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. Removing a keystone species from an ecosystem can cause a dramatic shift in the habitat structure and a loss of biodiversity, even if the species itself is not the most numerous. The concept highlights how certain organisms play a critical role in maintaining the balance of an ecosystem, much like the keystone in an arch that supports the entire structure.

Professor: Let me give you a classic example: the sea otter in the kelp forest ecosystem. Sea otters are not the most abundant animals in the Pacific Ocean, but they are a keystone species. They feed on sea urchins. Without otters, sea urchins multiply rapidly and graze on the kelp, destroying entire underwater forests. These kelp forests provide habitat and food for many fish, crabs, and other marine life. When otters were hunted to near extinction in the 19th century, kelp forests disappeared and biodiversity plummeted. After otters were protected and reintroduced, they controlled the urchin population, allowing kelp to regrow. Soon, the whole ecosystem—fish, invertebrates, and even seabirds—returned. So the otter’s presence, though small in number, is vital for the entire community.