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article imageWolves Enable Aspen Trees to Return to Yellowstone Park

By Bob Ewing     Jul 27, 2007 in Environment
A new study produced by forest researchers shows that the reintroduction of wolves into Yellowstone National Park has had a positive impact on the Park's ecosystems. The aspen tree has made a comeback, for example.
The wolves have returned to Yellowstone National Park and so have the aspen. Is there a connection? Well, according to a new study that was recently published in Biological Conservation, there is.
The study refers to a process that has been labeled the ecology of fear and when wolves were reintroduced into Yellowstone in 1995, this process was activated. The elk is a wolf’s favourite food and the elk is very aware of this fact.
Elk, in turn, enjoy a good aspen buffet but the area where aspen likes to grow makes the elk an easy target so the ecology of fear suggests that elk avoid these areas because they would rather find another snack than become lunch. This allows the aspen to grow.
The study was conducted by forestry researchers at Oregon State University and supports theories about “trophic cascades”. Trophic cascade refers to ecological damage that can be caused when key predators – in this case, wolves – are removed from an ecosystem.
The study also shows that recovery is possible when the predators are returned. Another note of encouragement is that the return of the wolves and the aspen will have a positive effect on the Park and has implications for other areas and other predators and their food chains.
The aspen had been in decline over a number of years and in some areas were near extinction and the study states that this can be traced back to the extirpation of the last known wolf packs in the 1920s.
Before the wolves were returned to the Park, scientists had found many small sprouting aspen shoots as well as numbers of large trees 70 years old or more. However, there was little in between the two extremes. The high numbers of grazing ungulates, primarily elk, had grazed on the small tree shoots at leisure because they were not afraid of ending up on anyone’s menu.
The new study has found significant numbers of aspen, especially in streamside “riparian” zones. These aspen have grown from tiny shoots in the past decade to heights of more than seven feet. At this height elk and other animals have difficulty in grazing them so move onto other items.
The aspen is not the only member of the Park ecosystem that is impacted by the wolves’ removal from the Park. Stream erosion increased when the trees were eaten. Beaver dams declined. “Food webs broke down, and the chain of effects rippled through birds, insects, fish and other plant and animal species.”
The OSU scientists say that the element of fear is a concept that is beginning to receive more attention in ecology. The ecology of fear considers not just the numbers or species of animals, but also their behavior and the reasons for that behavior.
According to the study the team has shown that predators, such as wolves or cougars, have the ability to strike fear into their prey and significantly change their behavior as a result.
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