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 trigger a cascade of changes, often leading to a dramatic decline in biodiversity. These species often play critical roles such as controlling predator-prey dynamics or maintaining habitat structure, and their presence is vital for the overall health and stability of the ecosystem.

Professor: Let me give you a classic example. In Pacific coastal waters, the sea otter is a keystone species. Sea otters feed on sea urchins. When otters are present, they keep the urchin population in check. This allows kelp forests to thrive because urchins would otherwise devour the kelp. Kelp forests provide shelter and food for many fish and invertebrates. In the 19th century, fur traders hunted otters nearly to extinction. Without otters, urchin populations exploded, and they destroyed the kelp forests. As a result, fish populations plummeted, and the entire ecosystem collapsed. When otters were reintroduced and protected, they once again controlled the urchins, and the kelp forests recovered, bringing back the fish and other species. This shows how a single predator can have a huge effect on the whole community.