托福口语 Task 3

In biology, the term 'keystone species' refers to a species that has a disproportionately large effect on its environment relative to its abundance. Removing a keystone species can cause a cascade of changes, dramatically altering the ecosystem's structure and biodiversity. Classic examples include predators that control prey populations, thereby maintaining balance among multiple species. The concept highlights how certain organisms are critical for the stability of their ecological community, even if they are not the most numerous.

Professor: So, let me give you a real example. In the Pacific Northwest, sea otters are a keystone species. They eat sea urchins. Without otters, urchin populations explode, and they devour kelp forests. Kelp provides habitat for fish, crabs, and other marine life. When otters were hunted to near extinction, the kelp forests disappeared, and the ecosystem collapsed. But when otters were reintroduced, they controlled the urchins, allowing kelp to regrow. Soon, fish and other species returned. So, you see, the presence or absence of just one species—the otter—completely changed the entire ecosystem. That's why we call it a keystone species.