TOEFLリスニング

🎧 リスニング原文

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 in the 1990s. For decades, the park had no wolves, mainly due to hunting and government eradication programs. Without wolves, the elk population exploded, and they overgrazed the willows and aspens along riverbanks. This led to soil erosion and a decline in biodiversity, because birds and beavers lost their habitat. Then, in 1995, scientists reintroduced 14 wolves from Canada. The immediate effect was a drop in elk numbers, but the more surprising effect was a change in elk behavior. They started avoiding open valleys, which allowed vegetation to recover. As trees grew taller, beavers returned and built dams, which created wetlands for fish and amphibians. Even the rivers' paths changed as the banks stabilized. This is what ecologists call a 'trophic cascade'—where top predators indirectly shape the entire ecosystem. And it shows that small interventions can have huge, cascading consequences.