TOEFL Speaking Task 3

In biology, the concept of 'keystone species' refers to a species whose presence and role within an ecosystem has a disproportionately large effect on other organisms within the ecosystem. Unlike dominant species, which are abundant and have high biomass, keystone species are often not the most numerous, yet their activities shape the habitat and influence the survival of many other species. Removing a keystone species can lead to significant changes in the ecosystem's structure and biodiversity, often causing a cascade of extinctions or population shifts.

Professor: Let me give you a classic example: the sea otter. In the kelp forest ecosystems off the Pacific coast, sea otters are not the most abundant creatures, but they are crucial. They prey on sea urchins. Now, sea urchins feed on kelp, which forms dense underwater forests that provide shelter and food for many fish and invertebrates. Without otters, urchin populations explode, and they devour the kelp, leaving barren areas. This destroys the habitat for countless species. But when otters are present, they keep urchin numbers in check, allowing kelp to thrive. This supports a rich community of species, from fish to crabs. So, despite being relatively few in number, the otter’s hunting behavior has a massive impact on the entire ecosystem’s health and diversity.