托福口语 Task 3

In biology, the term 'keystone species' refers to a species whose presence and role within an ecosystem have a disproportionately large effect on other organisms within the ecosystem. Unlike dominant species, keystone species are often not the most abundant, but their activities—such as predation, herbivory, or providing critical resources—shape the entire community structure. If a keystone species is removed, the ecosystem may undergo dramatic changes, often leading to a loss of biodiversity and even the collapse of the habitat.

Professor: Let me give you a classic example. In the Pacific coastal waters, sea otters are a keystone species. They feed on sea urchins, which in turn graze on kelp forests. When otters are present, they keep the urchin population in check, allowing the kelp to thrive. Kelp forests provide shelter and food for many fish and invertebrates. However, when otters were hunted to near extinction in some areas, the urchin population exploded. The urchins devoured the kelp, creating barren underwater areas. As a result, the fish and other species that depended on the kelp disappeared. So, you see, the otter's role was far greater than its numbers might suggest—removing it triggered a cascade of negative effects.