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. Such species play a critical role in maintaining the structure of an ecological community. Their presence influences the types and numbers of other species in the ecosystem. If a keystone species is removed, the ecosystem may drastically change or even collapse, leading to a loss of biodiversity. Classic examples include predators that control herbivore populations, thereby preventing overgrazing of vegetation.

Professor: Let's talk about a real example of a keystone species. Consider the sea otter, which lives in kelp forests along the Pacific coast. Sea otters prey on sea urchins. Without otters, the urchin population explodes, and they devour the kelp. Kelp provides habitat and food for many fish and invertebrates. In areas where otters were hunted to near extinction, the kelp forests disappeared, and the ecosystem became barren—fish and other marine life declined sharply. But when otters were protected and reintroduced, they controlled the urchins again, and the kelp forests recovered, bringing back the diverse community. So, the otter's role is small in number but huge in impact.