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 a collapse in biodiversity. Such species play a critical role in maintaining the structure of an ecological community, often through predation, competition, or providing resources for other species.

Professor: Let me give you a classic example. In the Pacific Northwest, the sea otter is a keystone species. Sea otters feed on sea urchins, which in turn graze on kelp forests. When otters were hunted to near extinction, sea urchin populations exploded, and they decimated the kelp forests. Without kelp, many fish and invertebrates lost their habitat and food sources, so the entire ecosystem collapsed. But when otters were reintroduced and protected, they controlled the urchin population, allowing kelp forests to recover. This brought back the fish and other species that depended on kelp. So the otter, though not abundant, has a huge impact on the whole community.