Geological Survey in Fort Collins found that the combination of intense elk browsing on willows and simulated beaver cuttings produced stunted willow stands. Conversely, simulated beaver cutting without elk browsing produced verdant, healthy stands of willow.
In the three-year experiment, willow stem biomass was 10 times greater on unbrowsed plants than on browsed plants. Unbrowsed plants recovered 84 percent of their pre-cut biomass after only two growing seasons, whereas browsed plants recovered only 6 percent. As the beavers spread and built new dams and ponds, the cascade effect continued, said Smith. Beaver dams have multiple effects on stream hydrology. They even out the seasonal pulses of runoff; store water for recharging the water table; and provide cold, shaded water for fish, while the now robust willow stands provide habitat for songbirds.
In addition to wolves changing the feeding habits of elk, the rebound of the beaver in Yellowstone may also have been affected by the Yellowstone fires, the ongoing drought, warmer and drier winters and other factors yet to be discovered, Smith said.
Biologists are often faced with the grim task of documenting the cascade effects of what happens when a species is removed from an ecosystem, by local extirpation or even extinction. In Yellowstone, biologists have the rare, almost unique, opportunity to document what happens when an ecosystem becomes whole again, what happens when a key species is added back into the ecosystem equation.
Scott Creel, an ecology professor at Montana State University, is hip-deep in that feeding frenzy. Wolves come and go, he said, enabling him to study what elk do in the presence and absence of wolves. Note: Content may be edited for style and length.
Science News. Journal Reference : Luke E. Painter, Michael T. Tall willow thickets return to northern Yellowstone. Ecosphere , ; 11 5 DOI: ScienceDaily, 28 May Oregon State University. Reintroduction of wolves tied to return of tall willows in Yellowstone National Park.
Retrieved November 13, from www. ScienceDaily shares links with sites in the TrendMD network and earns revenue from third-party advertisers, where indicated. In the past, dams made by beavers were ubiquitous features of Yellowstone's stream network. A third of mainstream reaches show evidence of sediment deposition as a result of beaver dams, a process that's happened for millennia.
That sediment offered willows a place to take root. Warren found beavers and their ponds scattered throughout the park. Near the Elk Creek Bench Colony, for example, Warren spotted "a group of beaver ponds which present interesting features," he stated in a report published in the s. The brook flows through a flat depression in a ridge, and it is in the swampy, springy ground just below the woods that most of the ponds are located. It's a rare if not non-existent sight in Yellowstone today, especially on the park's northern range where Hobbs' team conducted its research.
Restoring an ecologically complete ecosystem in Yellowstone requires the return of willows--and with them, beavers. There's a clear threshold for ecosystem recovery.
Willow stands must be more than 6 feet tall, the scientists found. That height is important, says Marshall. Then willows are beyond the reach of browsing elk, and can serve as seed sources for new young willows. Once willows have returned, beavers will gnaw down a certain number of them to build dams. The dams will further slow stream flow, allowing yet more willows to grow.
The results offer new insights on the role of wolf-driven trophic cascades in the Yellowstone ecosystem, says Hobbs.
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