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. Removing a keystone species from an ecosystem can cause a cascade of changes, often leading to a dramatic decline in biodiversity. This concept highlights how certain organisms play a critical role in maintaining the structure and health of their ecological community, much like a keystone in an arch that supports the entire structure.

Professor: Let me give you a classic example. In the Pacific Northwest, the sea otter is a keystone species in kelp forest ecosystems. Sea otters prey on sea urchins, which feed on kelp. When otters are present, they keep the urchin population in check, allowing kelp forests to thrive. These forests provide habitat and food for many fish, invertebrates, and other marine life. However, in areas where otters were hunted to near extinction, the urchin population exploded. The urchins devoured the kelp, creating barren underwater landscapes. As a result, fish and other species that depended on the kelp disappeared, drastically reducing biodiversity. When otters were later reintroduced, the kelp forests recovered, and biodiversity returned. This shows how a single predator can control the entire ecosystem.