TOEFL Speaking 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 many other organisms and helps determine the types and numbers of various other species in the ecosystem. If a keystone species is removed, the ecosystem may drastically change or even collapse, leading to a loss of biodiversity and habitat stability.

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 graze on kelp forests. When otters are present, they keep the sea urchin population in check, allowing kelp forests to thrive. These forests provide shelter and food for fish, crabs, and other marine life. But in areas where otters were hunted to near extinction, sea urchin populations exploded. The urchins devoured the kelp, creating barren underwater areas. With no kelp, many fish and invertebrates lost their habitat, and the entire ecosystem became less diverse and productive. When otters were reintroduced and protected, the kelp forests recovered, and so did the biodiversity. This shows how one species can have a huge impact on the whole community.