TOEFLスピーキング 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 the types and numbers of other species in the ecosystem. If a keystone species is removed, the ecosystem may drastically change or even collapse, often leading to a loss of biodiversity. Classic examples include predators that control prey populations, or species that provide essential resources for others.

Professor: Let me give you a classic example of a keystone species—the sea otter. In the kelp forest ecosystems along the Pacific coast, sea otters are not the most numerous animals, but they are crucial. They feed on sea urchins. Without otters, sea urchins multiply rapidly and devour the kelp. Kelp is a type of seaweed that provides shelter and food for many fish and invertebrates. When otters were hunted to near extinction in some areas, the urchin population exploded, and the kelp forests were destroyed. This led to a dramatic drop in fish and other marine life that depended on the kelp. But when otters were reintroduced and protected, they kept urchins in check, and the kelp forests recovered, bringing back the biodiversity. So, the otter's effect is huge compared to its numbers.