托福口语 Task 3

In biology, the concept of 'keystone species' refers to a species whose presence and role in an ecosystem have a disproportionately large effect on other organisms within the ecosystem. Keystone species help maintain the structure and health of their environment. If a keystone species is removed, the ecosystem can dramatically change or even collapse, leading to the loss of many other species. Their influence often comes from their unique feeding habits or behaviors that shape the habitat for others.

Professor: Let me give you a classic example: the sea otter along the Pacific coast. Sea otters eat sea urchins, which in turn feed on kelp forests. Without otters, sea urchins multiply rapidly and destroy the kelp, creating 'urchin barrens' where few other organisms can survive. But when otters are present, they keep urchin numbers in check, allowing kelp forests to thrive. Those forests then provide food and shelter for fish, crabs, and other marine life. So, the otter's role is small in terms of its numbers, but its impact is huge—it literally maintains the entire ecosystem's balance. This shows how a keystone species can shape its environment.