The return of the wolf
ABy the early twentieth century, grey wolves had been hunted, trapped and poisoned out of most of the western United States, including Yellowstone National Park. For decades the park had no wolves at all. In 1995 and 1996, after a long public debate, wolves captured in Canada were released into the park, and the population grew quickly.
BThe return of a top predator was expected to affect the animals it hunts, above all elk. What attracted wider attention was the claim that the wolves had changed the landscape itself. According to this account, elk began to avoid open river valleys where they could easily be caught, allowing young willow and aspen trees to grow back along the rivers. The trees, in turn, were said to support birds and beavers and even to stabilise riverbanks.
CThis chain of effects, known as a trophic cascade, became a popular example in films and textbooks. Many ecologists, however, believe the story has been simplified. Elk numbers also fell because of hunting by people, harsh winters and the return of other predators such as bears and cougars. Some studies found that trees recovered only in certain places, and that in areas where beavers had disappeared, streams had changed in ways that wolves alone could not reverse.
DNone of this means the wolves made no difference. Rather, the Yellowstone case shows how hard it is to separate the influence of one species from everything else happening in an ecosystem. It also shows how a good story can travel faster than the evidence behind it. For wildlife managers elsewhere, the more cautious lesson may be the most useful one: restoring a predator can be valuable, but it is unlikely to repair a damaged landscape on its own.