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 dramatic changes, often leading to a loss of biodiversity and even the collapse of the community. The concept highlights how certain organisms are pivotal to the survival of many others.

Professor: Let me give you an example from a rocky shore ecosystem. Consider the sea otter. Sea otters are not the most numerous creatures in the Pacific coastal waters, but they are a keystone species. They feed on sea urchins. When otters are present, they keep the sea urchin population in check. This allows kelp forests to thrive, because urchins would otherwise devour the kelp. Kelp forests provide habitat and food for fish, crabs, and many other organisms. Now, when sea otters were hunted nearly to extinction in some areas, the urchin population exploded. They destroyed the kelp forests, and the ecosystem became a barren area with very few species. But when otters were reintroduced and their numbers recovered, the kelp forests returned, and biodiversity increased again. So, the otter's impact is huge relative to its numbers, and its presence or absence completely changes the community.