Question 36·Hard·Command of Evidence
Mara Chen and colleagues modeled summer baseflow (the amount of water a stream continues to carry during dry months) in a set of salmon-bearing streams. Some modeled stream reaches included active beaver complexes, which can store water and release it gradually, while others did not. The team estimated baseflow under three precipitation scenarios (average conditions, moderate drought, and severe drought). The researchers concluded that beaver complexes could largely offset baseflow reductions under moderate drought but not under severe drought.
Which choice best describes data from the graph that support Chen and colleagues’ conclusion?
For command-of-evidence questions tied to a graph, translate the conclusion into 2–3 concrete comparisons you should see in the data. Then choose the option that states those comparisons with the correct conditions (here: with vs. without beavers, and moderate vs. severe drought) and avoids swapping baselines or using true-but-irrelevant comparisons.
Hints
Use the conclusion as a checklist
The correct choice must support both parts: beavers help under moderate drought, but can’t fully help under severe drought.
Look for a baseline comparison
To show “offset,” compare with beavers (moderate drought) to a without beavers condition that represents a baseline (average conditions).
Make sure the second clause matches “not severe”
For “not under severe drought,” the choice should show with beavers (severe drought) still below that same baseline.
Step-by-step Explanation
Identify what the conclusion needs
The conclusion has two parts:
- Moderate drought: beavers largely offset the reduction (so “with beavers” under moderate drought should be comparable to, or better than, a relevant non-beaver baseline).
- Severe drought: beavers do not offset the reduction (so “with beavers” under severe drought should still be clearly worse than that baseline).
Match each part to a comparison in the graph
From the bars:
- Under moderate drought, the with-beavers bar is higher than the without-beavers bar under average conditions.
- Under severe drought, the with-beavers bar is much lower than the without-beavers bar under average conditions.
Select the choice that states both comparisons
The only option that includes both the “offset under moderate drought” comparison and the “not offset under severe drought” comparison is:
Baseflow with beavers under moderate drought exceeded baseflow without beavers under average conditions, but the latter exceeded baseflow with beavers under severe drought.