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 trigger a cascade of changes, often leading to a drastic reduction in biodiversity. These species play a critical role in maintaining the structure and balance of the ecological community, often by controlling the population of other species or providing essential resources.

Professor: Let me give you a classic example of a keystone species—the sea otter. In the kelp forest ecosystems off the Pacific coast, sea otters are not the most numerous animals, but their presence is crucial. Sea otters feed on sea urchins, which in turn graze on kelp. When sea otters are present, they keep sea urchin populations in check, allowing kelp forests to thrive. These forests provide habitat and food for many fish, invertebrates, and other marine life. However, when sea otters were hunted to near extinction in the 19th century, sea urchin populations exploded. The urchins devoured the kelp, creating 'urchin barrens' where little else could survive. This led to a sharp decline in local biodiversity. When sea otters were reintroduced and protected, the kelp forests recovered, and biodiversity returned. So, the sea otter clearly demonstrates how a single species can have a massive impact on the entire ecosystem.