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. Removing a keystone species can cause a cascade of changes, drastically altering the ecosystem's structure and diversity. These species often maintain balance by controlling prey populations or providing critical resources for other organisms. Their presence is vital for the survival of many other species in the community.

Professor: Let me give you a classic example—the sea otter in the kelp forest ecosystem. Sea otters are not numerous, but they play a huge role. They prey on sea urchins, which feed on kelp. Without otters, urchin populations explode and devour the kelp forests. Kelp provides habitat and food for fish, crabs, and other marine life. When kelp disappears, those species decline, and the whole ecosystem becomes barren. But when otters are present, they keep urchin numbers low, allowing kelp to thrive and supporting high biodiversity. So, the otter is a keystone species because its impact is far greater than its small population size would suggest.