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 a dramatic shift in the habitat structure and a loss of biodiversity, often leading to the collapse of the entire community. The concept highlights how certain organisms play a critical role in maintaining ecological balance, even if they are not the most numerous or dominant in terms of biomass.

Professor: Let me give you a classic example—the sea otter in the Pacific kelp forests. Sea otters are not the most numerous creatures there, but they are keystone predators. They feed on sea urchins. When sea otters are present, they keep the urchin population in check. That allows kelp—giant seaweed—to grow thick and form underwater forests. These forests provide habitat and food for fish, crabs, and other marine life. But in the 19th century, fur traders hunted otters nearly to extinction. Without otters, the urchin population exploded. The urchins ate the kelp down to the roots, so the entire forest disappeared. Many fish and invertebrates lost their homes. The result was an underwater desert with very little life. Later, when otters were reintroduced, the kelp forests gradually returned, and biodiversity increased again. So you see, a single predator, though not abundant, can hold the whole ecosystem together.