托福口语 Task 3

In biology, the concept of 'keystone species' refers to a species whose impact on its ecosystem is disproportionately large relative to its abundance. The removal of a keystone species causes a dramatic shift in the ecosystem structure, often leading to a decline in biodiversity. This concept highlights that certain organisms play a critical role in maintaining the balance of their habitat, even if they are not the most numerous or dominant in terms of biomass.

Professor: Let me give you a classic example. In the Pacific Northwest, the sea otter is a keystone species. Sea otters eat sea urchins. When otters are present, they keep the urchin population in check. This allows kelp forests to thrive, because urchins would otherwise overgraze the kelp. These kelp forests provide shelter and food for many fish and invertebrates. However, when otters were hunted to near extinction in the 1800s, the urchin population exploded. They ate the kelp, destroying the forests. As a result, fish populations plummeted, and the ecosystem became nearly barren. When otters were reintroduced, the kelp forests recovered, and biodiversity returned. So, the otter, though not abundant, has a huge impact on the whole community.