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, which led to an overpopulation of elk. These elk overgrazed the vegetation, especially young willow and aspen trees along rivers. This had a cascading effect on other species, like beavers, which depend on those trees for food and building dams. When wolves were reintroduced in 1995, they not only reduced the elk population but also changed elk behavior—the elk avoided open river areas where they were more vulnerable to predators. This allowed the vegetation to recover. The regrowth of trees stabilized the riverbanks and provided habitat for beavers, birds, and other animals. Interestingly, even the rivers themselves changed—their channels became more stable and less prone to erosion. So, this shows how a single predator can have a profound impact on the entire ecosystem, a concept known as a trophic cascade.
❓ 質問 What was one of the main effects of the elk overpopulation in Yellowstone National Park before the wolves were reintroduced?
A. A. The elk population decreased due to lack of food.
B. B. Young trees along rivers were overgrazed, affecting other species.
C. C. Beavers increased because there were more trees.
D. D. Riverbanks became more stable due to abundant vegetation.
✅ 正解
B. Young trees along rivers were overgrazed, affecting other species.
TOEFL Listening Tips
Skim the questions first to predict the content
Watch for signal words in lectures and conversations (however, actually, in fact)
For main idea questions, focus on the beginning; for detail questions, focus on examples and data
For attitude questions, pay attention to the speaker's tone and word choice
Take notes on keywords; don't try to write down every word