That sediment offered willows a place to take root. A third of mainstream reaches show evidence of sediment deposition as a result of beaver dams, a process that's happened for millennia. In the past, dams made by beavers were ubiquitous features of Yellowstone's stream network. Once, beavers had been abundant anywhere streams flowed through Yellowstone. They needed the sluggish streams created by beavers. But removing elk browsing wasn't enough for the willows. Wolves hunted elk and brought down numbers of these ungulates. That's why bringing back wolves didn't work to quickly restore the ecosystem, the researchers believe. "But Yellowstone also needs beavers," says Hobbs. Scientists had thought that the return of the wolf, leading to a cutback on elk numbers and willow browsing, was central to restoring the Yellowstone ecosystem. "The results have immediate practical applications in restoring and protecting ecosystems such as that of Yellowstone." "This research illustrates the value of long-term ecological experiments to understanding how species interactions cascade through food webs to determine ecosystem resilience," says Alan Tessier, program director in the National Science Foundation's (NSF) Division of Environmental Biology, which funded the research. In addition to Hobbs, co-authors are Kristin Marshall, formerly of CSU and now of the National Oceanic and Atmospheric Administration, and David Cooper of CSU. The ecologists published results of their study this week in the journal Proceedings of the Royal Society B. "Putting wolves back isn't enough to reverse the extensive changes caused by their long absence," says Hobbs, who, along with other scientists discovered in a decade-long research project. Beavers prefer slow-moving waters, so they disappeared with the willows.
With no willows to slow stream flow, creeks flowed faster and faster. The loss of wolves caused far-reaching changes in the Yellowstone ecosystem: more elk and fewer willows. "Beavers are the missing piece in this ecosystem," says ecologist Tom Hobbs of Colorado State University (CSU) in Fort Collins.
Now, it turns out, they aren't alone on the ecological dance floor.Įlk and willows play a critical role in wolves' success in the Yellowstone National Park ecosystem, willows serving as browse for elk-and elk as food for wolves.īut there's another species involved, one that's instrumental to these well-choreographed steps: the beaver. In the public mind, and in nature, the two are inextricably linked. Find related stories on NSF's Long-Term Research in Environmental Biology Program at this link.
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