托福口语 Task 4

Today, we're going to talk about a fascinating phenomenon in animal behavior called 'trophic cascades.' Essentially, a trophic cascade occurs when a predator at the top of the food chain indirectly affects the population of species at lower levels, even plants. This happens because the predator's presence changes the behavior or numbers of its prey, which then affects the next level down. Let me give you two classic examples. First, consider the Yellowstone National Park case. When wolves were reintroduced in 1995, they preyed on elk. The elk, fearing wolves, avoided open river valleys where they were vulnerable. As a result, willow and aspen trees grew back along the rivers, which stabilized the banks and supported more biodiversity. Second, think of sea otters in the Pacific Ocean. Sea otters eat sea urchins. When otters are present, they keep urchin numbers low, allowing kelp forests to thrive. These forests provide habitat for many fish species. But when otters decline, urchins overgraze kelp, creating 'urchin barrens' and reducing fish populations. So, in both cases, the top predator's effect cascades down to benefit the entire ecosystem.