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Return Of The Wolves

Nursing Streams Back To Health In Yellowstone

New research in Yellowstone National Park shows that the preying of wolves on elk benefits aspen, an important tree species in streamside environments. The study was published in Conservation Biology by forestry researchers William Ripple and Robert Beschta of Oregon State University. They found that a concept called "The Ecology of Fear” is behind the aspen recovery. "Just as humans act cautiously in grizzly bear country, it appears that elk are also more alert in areas where wolves are present,” says Ripple. Consequently, elk avoid areas where they are more vulnerable to wolves, such as stream beds. As a result, elk browse less on aspen saplings near streams, allowing the trees to survive past their critical growth stage. This is great news for Yellowstone, says Ripple, since "aspen are considered key indicators of healthy ecosystems.”

Aspen play a pivotal role in preventing soil erosion and are also home and food for many insects and small herbivores. "Aspen plant communities are some of the most biodiverse you have in these ecosystems,” says Beschta. Over 100 vascular plants are associated with aspen communities; this diversity increases numbers of local beaver dams, birds, insects, and fish.

The new aspen growth is located primarily in areas where elk experience reduced visibility and cannot easily flee from wolves, the researchers found. Fallen logs, steep banks, gravel, and even the icy streams themselves can all become obstacles for fleeing elk. One part of the study involved contrasting new aspen growth in streamside areas, also known as riparian zones, with growth of those in adjacent uplands. "In riparian zones, where wolves can most easily sneak up on elk, and gullies or other features make it more difficult for elk to escape, we've seen the most aspen recovery,” Ripple says.

When the Beschta began researching Yellowstone's stream systems several years ago, the importance of wolves came as a bit of a shock. "I thought I was going to study an elk story,” remarks Beschta. Wolves weren't on anyone's minds. "They're just not part of my vocabulary,” he adds.

After wolves were eliminated from Yellowstone in the 1920s, as part of a broad effort to protect livestock in the West, elk population began to increasingly impact aspen. Even though the Park tried to control elk numbers, aspen continued to decline. With the reintroduction of wolves a decade ago, aspen are growing again for the first time in 50 years.

Ripple and Beschta have done similar work in other national parks. In Utah's Zion Canyon, the key predator is the cougar, which preys on red-tailed deer. Human interference has caused most cougars to relocate from the area. Without cougars, the deer population has increased dramatically, and cottonwoods have suffered from intensified browsing. Like aspen, cottonwoods are essential for maintaining streambank stability and also act as a food source for herbivores.

Because of the potential benefits for stream ecosystems, these findings have sparked an interest in the reintroduction of wolves in more places than national parks. As regards to the wolves' future, Ripple says, "This is the opportunity for policy makers and managers to use this information to develop wolf management programs.” Over the next few years, residents of Washington and Oregon can expect a reentry of wolves from Idaho and Montana into more rural areas.

But not everyone is pleased with the proposition. "There are quite a few wrinkles that need to be worked out,” says Jack Field of the Washington Cattlemen's Association. Cattle are easy prey for wolves, and existing fencing is not adequate to keep wolves out of pastures. Current law in Oregon already prohibits harming wolves for any reason other than protecting human life. Local farmers and wildlife personnel likely will need to work closely to develop a management plan that everyone can agree on.

Cat Adams is an undergraduate at the University of Washington studying microbiology and biology with an emphasis on ecology and evolution.

Images:

Top: Wolf running through snow near Blacktail Pond. Photo: Jim Peaco

Bottom: William Ripple measuring young aspen along Slough Creek during the summer of 2006. Photo: William Ripple

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