การฟัง TOEFL

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Listen to part of a lecture in an environmental science class. Professor: Today we're going to discuss a fascinating example of ecosystem restoration—the reintroduction of wolves to Yellowstone National Park in the 1990s. After wolves were eliminated from the park in the 1920s, the elk population exploded. Overgrazing by elk devastated young trees, especially willows and aspens along rivers. This caused erosion and reduced habitat for birds and beavers. In 1995, scientists reintroduced wolves. The wolves not only reduced elk numbers but also changed elk behavior—they avoided open river valleys, allowing vegetation to recover. Trees grew taller, which stabilized riverbanks. The return of willows provided food for beavers, and their dams created wetlands that attracted amphibians and fish. This cascade of effects shows how a single predator can reshape an entire ecosystem. However, the success wasn't immediate—it took decades, and some argue that other factors, like drought, also played a role. Still, it's a powerful example of trophic cascades.