托福口语 Task 4

Today, we're going to talk about a fascinating concept in ecology called 'keystone species.' A keystone species is a species that has a disproportionately large effect on its environment relative to its abundance. Remove it, and the entire ecosystem can collapse. Let me give you two classic examples. First, consider the sea otter in the Pacific kelp forests. Sea otters prey on sea urchins. Without otters, urchins multiply and devour the kelp, destroying the habitat for fish and other marine life. But when otters are present, they keep urchin numbers low, allowing kelp forests to thrive, supporting a rich biodiversity. Second, think about the gray wolf in Yellowstone National Park. Wolves control the elk population. Without wolves, elk overgraze willows and aspens, leading to erosion and loss of bird habitats. When wolves were reintroduced, they reduced elk numbers and changed their grazing behavior, allowing vegetation to recover, which stabilized riverbanks and brought back many species. So, in both cases, these predators act as keystone species, maintaining balance in their ecosystems.