medium · LSAT Reading Comprehension
Trophic cascades are powerful indirect interactions that can control entire ecosystems, occurring when a change in the population of a top predator ripples through successive lower levels of the food web. A classic example occurred in Yellowstone National Park after the reintroduction of gray wolves. Wolves prey on elk, whose population had surged in the wolves' absence, leading to overgrazing of willow and aspen trees along riverbanks. With the wolves back, elk behavior shifted as they avoided open areas where they were vulnerable to predation. This allowed the vegetation to recover, which in turn provided habitat for songbirds and beavers. The beavers built dams that created ponds for fish and amphibians, illustrating how the presence of a single predator species can fundamentally reshape the physical and biological landscape.
Based on the example of trophic cascades provided, which of the following hypothetical scenarios would be most consistent with the author's description of how ecosystems function?
- Removing a reef's top shark frees mid-sized fish to multiply, those fish wipe out the algae-grazers, and the reef ends up smothered in algae.
- A drought kills off the willows, and with that resource gone both beaver and elk populations decline.
- A growing beaver population builds more dams, which in turn draws wolves that prefer to hunt near water.
- Elk numbers hold steady whether or not wolves are present, because elk simply broaden their diet to new plants.
- Wolves directly hunt and reduce the beaver population, eliminating the dams that once created ponds.
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