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. The presence of a keystone species helps maintain the structure and diversity of an ecosystem. If such a species is removed, the ecosystem can undergo dramatic changes, often leading to a loss of biodiversity. Classic examples include predators that control prey populations, or species that provide critical resources for others.

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. When otters are present, they keep the urchin population in check. Urchins eat kelp, which forms underwater forests that shelter many fish and invertebrates. Without otters, urchin populations explode and they devour the kelp, creating 'urchin barrens'—areas with no kelp and much lower biodiversity. When otters were reintroduced, kelp forests recovered, and fish populations increased. So the otter's effect is huge relative to its numbers, exactly what we mean by a keystone species.