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, often leading to the collapse of an ecosystem. The concept emphasizes that certain organisms, though not the most numerous, play a critical role in maintaining the structure and diversity of their community.

Professor: Let me give you a classic example. In the Pacific Northwest, the sea otter is a keystone species. Otters eat sea urchins, which feed on kelp forests. When otters were hunted nearly to extinction, the sea urchin population exploded. The urchins devoured the kelp, creating barren underwater areas. As a result, many species that depended on kelp, like fish and crabs, lost their habitat and food sources. But when otters were protected and reintroduced, they controlled the urchin population. Kelp forests recovered, and biodiversity returned. So you see, the otter’s presence—though not numerous—keeps the whole ecosystem balanced.