托福口语 Task 4

Today, we're going to talk about a fascinating concept in animal behavior known as 'trophic cascades.' A trophic cascade occurs when the presence or absence of a top predator indirectly affects species at lower levels of the food chain, often altering the entire ecosystem. Let me give you two classic examples. First, consider the Yellowstone National Park. When wolves were reintroduced in 1995, they hunted elk, which reduced elk populations and changed their grazing behavior. This allowed overgrazed willow and aspen trees to recover, which in turn provided habitat for songbirds and beavers. The beavers built dams, creating wetlands that supported fish and amphibians. So, the wolves' effect trickled all the way down to plants and smaller animals. Second, think about sea otters in the Pacific Ocean. When otters are present, they prey on sea urchins. Without otters, urchins overgraze kelp forests, destroying the habitat for many fish and invertebrates. But where otters thrive, kelp forests flourish, supporting a rich diversity of marine life. In both cases, the top predator's role was crucial for maintaining the health and balance of the entire ecosystem.