托福口语 Task 3

In biology, the term "keystone species" refers to a species whose impact on its ecosystem is disproportionately large relative to its abundance. Removing a keystone species can cause dramatic changes in the ecosystem's structure, often leading to a collapse or a cascade of extinctions. The concept emphasizes that certain organisms play a critical role in maintaining biodiversity and ecological balance, even if they are not the most numerous.

Professor: Let me give you an example of a keystone species: the sea otter. In kelp forest ecosystems along the Pacific coast, sea otters are not the most abundant animals, but they have a huge effect. They eat sea urchins. Without otters, sea urchins multiply and eat the kelp at the base, destroying the entire forest. Kelp forests provide habitat and food for many species like fish and crabs. When otters are present, they control the urchin population, allowing kelp to thrive, which in turn supports a rich community. So the presence of a few otters keeps the whole ecosystem healthy and diverse, demonstrating the concept perfectly.