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 this species is removed, the ecosystem may undergo significant changes, often leading to a collapse in biodiversity. Common examples include predators that control prey populations, or species that provide critical resources for others.

Professor: Let me give you a classic example of a keystone species: the sea otter. In the kelp forest ecosystems off the Pacific coast, sea otters are not the most numerous creatures, but they play a huge role. They eat sea urchins, which are herbivores that feed on kelp. Without sea otters, the sea urchin population explodes, and they devour the kelp forests, destroying the habitat for many fish and invertebrates. When sea otters are present, they keep urchin numbers in check, allowing kelp to thrive, which supports a rich community of species. So, even though otters are relatively few, their impact is enormous—they truly are a keystone species.