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Question

Difficulty: HardWeaken

Forestry Researcher: Over the past decade, managers of the Eldridge Timber Reserve introduced the predatory European pine beetle (*Rhizophagus grandis*) to suppress populations of the destructive great spruce bark beetle (*Dendroctonus micans*). Surveys indicate that spruce tree mortality in the reserve plummeted by 45 percent over this period. Because no chemical pesticides or other management interventions were deployed, the introduction of the predatory beetle was clearly the primary factor responsible for the dramatic recovery of the spruce forest.

Which of the following, if true, most seriously weakens the forestry researcher's argument?

  1. A
    Laboratory feeding trials demonstrate that adult European pine beetles consume up to three times their body weight in spruce bark beetle larvae daily.
  2. B
    Historical reserve records indicate that annual spruce tree mortality had occasionally fluctuated by as much as five percent prior to the introduction.
  3. Throughout the same decade, unmanaged spruce forests in adjacent regions with no predatory beetle introductions experienced an identical decline in spruce tree mortality following a succession of unusually cold winters.Answer
  4. D
    The European pine beetle is entirely incapable of establishing breeding populations in any forest dominated by deciduous tree species.
  5. E
    Spruce bark beetles were not the sole insect species causing minor foliage damage within the timber reserve before the study began.

Answer

The finding that unmanaged spruce forests in adjacent regions without predatory beetle introductions experienced an identical mortality decline following unusually cold winters most seriously weakens the argument by demonstrating that the recovery occurred independently of the predator introduction.
The correct answer provides a natural control group by demonstrating that spruce forests in neighboring regions with no introduced predators experienced the identical forty-five percent decline in mortality due to severe cold winters. This provides a strong, comprehensive alternative explanation that accounts for the observed effect across the region, severely undermining the claim that the introduction of the predatory beetle was the primary cause of the recovery.

Step-by-Step Solution

1
Isolate the conclusion and supporting premises in the stimulus.
The researcher concludes that the introduction of the predatory beetle caused the 45 percent drop in spruce mortality, inferring this solely from the timeline and the absence of chemical pesticide interventions.
Accurately identifying the central causal claim is the necessary first step in evaluating a weaken item.
2
Identify the critical logical assumption and vulnerability in the causal inference.
The author assumes that no regional climatic or ecological factor independently caused the decrease in spruce bark beetles and tree mortality over the same decade.
Causal conclusions based on simple before-and-after correlation are vulnerable to confounding external variables.
3
Evaluate the answer choices for evidence of an alternative cause or a control group where the effect occurs without the cause.
The information that adjacent unmanaged spruce forests without predatory beetles experienced the exact same 45 percent drop due to cold winters shows the effect happens without the proposed cause, establishing a compelling alternative explanation.
Demonstrating that the effect occurs equally in the absence of the hypothesized cause effectively breaks the causal connection.

Key Concept

Undermining a causal conclusion by introducing a control group and an alternative explanatory variable
Estimated Time:1m 30s
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Weaken

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