托福口语 Task 3

In ecology, the concept of '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 drastic reduction in biodiversity. These species play a critical role in maintaining the structure and balance of their habitat, much like a keystone in an arch—if removed, the entire structure collapses. Examples often include predators that control prey populations or species that provide essential resources for others.

Professor: Let me give you a classic example: the sea otter. In the kelp forest ecosystems along the Pacific coast, sea otters are a keystone species. They prey on sea urchins, which are herbivores that feed on kelp. Without sea otters, the urchin population explodes and they devour the kelp forests, leaving behind 'urchin barrens'—areas with almost no kelp. This destroys the habitat for fish, crabs, and other organisms that depend on kelp. However, when sea otters are present, they keep urchin numbers in check, allowing kelp to thrive. As a result, the entire ecosystem stays rich and diverse. So, the sea otter, though not the most numerous species, has a huge impact on its environment—exactly what defines a keystone species.