When wolves were reintroduced to Yellowstone National Park in 1995, scientists expected them to reduce elk populations. What they didn't anticipate was the cascade of effects: fewer elk meant less overgrazing, which allowed willows and aspens to recover. More trees stabilized riverbanks and created habitat for beavers, whose dams further transformed the landscape. Researchers now study this as a textbook example of 'trophic cascades.'
Based on the passage, it can be inferred that
wolves directly created habitats for beavers
ecosystems may contain interconnected relationships that produce far-reaching effects from single changes
scientists perfectly predicted all consequences of wolf reintroduction
elk populations in Yellowstone are now extinct
Correct Answer: B
Choice B is the best answer. One change (wolves) triggered a chain of connected effects.
- Context clues: Wolves → elk reduction → plant recovery → riverbank changes → beaver habitat—a cascade.
- Meaning: The chain of effects shows profound interconnection in the ecosystem.
- Verify: "Trophic cascades" describes exactly this kind of interconnected chain reaction.
💡 Strategy: When a passage tracks a chain of cause-and-effect, infer the interconnected nature of the system.
Choice A is incorrect because wolves affected elk, which affected plants, which affected beavers—indirect, not direct. Choice C is incorrect because scientists "didn't anticipate" the cascade. Choice D is incorrect because elk populations were reduced, not eliminated.