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. Removing a keystone species can cause significant changes in the ecosystem's structure and biodiversity, often leading to the collapse of the entire system. These species play a critical role in maintaining the balance among other species, and their presence ensures the survival of many others.

Professor: Consider the sea otter, a classic example of a keystone species. In kelp forest ecosystems along the Pacific coast, sea otters feed on sea urchins. Without otters, the urchin population explodes and consumes vast amounts of kelp, destroying the forest. This kelp provides habitat and food for many fish and invertebrates. When otters were hunted to near extinction in the 1800s, the kelp forests disappeared, and the fish populations crashed. But when otters were reintroduced, they controlled the urchins, and the kelp forests recovered, bringing back the entire ecosystem. So, the otter's presence or absence dramatically shaped the whole community, despite being relatively few in number compared to urchins and kelp.