Listen to part of a lecture in an environmental science class. Professor: Today we're going to discuss a fascinating example of ecological restoration: the reintroduction of gray wolves to Yellowstone National Park in the 1990s. As you know, wolves were hunted to extinction in the park by the 1920s. Without wolves, the elk population exploded. The elk overgrazed the willow and aspen trees along riverbanks. This led to erosion and a decline in bird populations that depended on those trees. In 1995, scientists reintroduced 14 wolves from Canada. The effects were dramatic. The elk changed their behavior, avoiding areas where wolves were active. This allowed vegetation to recover. With trees growing again, beavers returned, and their dams created habitats for fish and amphibians. Even the rivers' paths became more stable. This process, known as a trophic cascade, shows how a top predator can shape an entire ecosystem. The park is now more balanced, though debates continue about wolf management in the region.
❓ 题目 What was the main consequence of the elk population explosion in Yellowstone before the wolf reintroduction?