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Difficulty: HardStrengthen

Entomologist: The Corvid butterfly population in the Aral Valley has plummeted over the last decade. Concurrently, there has been a sharp increase in the invasive thread-wasp, a predator of Corvid caterpillars. Nevertheless, the butterfly's decline is almost certainly due to a newly introduced agricultural herbicide, not the wasp. The herbicide, while directly harmless to the insects, accumulates in the soil and severely stunts the milkweed plants that Corvid caterpillars rely on exclusively for food.

Which one of the following, if true, most strengthens the entomologist's argument?

  1. A
    The thread-wasp is highly susceptible to the new agricultural herbicide, and its numbers drop significantly in localized areas where the herbicide is heavily applied.
  2. B
    Farmers in the Aral Valley who use the greatest amounts of the new herbicide are also the most likely to grow crops that inadvertently attract the thread-wasp.
  3. In neighboring valleys where the thread-wasp population has increased just as sharply but the new herbicide is not used, the Corvid butterfly population has not declined.Answer
  4. D
    The agricultural herbicide is known to be directly fatal to several other species of insects in the Aral Valley that do not feed on milkweed.
  5. E
    The overall percentage of milkweed plants stunted by the herbicide is smaller than the absolute number of Corvid butterflies that have disappeared over the decade.

Answer

The option stating that in neighboring valleys where the thread-wasp population has increased just as sharply but the new herbicide is not used, the Corvid butterfly population has not declined.
The correct answer provides a powerful 'cause without effect' control scenario. By demonstrating that an influx of thread-wasps does not cause a decline in the butterfly population as long as the herbicide is absent, this option effectively eliminates the thread-wasp as the alternate cause. This strongly reinforces the entomologist's conclusion that the herbicide is the true culprit.

Step-by-Step Solution

1
Identify the main conclusion and the premises of the argument.
Conclusion: The Corvid butterfly's decline is due to the new herbicide, not the thread-wasp. Premises: The butterfly population plummeted while the wasp population increased; however, the herbicide stunts the milkweed plants the caterpillars exclusively rely on.
Understanding the core argument structure is essential for determining what type of evidence is required to strengthen the specific causal claim.
2
Analyze the causal relationship being asserted.
The entomologist asserts that Cause A (the herbicide) is responsible for the Effect (butterfly decline), and explicitly argues against Cause B (the thread-wasp) being responsible.
Identifying the primary cause and the competing alternate cause allows us to anticipate what kind of evidence would support the author's preferred cause over the alternative.
3
Evaluate the options to find one that supports Cause A or definitively rules out Cause B.
The correct option provides a scenario where Cause B (wasps) is present but Cause A (herbicide) is absent, and the Effect (decline) does not occur.
In logical reasoning, showing that an effect disappears when the suspected cause is removed—even if the alternate cause remains present—is one of the most powerful ways to rule out the alternate cause and strengthen the conclusion.

Key Concept

Strengthening Causal Arguments via Control Groups
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