Side note, does this mean they created an "herbivore pit?"
"Questions have also emerged about the well-publicized relationship between wolves and willows. Marshall and two colleagues investigated the controls on willow shrubs by examining ten years’ worth of data from open plots and plots surrounded by cages to keep the elk out. Her team found8 that the willows were not thriving in all the protected sites. The only plants that grew above 2 metres — beyond the reach of browsing elk — were those in areas where simulated beaver dams had raised the water table.
If beavers have a key role in helping willows to thrive, as Marshall’s study suggests, the shrubs face a tough future because the park’s beaver populations have dropped. Researchers speculate that the removal of wolves in the 1920s allowed elk to eat so much willow that there was none left for the beavers, causing an irreversible decline.
“The predator was gone for at least 70 years,” says Marshall. “Removing it has changed the ecosystem in fundamental ways.” This work suggests that wolves did meaningfully structure the Yellowstone ecosystem a century ago, but that reintroducing them cannot restore the old arrangement.
Arthur Middleton, a Yale ecologist who works on Yellowstone elk, says that such studies have disproved the simple version of the trophic cascade story. The wolves, elk and vegetation exist in an ecosystem with hundreds of other factors, many of which seem to be important, he says."
http://www.nature.com/news/rethinking-predators-legend-of-the-wolf-1.14841
