Narrator: Listen to part of a lecture in an environmental science class. Professor: Today we're going to talk about a fascinating example of ecological restoration—the reintroduction of gray wolves to Yellowstone National Park. When wolves were removed in the 1920s, elk populations exploded, and they overgrazed the young willow and aspen trees. This caused a decline in beavers, because beavers rely on those trees for food and building dams. Without beavers, the waterways changed, and many aquatic species suffered. In 1995, scientists reintroduced wolves. The wolves changed elk behavior—the elk avoided open valleys, allowing trees to regrow. This regrowth stabilized riverbanks, provided habitat for birds, and brought beavers back. The beavers' dams created ponds that supported fish and amphibians. This process is called a 'trophic cascade,' where a top predator indirectly benefits many other species. Interestingly, the wolves also helped scavengers like bears and ravens by leaving carcasses. So, a single species can have a huge ripple effect on the entire ecosystem. Any questions?
❓ 题目 According to the professor, what was a direct consequence of the wolves' removal from Yellowstone in the 1920s?
A. A. The beaver population increased due to more available trees.
B. B. Elk overgrazed young trees, which led to a decline in beavers.
C. C. Riverbanks became more stable because of increased vegetation.
D. D. Aquatic species thrived due to changes in the waterways.
✅ 正确答案
B. Elk overgrazed young trees, which led to a decline in beavers.