TOEFL Speaking Task 3

In biology, the term "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 a large biomass, keystone species are often not the most numerous, but their activities, such as predation or providing critical resources, shape the entire community. Removing a keystone species can lead to a cascade of changes, often resulting in a dramatic loss of biodiversity and the collapse of the ecosystem's structure.

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 numerous species, but they are crucial. You see, sea otters prey on sea urchins. Without otters, the urchin population explodes and they eat the kelp forests down to the roots, leaving barren areas with almost no life. But when otters are present, they keep urchin numbers in check, allowing kelp to thrive. Kelp provides habitat and food for fish, crabs, and many other organisms. So, by simply eating urchins, sea otters indirectly support the whole community. When otters were hunted to near extinction in some areas, the kelp disappeared, and so did the fish. When otters were reintroduced, kelp and biodiversity rebounded. That shows how a single predator, not abundant in numbers, can control the entire ecosystem.