Single correct option
Directions: The passage below is followed by four alternate summaries. Choose the option that best captures the essence of the passage.
Predators are often imagined as engines of scarcity, thinning prey populations that would otherwise overrun their habitat. But the reintroduction of wolves to Yellowstone in 1995 revealed a subtler mechanism: it was not merely the number of elk that wolves reduced, but elk behaviour. Elk began avoiding open valley floors where wolves could easily hunt them, allowing willow and aspen saplings in those areas to grow past the height at which browsing elk had previously destroyed them each year. The resulting vegetation recovery altered riverbank erosion patterns and, eventually, even the course of some streams — effects that owed as much to fear-driven behavioural change as to predation itself.
Predators are often imagined as engines of scarcity, thinning prey populations that would otherwise overrun their habitat. But the reintroduction of wolves to Yellowstone in 1995 revealed a subtler mechanism: it was not merely the number of elk that wolves reduced, but elk behaviour. Elk began avoiding open valley floors where wolves could easily hunt them, allowing willow and aspen saplings in those areas to grow past the height at which browsing elk had previously destroyed them each year. The resulting vegetation recovery altered riverbank erosion patterns and, eventually, even the course of some streams — effects that owed as much to fear-driven behavioural change as to predation itself.
- A. Wolves in Yellowstone reduced elk populations so significantly that riverbank vegetation was able to recover fully.
- B. Predator reintroduction can reshape ecosystems through behavioural changes in prey, not just through reducing prey numbers.
- C. Yellowstone's river courses changed entirely because of the reintroduction of wolves in 1995.
- D. Elk populations avoid open valley floors regardless of whether predators are present in an ecosystem.
Show answer and solution
Answer: (B) Predator reintroduction can reshape ecosystems through behavioural changes in prey, not just through reducing prey numbers.
The passage's key point is that the ecological effect worked through changed elk behaviour (fear of open ground), not simply reduced elk numbers — captured by option B. Option A wrongly attributes the effect to population reduction alone; C overstates a partial, gradual change into a complete one; D contradicts the passage, which says avoidance was driven by wolf presence.
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