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 the collapse of the ecosystem's structure and diversity. These species often control the population of other organisms, maintain habitat, or provide critical resources. Their presence is essential for maintaining the balance of the ecosystem, even if they are not the most numerous organisms.

Professor: Let me give you a classic example: the sea otter. In the kelp forest ecosystems off the coast of the Pacific Ocean, sea otters are a keystone species. They eat sea urchins, which are herbivores that feed on kelp. Without otters, the sea urchin population explodes, and they devour the kelp forests. This destroys the habitat for many fish, invertebrates, and other organisms that rely on the kelp. When otters are present, they keep the urchin numbers in check, allowing kelp forests to thrive, which supports high biodiversity. So even though otters are not the most abundant species, their presence or absence dramatically alters the whole ecosystem.