托福口语 Task 4

Today, we're going to talk about a fascinating behavior in the animal kingdom called 'trophic cascades.' A trophic cascade is an ecological process that starts at the top of the food chain and ripples down to affect the entire ecosystem. The main point I want you to grasp is that the presence or absence of a top predator can dramatically alter not just the prey population, but also the vegetation and even the physical landscape. Let me give you two classic examples. First, consider the Yellowstone National Park wolves. When wolves were reintroduced in 1995, they preyed on elk, which reduced elk numbers and changed their grazing behavior. As a result, overgrazed willow and aspen trees recovered, which then attracted beavers and increased biodiversity along rivers. Second, think about sea otters in the Pacific coastal waters. Otters prey on sea urchins. When otters were hunted to near extinction, urchin populations exploded, and they grazed down entire kelp forests. The kelp forests, which provided habitat for many fish species, disappeared, leading to a collapse in local fish populations. But when otters were protected and recovered, they controlled urchins again, allowing kelp forests to thrive and fish to return.