托福口语 Task 3

In biology, the term 'keystone species' refers to a species whose impact on its ecosystem is disproportionately large relative to its abundance. Removing a keystone species can cause a cascade of changes, often leading to the collapse of the ecosystem's structure. Classic examples include predators that control prey populations, thereby maintaining the balance among multiple species. The concept highlights how a single organism can be vital for the survival of many others.

Professor: Let me give you a classic example from the Pacific coast. The sea otter is a keystone species. In areas where sea otters are present, they eat sea urchins. Sea urchins, in turn, feed on kelp forests. Without otters, urchin populations explode, and they devour the kelp, turning the lush underwater forest into a barren area. This destroys the habitat for fish, crabs, and other organisms that rely on kelp. But when otters are present, they keep urchin numbers in check, allowing kelp to thrive, which supports a diverse community. So, a relatively small number of otters has a huge effect on the entire ecosystem's health and biodiversity.