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The Wolf Effect
Where elk fear predation, an ecosystem returns

It seemed obvious. Because wolves prey on elk, and elk feed on plants, the wolves' reintroduction to Yellowstone National Park in 1995 should have led to a decline in elk numbers. That would then explain why some plants elk eat are suddenly thriving.

But when Robert Beschta and William Ripple of Oregon State University began to study plant recovery in the park, they found a different twist. "What we're actually seeing is that the size of the elk population hasn't changed significantly," Beschta says, "and isn't the biggest factor" in the revival of certain plants—plants that impact the health of the entire ecosystem from bugs to birds to mammals.
Instead, it seems that fear of predation, not elk numbers, is driving floral recovery—by changing the ungulates' behavior. In some areas where wolves now prowl, "elk no longer hang out at streamside, browsing," Beschta says. "They're more cautious and spend less time where there's a high risk of predation. In those areas, river-loving woody plants like young cottonwoods and willows, once overbrowsed by elk, are taller than they've been in decades. Some are going gangbusters."

This suggests, Beschta says, that the extermination of wolves from the park nearly a century ago—creating an Eden for wild ungulates—may have led to the long-term decline of certain plant species. And while wolf reintroduction wasn't done to improve forest health, putting wolves back in the ecosystem may prove key to the survival of riverside plant communities, which in turn strengthen stream banks and provide shade and wildlife habitat.
"How often do you get to connect wolves with warblers?" asks National Park Service biologist Doug Smith, who has headed up the wolf reintroductions. "Here you can: Wolves are, indirectly, helping to bring back nesting habitat for songbirds. As the willows recover, beavers create new aquatic habitats around which life just skyrockets." To regulate the ecosystem and maintain biodiversity, Smith says Yellowstone needs its top dog to keep the elk on their toes.
Beschta agrees. "It's one thing for us to put wolves back into Yellowstone because we took them out," he says. "It's another to put them back because the ecosystem requires it." Without wolves in the park today to scare the elk off overbrowsing, "the clock would be running out on remaining cottonwoods," he warns. "They'd eventually disappear. Probably aspens too. Bringing the wolves back has been an incredible plus."
Further research should tell whether climate and fire history are also affecting floral revival, but some scientists believe they've already found the key—and not just to the Yellowstone ecosystem. "Wolves seem to have an inordinate impact here, and I'm sure the same is true of top predators that have been lost elsewhere," says Beschta. "Perhaps now we can start to appreciate just how important a role these kinds of carnivores play."
—Jennifer S. Holland

Web Links

Wolf Reintroduction Program
This official Yellowstone National Park website details the history of wolf reintroduction to the park. 
International Wolf Center
If you want to learn more about the behavior of wolves and maybe even plan a trip to see wolves in the wild, visit this Minnesota-based organization's site. 
Rocky Mountain Elk Foundation
The Elk Foundation is an international conservation organization devoted to ensuring the future health and survival of the elk.

Free World Map

Paine, Robert T. "Food Web Complexity and Species Diversity," The American Naturalist (January-February 1966), 65-77.
Pickrell, John. "Wolves' Leftovers Are Yellowstone's Gain, Study Says," National Geographic News. Available online at
Powell, Dave. "Reintroduction of Wolves May End Tree and Shrub Decline in Yellowstone," The Forestry Source (December 2003). 
Ripple, William J., and Robert L. Beschta. "Wolf Recovery, Predation Risk, and Cottonwood Recovery in Yellowstone National Park," Forest Ecology and Management (2003), 299-313.


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