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 an organism 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 kelp forest ecosystems of the Pacific coast. Sea otters prey on sea urchins, which in turn feed on kelp. When otters are present, they keep urchin populations in check, allowing kelp forests to thrive. These forests provide habitat and food for many fish and invertebrates. However, when otters were hunted to near extinction, urchin populations exploded, decimating the kelp, leading to what we call 'urchin barrens'—areas devoid of life. Second, think about the gray wolf in Yellowstone National Park. Wolves were reintroduced in 1995 after being absent for 70 years. They prey on elk, and with wolves around, elk avoid overgrazing riverbanks. This allowed willow and aspen trees to regrow, which stabilized the riverbanks and brought back beavers, songbirds, and other species. Both examples show how a single predator can shape an entire ecosystem.