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Difficulty: MediumStrengthen

Ecologist: Over the past decade, populations of the yellow-bellied marmot in a subalpine valley have emerged from hibernation an average of three weeks earlier than they did thirty years ago. Some researchers attribute this shift primarily to rising spring temperatures. However, during the same thirty-year period, local winter snowpack depth decreased by nearly forty percent, exposing burrow entrances to direct sunlight much earlier in the season. Because marmots awaken primarily in response to ambient light penetrating their burrow entrances rather than ambient air temperature, the earlier emergence is almost certainly a result of reduced snowpack rather than warmer air.

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

  1. A
    Subalpine soil temperature measurements indicate that rising spring air temperatures warm subterranean nesting chambers weeks before surface snowpack melts.
  2. B
    All mammalian species inhabiting subalpine ecosystems rely exclusively on light cues rather than thermal cues to regulate dormancy cycles.
  3. In neighboring subalpine valleys where spring temperatures increased at similar rates but winter snowpack depth remained constant, marmot emergence dates did not advance significantly.Answer
  4. D
    Decreased winter snowpack has significantly reduced the seasonal runoff needed to sustain alpine vegetation consumed by marmots after emerging from hibernation.
  5. E
    Marmot populations in lower-elevation temperate forests typically experience higher average annual temperatures than populations located in high subalpine valleys.

Answer

The statement showing that marmot emergence dates did not change in valleys where temperatures rose but snowpack remained constant most strengthens the argument.
The argument concludes that reduced snowpack (which permits light penetration), rather than rising air temperature, is the cause of earlier marmot emergence. The correct choice provides a comparative control case: where air temperature rose but snowpack depth remained historically constant, marmot emergence did not shift. By showing that the effect does not occur in the absence of the author's proposed cause, even when the rival cause is present, this choice effectively rules out air temperature as the driving factor and bolsters the ecologist's conclusion.

Step-by-Step Solution

1
Identify the conclusion and premises of the argument
Conclusion: The earlier emergence is almost certainly caused by reduced snowpack rather than warmer air. Premises: Marmots emerge 3 weeks earlier; snowpack decreased by 40%; marmot awakening is triggered primarily by light penetrating burrow entrances rather than ambient air temperature.
Understanding the precise causal relationship claimed by the author is essential to finding the most supportive evidence.
2
Identify the logical gap or competing explanation
The rival explanation claims that rising spring temperatures caused the earlier emergence. The author dismisses this, attributing the change to reduced snowpack/light.
Strengthening a causal argument typically involves ruling out an alternative cause or demonstrating that the effect does not occur when the proposed cause is absent.
3
Evaluate the answer choices to identify the strongest supportive claim
The choice providing a control group (temperature increased but snowpack depth remained constant, resulting in no shift in emergence) directly demonstrates that temperature increases alone are insufficient to advance emergence dates.
Showing that the effect is absent when the suspected cause (reduced snowpack) is absent, despite the presence of the rival cause (warm temperatures), strongly validates the author's causal conclusion.

Key Concept

Strengthening a Causal Claim via Elimination of an Alternative Cause
Estimated Time:1m 15s
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