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, often leading to the decline or extinction of other species. The concept highlights how certain organisms play a critical role in maintaining the structure and balance of their habitat, even if they are not the most numerous.

Professor: Let me give you a classic example of a keystone species: the sea otter. In kelp forest ecosystems along the Pacific coast, sea otters are not the most numerous creatures, but they have a huge effect. They prey on sea urchins, which feed on kelp. Without otters, sea urchin populations explode, and they devour the kelp forests. Kelp provides shelter and food for many fish and invertebrates. When kelp disappears, those animals lose their habitat and their numbers drop sharply. But when otters are present, they keep urchin numbers in check, allowing kelp to thrive, and thus the whole ecosystem remains diverse and healthy. So, a single predator, though not abundant, is essential.