托福口语 Task 3

In biology, the term 'keystone species' refers to a species that has a disproportionately large effect on its environment relative to its abundance. Unlike dominant species, which are numerous and directly shape the habitat, keystone species exert their influence through key interactions, such as predation or competition. Their presence maintains the structure and diversity of the ecosystem. If a keystone species is removed, the ecosystem can undergo dramatic changes, often leading to a collapse or a significant loss of biodiversity.

Professor: Let me give you a classic example. In the Pacific Northwest, sea otters are a keystone species. They feed on sea urchins. Without otters, sea urchins multiply and devour kelp forests. Kelp provides shelter and food for many fish and invertebrates. In the 1800s, fur traders hunted otters nearly to extinction. As a result, urchin populations exploded, and they destroyed the kelp forests, causing the fish and other marine life to disappear. When otters were later protected and reintroduced, they ate the urchins again. The kelp forests recovered, and the fish and other species returned. So you see, the otter's presence—though not numerous—kept the whole ecosystem balanced.