TOEFLスピーキング Task 3

In biology, the term "keystone species" refers to a species whose presence and role in an ecosystem have a disproportionately large effect on other organisms within the system. Unlike dominant species, keystone species are often not abundant, but their activities—such as predation, herbivory, or providing critical resources—shape the structure and diversity of the entire community. If a keystone species is removed, the ecosystem may undergo dramatic changes, often leading to a loss of biodiversity or even ecosystem collapse.

Professor: Let me give you a classic example: the sea otter in the kelp forest ecosystem of the Pacific Ocean. Sea otters are not the most numerous creatures there, but they are vital. They feed on sea urchins. Now, sea urchins love to eat kelp, the large seaweed that forms underwater forests. Without otters, sea urchin populations explode, and they devour the kelp, destroying the forest. This kills the habitat for fish, crabs, and other marine life. But when sea otters are present, they keep the urchin numbers in check. Kelp thrives, providing shelter and food for many species. So, by controlling the urchin population, sea otters maintain the health and diversity of the entire ecosystem. That's why they are a keystone species.