medium · LSAT Logical Reasoning
Ecologist: For decades the riverbank ecosystem in this valley had no wolves, its elk herd grew enormous, and the elk overgrazed the willows and aspens along the streams. Several years after wolves were reintroduced, the elk population dropped to roughly its historical size and the streamside vegetation returned to its former abundance. The reintroduction of wolves has therefore restored the ecosystem's balance.
Which one of the following principles, if valid, most helps to justify the ecologist's reasoning?
- When a native predator's return brings an ecosystem's prey numbers and vegetation back to their historical levels, that ecosystem counts as restored.
- An ecosystem can never recover its lost streamside vegetation for as long as a top-level predator such as the gray wolf remains entirely absent from it.
- Wolves ought to be reintroduced to each and every national park in which they once lived but from which they have since been wholly driven out.
- Predators are by a wide margin the single most important factor in maintaining the long-term ecological health of any riverbank ecosystem at all.
- Once a natural ecosystem has been seriously disrupted, no later human intervention can ever fully return it to its original prior condition.
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