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 prolonged drought kills off the riverbank willows, and with that shared food resource gone both the beaver and the elk populations steadily decline.
- A growing beaver population builds more dams, and the resulting ponds in turn draw in wolves that prefer to hunt their prey near open water.
- Elk numbers hold perfectly steady whether or not wolves are present, because the elk simply broaden their diet to a range of new plants.
- Wolves begin to directly hunt and sharply reduce the beaver population, eliminating the very dams that had once created the park's ponds.
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