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 from an ecosystem can lead to drastic changes, often causing a cascade of negative effects that destabilize the entire community. These species play a critical role in maintaining the structure, diversity, and balance of their habitats, much like a keystone in an arch that supports the whole structure.

Professor: Let me give you a classic example—the sea otter. In the kelp forest ecosystems off the Pacific coast, sea otters are a keystone species. They prey on sea urchins. When sea otters are present, they keep the urchin population in check. This allows kelp to thrive, creating a dense underwater forest that provides habitat and food for fish, crabs, and other marine life. But when sea otters were hunted to near extinction in the 1800s, urchin populations exploded. The urchins devoured the kelp, leaving barren areas with no plant life. Fish and other species disappeared, and the whole ecosystem collapsed. When otters were reintroduced and protected, the kelp forests returned, and biodiversity recovered. This shows how one species can control the entire ecosystem.