TOEFL Listening

🎧 Listening Transcript

Listen to part of a lecture in an environmental science class. Professor: Today, we're going to discuss a fascinating example of ecological restoration: the reintroduction of wolves to Yellowstone National Park in the 1990s. When wolves were removed from the park in the 1920s, the elk population exploded, leading to overgrazing of young willow and aspen trees. This devastated the riparian habitats, which many birds and beavers depend on. But after wolves were reintroduced in 1995, something remarkable happened. The wolves not only reduced the elk population but also changed elk behavior. Elk became more cautious, avoiding open areas where they could be ambushed. As a result, vegetation began to recover along riverbanks. This regrowth stabilized the soil, reduced erosion, and even altered the river's course in some places. The return of wolves also benefited scavengers like bears and ravens, who fed on wolf kills. This cascade of effects, known as a 'trophic cascade,' shows how a single predator can reshape an entire ecosystem. So, what does this teach us? It reminds us that ecosystems are interconnected, and removing or adding a species can have far-reaching consequences.