TOEFL Speaking 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 causes significant changes in the population numbers of other species, often leading to a collapse or drastic alteration of the entire community structure. Classic examples include predators that control herbivore populations, or species that provide critical resources for many others.

Professor: Let me give you a concrete example. In the Pacific Northwest, the sea otter is a keystone species. Sea otters feed on sea urchins, which in turn eat kelp. When otters are present, they keep the urchin population in check, allowing kelp forests to thrive. These kelp forests provide habitat and food for fish, crabs, and other marine life. But when otters were hunted nearly to extinction in the 1800s, urchin populations exploded. They devoured the kelp, creating 'urchin barrens' where few other species could survive. Fish and invertebrates that depended on kelp disappeared, and the whole ecosystem changed dramatically. When otters were later reintroduced, the kelp forests recovered, and biodiversity returned. So the otter's influence far exceeds its numbers.