托福口语 Task 4

Today, we're going to talk about a fascinating concept in animal behavior called 'trophic cascades.' A trophic cascade occurs when a predator at the top of the food chain indirectly affects the abundance of plants or other species at lower levels, often by controlling the population of an intermediate consumer. Let me give you two classic examples. First, consider the Yellowstone National Park story. When wolves were reintroduced in 1995, they preyed on elk. With fewer elk, the overgrazed willow and aspen trees began to recover. This not only stabilized riverbanks but also brought back beavers and songbirds that depend on these trees. Second, think about sea otters in the kelp forests of the Pacific Ocean. Sea otters eat sea urchins, which feed on kelp. Without otters, urchin populations explode and destroy the kelp forest, harming fish and other marine life. But when otters are present, they keep urchins in check, allowing kelp to thrive, which supports a rich ecosystem. So in both cases, a top predator triggers a chain of effects that benefits the entire habitat.