TOEFL Speaking Task 3

In biology, 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 dramatic loss of biodiversity or collapse of the ecosystem. Classic examples include predators that control prey populations, or species that create or modify habitats, such as beavers building dams, which create wetlands for many other organisms.

Professor: Today, let’s look at a real 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 their role is huge. Sea otters feed on sea urchins. Without otters, sea urchin populations explode, and they devour the kelp forests. When kelp disappears, many fish, crabs, and other organisms lose their habitat and food source, so biodiversity drops sharply. But when otters are present, they keep urchin numbers in check, allowing kelp to thrive. That kelp provides shelter and food for countless species, maintaining a healthy, diverse ecosystem. So, you see, even though otters are relatively few in number, their presence or absence makes a massive difference.