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 dramatic changes in the population of other species, often leading to a collapse in biodiversity. These species play a critical role in maintaining the structure of a community, often by controlling the population of dominant competitors or providing essential resources for other organisms.

Professor: Let me give you a classic example: the sea otter in the Pacific coastal waters. Sea otters are not the most numerous creatures in that habitat, but they have a huge effect. They feed on sea urchins. Now, sea urchins eat kelp, which forms dense underwater forests. When otters are present, they keep the urchin population in check. This allows kelp to thrive, providing shelter and food for fish, crabs, and other marine life. But consider what happens when otters are removed—due to hunting or orca predation. Without otters, urchin populations explode. They devour the kelp, turning the lush forest into a barren area, called an urchin barren. Fish and other species lose their habitat, and biodiversity plummets. So a single species, the otter, controls the entire ecosystem's health, even though it's not the most abundant.