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. The presence of a keystone species helps maintain the structure and diversity of an ecosystem. If such a species is removed, the ecosystem may undergo dramatic changes, often leading to a loss of biodiversity and the collapse of the community. Classic examples include predators that control prey populations, thereby preventing overgrazing and allowing various plant and animal species to thrive.

Professor: Let me give you a concrete example. In the Pacific Northwest, there's a species of sea otter that lives in kelp forests. Sea otters are considered a keystone species because they prey on sea urchins. Sea urchins feed on kelp, and if their population grows unchecked, they can destroy entire kelp forests. When otters are present, they keep the urchin population low, which allows the kelp to flourish. The kelp then provides habitat and food for fish, crabs, and other marine life. In areas where otters were hunted to near extinction, the urchin population exploded, and the kelp forests disappeared. As a result, the fish and other species that depended on the kelp also vanished. So, the otters have a huge effect on the ecosystem, far out of proportion to their numbers.