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 cause a cascade of changes, often leading to the collapse of the ecosystem's structure. Classic examples include predators that control prey populations, thereby maintaining biodiversity. The concept emphasizes that certain species play pivotal roles that cannot be replaced by others, and their presence is critical for the stability of the community.

Professor: Let me give you a real example: the sea otter along the Pacific coast. In the past, hunters nearly wiped out sea otters for their fur. When otters disappeared, their main prey—sea urchins—multiplied explosively. The urchins then ate the kelp forests down to the roots, destroying the habitat for fish, crabs, and other organisms. The whole underwater community collapsed. Later, when otters were reintroduced and protected, they preyed on the urchins again, allowing kelp to regrow. Soon, fish and other species returned, and the ecosystem recovered. Notice that the otters were not numerous—they were far fewer than the urchins—but their effect was enormous. That’s what makes them a keystone species.