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 other species in the ecosystem. If a keystone species is removed, the ecosystem may undergo significant changes, often leading to a loss of biodiversity and collapse of the habitat structure. Classic examples include predators that control prey populations or species that provide critical resources for others.

Professor: Let me give you a concrete example from a marine ecosystem. Consider the sea otter, a keystone species in kelp forest habitats along the Pacific coast. Sea otters feed on sea urchins. Without otters, sea urchins multiply rapidly and overgraze on kelp, which forms dense underwater forests. These kelp forests provide food and shelter for many fish, invertebrates, and other marine life. When otters are present, they keep the sea urchin population in check, allowing kelp to thrive. This supports a rich biodiversity. However, when sea otters were hunted nearly to extinction in some areas, the kelp forests disappeared, and the ecosystem collapsed. With otter conservation efforts, the otters returned, and the kelp forests and their associated species recovered. This clearly shows how a single species can drastically influence the entire ecosystem.