College of Forestry

Global Trophic Cascades Program

Observatory: Fates of Wolf and Aspen

New York Times, September 26, 2000

The hip bone's connected to the thigh bone, as the old children's song goes, and in ecology, things are tied together as well. The latest case in point comes from Yellowstone National Park, where researchers suggest that a sharp decline in aspen trees over the past century can be linked to the elimination of wolves.

The researchers, from Oregon State University, have been studying the decline in aspen groves in the park, using historical documents, aerial photographs and ring-dating. As they report in the journal Biological Conservation, they determined that for more than a century and a half, until the late 1920's, young trees were able to survive and mature within existing groves. Things changed around the time that the wolves in and around the park were eliminated (the last wolves were seen in 1926).

Here is the connection, as theorized by the researchers. Wolves are natural predators of elk, and elk like to munch on aspen suckers in the winter, stunting the trees. When the wolves disappeared, the numbers of elk increased, and their behavior changed — they no longer avoided areas, like aspen groves, where they were vulnerable to wolf attack. So this might be one reason the trees suffered.

The researchers are testing this theory. And of course, wolves were reintroduced to the park in 1995. But it is too early to say whether their reintroduction will help the aspens.