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 biodiversity and the structure of an ecosystem. When a keystone species is removed, the ecosystem can dramatically change, often leading to the decline or extinction of other species. These species are not necessarily the most numerous, but they play a critical role in the food web or habitat structure, such as by controlling predator-prey relationships or providing essential resources for other organisms.

Professor: Let's look at a classic example from the Pacific Northwest—the sea otter. In coastal kelp forests, sea otters are a keystone species. They prey on sea urchins, which are herbivores that feed on kelp. When sea otters are present, they keep the sea urchin population in check, allowing kelp forests to thrive. Kelp provides habitat and food for many fish, invertebrates, and other marine life. However, when sea otters were hunted to near extinction in the 1800s, the sea urchin population exploded. The urchins consumed vast amounts of kelp, turning lush forests into barren areas, and many species that depended on the kelp disappeared. When sea otters were later protected and reintroduced, the kelp forests recovered, and biodiversity returned. This shows how a single predator can shape an entire ecosystem.