托福口语 Task 4

Today, we're going to talk about a fascinating phenomenon in animal behavior called 'trophic cascades.' A trophic cascade occurs when a predator at the top of the food chain indirectly affects the population of species at lower levels, often by controlling the population of an intermediate predator or herbivore. This can have surprising effects on the entire ecosystem. Let me give you two classic examples. First, consider the reintroduction of gray wolves to Yellowstone National Park in the 1990s. The wolves preyed on elk, which had been overgrazing the park's vegetation. With fewer elk, the willow and aspen trees began to recover, which in turn provided habitat for birds and beavers. This shows how a predator can benefit plant life and other animals. Second, think about sea otters in kelp forest ecosystems. When sea otters are present, they hunt sea urchins, which feed on kelp. Without otters, urchins can destroy the kelp forest, reducing biodiversity. But with otters, kelp thrives, supporting fish and other marine life. So, in both cases, the top predator's presence maintains ecosystem health.