Unfortunately, it appears that simply bringing wolves back into Yellowstone is not enough to repair the the beaver population:
Let me explain from the beginning. Wolves had been completely extirpated from Yellowstone by 1926. This had a two-fold effect on elk within the park.
First off, the elk population skyrocketed. With no predators to cull the herd, the population grew unchecked. This was in fact the intention at the time, but is an ill founded concept. It was shown by a famous account from Aldo Leopold that the mule deer population exploded after the removal of predators from the Grand Canyon in the early 1900's. Over browsing destroyed the ecosystem and the population soon crashed. To further exacerbate the issue, the land has now been so damaged that the overall carrying capacity for mule deer and other herbivores has been greatly diminished.
Aldo Leopold on Trophic Cascades:
http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/22138/RippleWilliam.FERM.LinkingWolvesPlants.pdf?sequence=1
A similar incident has taken place in Zion National Park. Riparian cottonwood forests have been greatly diminished and the ability of the park to support grazing and browsing animals has been damaged.
Impact of a cougar decline on Zion Canyon, Zion National Park:
http://www.nature.nps.gov/parkscience/index.cfm?ArticleID=318&ArticleTypeID=28
Similar studies have taken place in Wisconsin. So this is a well studied issue.
The second effect was that the eating habits of the elk changed. Previously, they had avoided areas with thick cover where predators could hide, such as aspen and willow forests. Sticking to open pastures and meadows allowed them more time to react to approaching wolves. Without fear of predators, elk were free to move in and apply additional pressure to sapling regeneration. As the trees aged and began to decline, they were not replaced by younger trees.
Of special importance in this instance is willow. Willow are a species of tree which help to engineer their own environment. Willow slows down streams and prevents erosion. Their absence has caused streams to create deeper gullies and speed up as erosion increases. Willows need slow moving water to establish themselves. So this cycle is self reinforcing, without slow water, there is no willow and without willow, there is no slow water.
What does this mean? This affects ground water and stream side plant communities. Water quality is diminished by more silt and temperatures rise without tree cover. This affects the ecology within the streams all the way to the microbial level.
But most of all it affects the ability of beavers to move back into the environment. Beavers cannot function in fast moving streams and must have a certain set of circumstances present before they can do their job. Without willow, the right conditions for beavers are not present. Since they help engineer their own environment in the same way that willow does, their absence also negatively reinforces their ability to move back into the environment once gone.
It is not just a simple matter of reintroducing wolves and watching everything go back to the way it was. Trophic Cascades are far more complicated than that. In this case the lands ability to 'carry' beavers has been greatly diminished. That is not to say that there has not been SOME recovery, but it just has not been what they hoped for. Some areas may take longer and others may be permanently unable to provide beaver habitat.
Since beavers have such a monumental effect on riparian ecology and forest dynamics, this affects the entire system. Populations of aquatic invertebrates and fish like trout may be affected. Migrating birds and waterfowl like geese have less habitat:
http://www.sciencedaily.com/releases/2013/02/130213132328.htm
Other terrestrial birds, rodents, and invertebrates may be affected by loss of nesting trees and food sources. Vegetation regimes are greatly altered and the list goes on and on.
This is the essence of a Trophic Cascade.
See our previous entry on the importance of beavers in their ecosystem:
http://agroregeneration.blogspot.com/2013/01/beavers-are-engineers-of-structural.html
Picture Citation: http://commons.wikimedia.org/wiki/File:Longford_Stream_-_geograph.org.uk_-_1398014.jpg
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