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 can cause a cascade of changes, often leading to a dramatic reduction in biodiversity or even the collapse of the ecosystem. The concept highlights how certain organisms play a critical role in maintaining the structure and balance of their habitat, much like a keystone in an arch that supports the entire structure.

Professor: Let me give you a classic example—the sea otter in the Pacific kelp forests. Sea otters are not the most numerous species in that ecosystem, but they have a huge effect. They prey on sea urchins. Sea urchins feed on kelp, which forms dense underwater forests that provide shelter and food for many fish and other organisms. When sea otters are present, they keep the urchin population in check, allowing kelp forests to thrive. But in areas where otters were hunted to near extinction, urchin populations exploded. The urchins ate the kelp down to the roots, destroying the entire forest. As a result, fish and other species lost their habitat, and biodiversity plummeted. So, by controlling the urchin population, the sea otter acts as a keystone species—its absence triggers a chain reaction that collapses the whole ecosystem.