Question 39·Hard·Command of Evidence
Marine biologist Dr. Elena Morales argues that establishing dense kelp forests along temperate coastlines can markedly lessen local ocean acidity and thereby improve the calcification of shell-forming organisms such as mussels and oysters. To test her idea, Morales’s team selected two nearly identical coves: an experimental cove, where thousands of juvenile kelp plants were attached to submerged lines, and a control cove, which was left unchanged. Over the next six months, researchers recorded seawater pH every week and measured the shell thickness of caged mussels at the beginning and end of the study.
Which outcome of the pilot project, if verified, would most strongly support Morales’s claim?
For “most strongly supports the claim” questions, translate the claim into a checklist of predicted, measurable differences between the experimental group and the control. Then choose the option that matches all predictions while ruling out alternative explanations (like preexisting differences at the start or changes that happen equally in both groups).
Hints
Split the claim into parts
Identify the two things Morales predicts: a change in pH in the kelp cove and a change in shell thickness/growth in the kelp cove, each compared to the control.
Watch for confounds from the start-of-study measurement
The researchers measured shell thickness at the beginning and end. Strong support should show the kelp cove didn’t just start out with thicker shells.
Compare experimental vs. control, not just overall change
If both coves change similarly, that points to a regional trend rather than an effect caused by adding kelp.
Step-by-step Explanation
Turn the claim into two testable predictions
Morales’s claim implies two linked predictions:
- Adding kelp should make seawater less acidic, meaning higher pH, in the experimental cove compared with the control.
- With reduced acidity, shell-forming organisms should show better calcification, meaning greater shell growth in the experimental cove compared with the control.
Prioritize outcomes that isolate the kelp effect
Because the study has an experimental cove and a control cove, the best supporting outcome should:
- Show a difference between coves (not the same change in both), and
- Use the shell-thickness measurements at the beginning and end to rule out the possibility that one cove’s mussels simply started with thicker shells.
Evaluate the choices for pH evidence + shell-growth evidence
Eliminate choices that:
- Show no pH advantage in the kelp cove (doesn’t support reduced acidity), or
- Show no greater shell growth in the kelp cove (doesn’t support improved calcification), or
- Allow an alternative explanation such as the kelp-cove mussels being thicker at the start (a confound rather than an effect).
Select the outcome that best matches both predictions and avoids confounds
The best-supported outcome is:
Over the six-month period, average seawater pH in the kelp-enhanced cove was about 0.15 units higher than in the control cove, and mussels in both coves started with similar shell thickness but ended with shells that were, on average, 18 percent thicker in the kelp-enhanced cove than in the control cove.
This shows a kelp-associated pH increase (less acidity) and stronger shell growth in the experimental cove, while also indicating the groups started from a comparable baseline.