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. Such species play a critical role in maintaining the structure of an ecological community. Their presence influences the types and numbers of other species in the ecosystem. When a keystone species is removed, the ecosystem may undergo significant changes, often leading to a loss of biodiversity and even the collapse of the community. Classic examples include predators that control prey populations, thereby preventing overgrazing or overpopulation.

Professor: Let me give you a real-world example of a keystone species – the sea otter. In the kelp forest ecosystems off the Pacific coast, sea otters are the top predators. They feed on sea urchins. Without otters, the urchin population explodes, and they devour the kelp, creating barren areas with no kelp. Kelp is crucial because it provides habitat and food for many fish and other marine organisms. When otters are present, they keep urchin numbers in check, allowing kelp to thrive. This supports a diverse community of fish, crabs, and other species. So, in this case, the sea otter has a huge impact on the ecosystem despite being just one species, which perfectly illustrates the concept of a keystone species.