Listen to part of a lecture in an environmental science class. Professor: Today we're going to discuss a concept that's crucial for understanding how ecosystems function: the trophic cascade. A trophic cascade occurs when predators at the top of a food chain indirectly affect the abundance of organisms at lower levels, not just by eating them, but by influencing their behavior. A classic example is the reintroduction of gray wolves to Yellowstone National Park in the 1990s. Before the wolves were reintroduced, elk populations had grown very large, and they overgrazed young aspen and willow trees along rivers. This caused a decline in beaver populations because beavers rely on willows for food and building dams. When wolves returned, they changed the elk's behavior—the elk avoided areas where they were vulnerable to attack, especially valleys and riverbanks. This allowed the trees to recover. The recovering willows provided habitat for songbirds and food for beavers, which in turn created ponds that supported fish and amphibians. So you see, a single top predator can have effects that ripple all the way down through the ecosystem.
❓ คำถาม What is the main point the professor makes about trophic cascades?
A. They only occur in large national parks like Yellowstone.
B. They involve indirect effects of top predators on lower trophic levels.
C. They are caused by changes in plant populations rather than animal populations.
D. They primarily affect beaver populations rather than other species.
✅ คำตอบที่ถูกต้อง
They involve indirect effects of top predators on lower trophic levels.
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
