TOEFL Listening

🎧 Listening Transcript

Listen to part of a lecture in an environmental science class. Professor: Today we're going to talk about a fascinating example of ecological restoration: the reintroduction of gray wolves to Yellowstone National Park in the 1990s. For decades, the park had no wolves, which led to an overpopulation of elk. These elk overgrazed the vegetation, especially young willow and aspen trees along rivers. This had a cascading effect on other species, like beavers, which depend on those trees for food and building dams. When wolves were reintroduced in 1995, they not only reduced the elk population but also changed elk behavior—the elk avoided open river areas where they were more vulnerable to predators. This allowed the vegetation to recover. The regrowth of trees stabilized the riverbanks and provided habitat for beavers, birds, and other animals. Interestingly, even the rivers themselves changed—their channels became more stable and less prone to erosion. So, this shows how a single predator can have a profound impact on the entire ecosystem, a concept known as a trophic cascade.