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. These species play a critical role in maintaining the structure of an ecological community. Their presence influences the types and numbers of other species in the ecosystem. When a keystone species is removed, the ecosystem can undergo significant changes, often leading to a decline in biodiversity and the collapse of the community structure. The concept highlights how certain organisms are pivotal to the health and stability of their habitats.

Professor: Let me give you a classic example. In the Pacific Northwest, the sea otter is a keystone species. Sea otters feed on sea urchins, which in turn graze on kelp forests. 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 and invertebrates. However, in areas where otters were hunted to near extinction, urchin populations exploded. The urchins devoured the kelp, creating barren underwater areas. With the kelp gone, fish and other species lost their homes, and biodiversity plummeted. This shows how removing one species—the otter—triggered a cascade of effects that drastically altered the entire ecosystem.