托福口语 Task 3

In biology, the term 'keystone species' refers to a species whose presence and role within an ecosystem have a disproportionately large effect on other organisms within the ecosystem. This concept highlights that certain species, though not necessarily abundant, maintain the structure, diversity, and balance of their community. If a keystone species is removed, the ecosystem can dramatically change, often leading to a cascade of negative effects, including the loss of other species and alterations in habitat.

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 graze on kelp forests. When sea otters are present, they keep the sea urchin population in check, allowing kelp forests to thrive. These kelp forests provide shelter and food for many fish and other marine life. However, when sea otters were hunted almost to extinction, the sea urchin population exploded. The urchins devoured the kelp, destroying the underwater forest. As a result, fish species that depended on kelp disappeared, and the entire ecosystem became barren. Once sea otters were reintroduced and protected, the kelp forests recovered, and the fish returned. This shows how a single species, though not very numerous, can control the whole ecosystem.