托福口语 Task 3

In environmental science, the concept of 'keystone species' refers to a species that has a disproportionately large effect on its ecosystem relative to its abundance. Removing a keystone species can trigger a cascade of changes, drastically altering the habitat and reducing biodiversity. Classic examples include predators that control herbivore populations, thereby protecting plant communities. The concept emphasizes that certain species are more critical to ecosystem stability than others, and their presence is vital for maintaining the structure and function of the community.

Professor: Let me illustrate with a well-known example from the Pacific coast—the sea otter. In kelp forests, sea otters prey on sea urchins. Without otters, urchin populations explode, and they devour the kelp, creating 'urchin barrens' where almost no marine life can thrive. But when otters are present, they keep urchins in check, allowing kelp to grow into dense underwater forests. These forests provide shelter and food for fish, crabs, and other organisms. In fact, researchers found that areas with otters had much higher biodiversity than areas without them. So the sea otter, despite being relatively few in number, is a classic keystone species—its removal leads to a dramatic collapse of the entire ecosystem.