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. Such species play a critical role in maintaining the structure of an ecological community. Their presence influences many other organisms in the ecosystem and helps to determine the types and numbers of various other species. The removal of a keystone species can cause a dramatic shift in the habitat, often leading to the collapse or significant alteration of the ecosystem.

Professor: So, let's talk about a classic example of a keystone species: the sea otter. In the kelp forest ecosystems off the Pacific coast, sea otters are not very numerous, but they have a huge impact. You see, sea otters feed on sea urchins. Without otters, sea urchins multiply quickly and devour the kelp, creating what we call an 'urchin barren' – a lifeless area where kelp can't grow. But when otters are present, they keep the urchin population in check. This allows kelp forests to thrive, which then provide habitat and food for fish, crabs, and other marine life. So, even though otters are only a small fraction of the total biomass, their hunting behavior maintains the entire ecosystem's health and biodiversity.