托福口语 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 decline in biodiversity. Such species often control the population of dominant competitors or provide critical resources for other organisms. The concept highlights that not all species are equally important in maintaining ecosystem structure and function, and conservation efforts often focus on protecting these pivotal species.

Professor: Let me give you a classic example—sea otters along the Pacific coast of North America. In areas where sea otters are present, they prey on sea urchins. Now, sea urchins feed on kelp, and kelp forests provide habitat and food for many fish and invertebrates. Without otters, urchin populations explode, and they overgraze the kelp, turning lush forests into barren areas called urchin barrens. This devastates the fish and other species that depend on kelp. However, where otters are protected and thrive, kelp forests flourish, and biodiversity is high. The otters are not numerous compared to the urchins or fish, but their presence has a huge effect on the entire community. So, the otter is a perfect example of a keystone species.