托福口语 Task 3

In biology, the term 'keystone species' refers to a species whose impact on its ecosystem is disproportionately large relative to its abundance. Removing a keystone species can trigger a cascade of changes, often leading to the collapse or drastic alteration of the ecosystem. This concept highlights how certain organisms play a critical role in maintaining the structure, diversity, and balance of their habitats, even if they are not the most numerous or obvious species present.

Professor: Let me give you a classic example. In the Pacific Northwest, the sea otter is a keystone species. Sea otters feed on sea urchins, which in turn eat kelp forests. When otters are present, they keep the urchin population in check, allowing kelp forests to thrive. These forests provide shelter and food for fish, crabs, and other marine life. In the 1800s, fur traders hunted otters nearly to extinction. Without otters, urchin populations exploded, and they devoured the kelp. The kelp forests disappeared, and with them, many fish species and the ecosystem became barren. Later, when otters were reintroduced and protected, they reduced urchin numbers, kelp forests regrew, and the whole community returned. So, a small number of otters—far fewer than the urchins or fish—had a huge effect on the entire ecosystem.