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 “Which finding would most directly support the hypothesis?” questions, start by carefully restating the hypothesis in your own words, paying attention to strong logical words like “only when,” “if,” and “because.” Then ask: What pattern of data would we expect if this hypothesis were true? Look for an answer that directly reports that pattern—usually by comparing outcomes under different conditions—rather than one that merely sounds related, explains background, or supports only part of the idea (like just the mechanism or just an alternative cause. Eliminate any option that doesn’t mention the key variables and the predicted outcome together.
Hints
Focus on the relationships in the hypothesis
Before looking at the choices, restate Li Na’s idea: it links vole outbreaks, unusually high summer rainfall, and shrub expansion, with a strong condition word: “only when.”
Think about what “only when” requires
If something happens only when a condition is present, strong evidence will compare cases with that condition to similar cases without it and show a clear difference in the outcome.
Look for data about shrub cover under different conditions
Scan the answer choices for one that reports actual results on shrub cover in years (or places) that differ in both vole levels and summer rainfall, not just side details like grass damage or temperature trends.
Eliminate partial or off-topic evidence
Reject any option that talks only about rainfall frequency, only about grass, or only about temperature, without directly connecting vole outbreaks, wet summers, and changes in shrub cover.
Step-by-step Explanation
Restate the hypothesis in simple terms
Li Na’s hypothesis is that vole outbreaks lead to shrub expansion only if they are followed by unusually rainy summers. In other words:
- When there is a vole outbreak and a very wet summer after it, shrubs should expand.
- When there is a vole outbreak without a very wet summer after it (just normal or dry), shrubs should not expand much.
So rainfall is a required condition for vole outbreaks to have the shrub-expanding effect.
Decide what kind of evidence would be strongest
To directly support this hypothesis, we want results from Li Na’s study that:
- Compare years with vole outbreaks followed by wet summers to years with vole outbreaks but normal/dry summers.
- Report what happened to shrub cover under those different conditions.
The best evidence will show a clear difference in shrub expansion depending on whether the post-outbreak summer was unusually wet or not.
Check each option against that evidence pattern
Now test each choice:
- A talks about how often wet summers occur with vs. without vole outbreaks. It never mentions shrub cover or compares shrub changes under different rainfall conditions, so it does not show that wet summers change the effect of outbreaks.
- B compares grass damage in plots with and without voles during wet summers. This shows voles hurt grass, but it does not say what happens to shrubs, and it does not compare wet vs. non-wet summers.
- C talks about temperature records, confirming that warming is not the cause. This is background information and does not test the rainfall-plus-voles idea.
- D is the only option that gives shrub-cover results under different combinations of vole outbreaks and rainfall levels.
Match the strongest evidence to the hypothesis
Choice D says that across nine sites, years with both vole outbreaks and above-average summer rainfall showed big increases in shrub cover, while years with vole outbreaks but normal or below-average rainfall did not. This exactly matches the hypothesis that vole outbreaks promote shrub expansion only when followed by unusually wet summers, so D is correct.