A species that is harmless at home can be ruinous somewhere else, and the reason is usually absence rather than presence. In its native range a plant or animal is held in check by the diseases, predators, and competitors it evolved alongside. Moved to a new continent without them, the same organism may spread at a rate it never managed at home. This is why control programmes so often begin by looking for a natural enemy from the original range, and why that step is approached with great caution. The history of such introductions includes several that became worse problems than the species they were meant to control.
It can be inferred from the passage that introducing a natural enemy is risky because
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Correct answer to question 54 (Paragraph Comprehension): the enemy may itself spread beyond its intended target
Method
The passage has just explained that any organism moved from its native range arrives without its own controls. The introduced enemy is subject to the same logic, which is why some became worse problems than the species they targeted.
Updated: November 2026
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