Question 66·Hard·Command of Evidence
Ecologists are investigating why certain alpine meadows in Central Asia rapidly convert to shrub-dominated landscapes even though mean annual temperatures have remained relatively stable. Long-term monitoring in the Dzhungarian Range shows that after several consecutive mild winters, populations of the mountain vole (Alticola argentatus) spike, coinciding with bursts of shrub establishment. However, similar shrub expansion has also occurred at times and locations where vole numbers stayed low.
To explain the inconsistency, researcher Li Na hypothesizes that vole outbreaks promote shrub expansion only when they are followed by summers of unusually high rainfall. The reasoning is that intense grazing by the expanded vole population suppresses grasses, and the subsequent wet summer enables the heavily browsed shrubs to regenerate quickly while grasses struggle to recover.
Which additional finding from Li Na’s study would most directly support her hypothesis?
For hypothesis-support questions, restate the hypothesis as an if/then claim and underline constraint words like “only when,” which signal a required condition. Then select the option that reports the exact predicted outcome pattern (especially comparisons showing the outcome appears when the required condition is present and disappears when it’s absent), and eliminate options that test only one factor or point to an alternative condition.
Hints
Focus on the relationships in the hypothesis
Restate Li Na’s idea using the key logic word: vole outbreaks promote shrub expansion only when they’re followed by unusually wet summers.
Translate “only when” into evidence to look for
“Only when” means the condition (wet summer) is required. Good support shows shrub expansion in cases with that condition and no expansion in similar cases without it.
Look for a comparison across condition combinations
Pick the choice that distinguishes outbreak+wet years from outbreak+not-wet years (and ideally also considers wet years without outbreaks).
Eliminate findings that make one factor irrelevant
If an option suggests rainfall alone explains shrub expansion (or that outbreaks work even without wet summers), it doesn’t support Li Na’s interaction claim.
Step-by-step Explanation
Restate the hypothesis as a testable pattern
Li Na claims vole outbreaks promote shrub expansion only when they are followed by unusually high summer rainfall. So wet summers are required for the outbreak-to-shrub link.
Determine what evidence would be most direct
Direct support should compare shrub cover across combinations of conditions (outbreak vs. no outbreak; wet summer vs. not wet) and show increased shrub cover only in the outbreak + wet-summer case.
Choose the option that matches that interaction
The only choice that reports shrub cover increasing only when a vole outbreak is followed by above-average summer rainfall—and not increasing when either condition is missing—is: "Across nine study sites, shrub cover increased only when a vole outbreak was followed by above-average summer rainfall; it did not increase after outbreaks with average rainfall or in wet summers without outbreaks."