TOEFLスピーキング 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 dramatic changes in the structure of the ecosystem, often leading to the decline or extinction of other species. The concept highlights that certain species play a critical role in maintaining the balance and diversity of their habitat, even if they are not the most numerous organisms present.

Professor: Let me give you a classic example from the Pacific coast. The sea otter is a keystone species in kelp forest ecosystems. Sea otters eat sea urchins, which in turn feed on kelp. When otters are present, they keep the urchin population in check, allowing kelp forests to thrive. These forests provide shelter and food for many fish and invertebrates. However, in areas where otters were hunted to near extinction in the 1800s, the urchin population exploded. The urchins overgrazed the kelp, destroying the forests. As a result, many fish species lost their habitat and disappeared from those areas. Later, when otters were reintroduced, the kelp forests gradually recovered, and fish populations returned. So the otter, even though it is not the most numerous animal, has a huge effect on the whole ecosystem.