托福口语 Task 3

In ecology, the concept of '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 a dramatic loss of biodiversity or habitat structure. Such species play a critical role in maintaining the balance of the community, often through predation, competition, or providing resources that other organisms depend on.

Professor: Let me give you a classic example. In the Pacific Northwest, sea otters are a keystone species. They feed on sea urchins. Without otters, sea urchin populations explode. The urchins then overgraze on kelp forests, which provide habitat and food for fish, crabs, and other marine life. Once the kelp is gone, those species disappear, and the entire underwater ecosystem collapses. But when otters are present, they keep urchin numbers in check, allowing kelp forests to thrive. This supports a rich diversity of species. So, even though otters aren't numerous, their presence has a huge effect on the whole ecosystem.