TOEFLスピーキング Task 3

In biology, the concept of 'keystone species' refers to a species that has a disproportionately large effect on its environment relative to its abundance. Removing a keystone species from an ecosystem can cause a cascade of changes, often leading to a dramatic shift in the habitat's structure and the loss of biodiversity. These species are critical for maintaining the balance of their ecological community, as they influence the survival and population of many other organisms. Their presence or absence can determine the overall health and stability of the ecosystem.

Professor: So, let me give you a classic example of a keystone species—the sea otter. In the kelp forest ecosystems along the Pacific coast, sea otters are not the most numerous animals, but they play a crucial role. They feed on sea urchins, which are herbivores that graze on kelp. Without sea otters, the sea urchin population explodes, and they devour the kelp, creating what we call 'urchin barrens.' This destroys the habitat for many fish and invertebrates that rely on kelp for shelter and food. But when sea otters are present, they keep the urchin population in check, allowing kelp forests to thrive. This supports a rich diversity of marine life. So, the sea otter has a huge impact on the ecosystem despite being relatively few in number.