State University in Corvallis, Oregon, and Roy Renkin and Douglas Smith of Yellowstone Center for Resources in Yellowstone National Park, Wyoming, in the December 2001 issue of Biological Conservation.

Individual aspen are genetically identical stems that originate as suckers growing from a common root system. However, there has been virtually no aspen regeneration in Yellowstone’s northern range since 1930. The only exception is within sites protected from elk browsing.

The cessation of aspen regeneration coincided with the loss of wolves (which were eradicated from Yellow-stone by 1926), leading two of the researchers (Ripple and Larsen) to hypothesize that they may have been crucial to maintaining the stands. The idea that wolves can protect aspen from elk is supported by the fact that since wolves recolonized Canada’s Banff and Jaspar National Parks in the early 1970s, new aspen stems have been growing again. Today these new stands are as tall as 5 m, and aspen regeneration has been particularly vigorous in high wolf-use areas.

Wolves were reintroduced to Yellowstone in 1996, and Ripple and his colleagues investigated whether they are benefiting aspen there. The researchers monitored aspen stands in high and low wolf-use areas in the northern range. They considered stands in three types of habitat: dry upland, which included bunchgrasses and yarrow; moist upland, which included timothy grass and goldenrod; and wet meadow/riparian, which included carex and other sedges.

Ripple and his colleagues found that elk may be avoiding wetter habitat types that are heavily used by wolves: aspen stands there tended to have fewer elk pellet groups and taller suckers compared to those in low wolf-use areas. Specifically, there were about half as many elk pellet groups in the high wolf-use areas in the moist upland and wet meadow/riparian habitats, and aspen suckers were about a fourth taller in high wolf-use areas in the wet meadow/riparian habitat.

The researchers caution that their findings are preliminary and that only long-term studies will show whether wolves really can help protect aspen from elk-overbrowsing. Such studies will “enhance decision-making regarding park resources,” say Ripple and his colleagues. For instance, wolf reintroductions might be incorporated into restoring aspen habitat and riparian ecosystems elsewhere in the western U.S. and worldwide.

For more Information
Ripple, W.J. et al. 2001. Trophic cascades among wolves, elk and aspen on Yellow-stone National Park’s northern range. Biological Conservation 102:227-234.

The Aspen Project

www.cof.orst.edu/cof/fr/research/aspen/

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