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, often leading to the collapse of the ecosystem. Classic examples include predators that control prey populations, thereby maintaining the balance of other species in the habitat.

Professor: Let me give you a classic example—sea otters in the Pacific kelp forests. Sea otters are a keystone species. They eat sea urchins, which normally graze on kelp. Without otters, urchin populations explode, and they devour the kelp, destroying the forest. This kills the habitat for fish and other organisms. When otters were hunted to near extinction in some areas, the kelp forests vanished. But when otters were reintroduced, they controlled the urchins again, and the kelp forests recovered. This shows how a single predator can shape an entire ecosystem.