托福口语 Task 3

In environmental science, the concept of '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, drastically altering the habitat and reducing biodiversity. The concept emphasizes that certain organisms play a critical role in maintaining the structure and balance of an ecosystem, even if they are not the most numerous or obvious members.

Professor: Let me give you a classic example—sea otters along the Pacific coast. In areas where sea otters were hunted to near extinction, the population of sea urchins exploded. These urchins grazed on kelp forests, turning once-thriving underwater forests into barren, rocky areas. As a result, fish and other marine life that depended on kelp disappeared. But when sea otters were reintroduced, they preyed on the urchins, keeping their numbers in check. This allowed kelp forests to recover, and soon the ecosystem flourished again with a rich diversity of species. So, although sea otters are not the largest or most numerous creatures, their presence or absence makes a huge difference—exactly what we mean by a keystone species.