托福口语 Task 3

In biology, the concept of 'keystone species' refers to a species whose presence and role within an ecosystem has a disproportionately large effect on other organisms within the ecosystem. The removal of a keystone species can cause a cascade of changes, leading to a significant decline in biodiversity. Classic examples include predators that control prey populations, thereby preventing any single species from dominating and allowing many others to thrive. Their influence is not based on abundance but on their ecological function.

Professor: Let me give you a concrete example from a Pacific coast rocky shore. The keystone species there is the sea otter. Sea otters feed on sea urchins. Now, if you remove the otters, what happens? Sea urchin populations explode. They overgraze on kelp forests, destroying the habitat that shelters fish, crabs, and other invertebrates. Within a few years, the once-rich kelp forest becomes a barren area with almost no marine life. But when otters are present, they keep urchin numbers low, allowing kelp to grow tall and dense. This kelp forest then supports a diverse community of species. So, you see, the otter—though not the most abundant—has a massive impact on the whole ecosystem's health and variety.