Question 81·Hard·Command of Evidence
Marine biologist Stephen Palumbi contends that widespread coral bleaching is driven more by local stressors (such as nutrient runoff and sedimentation) than by elevated sea‐surface temperatures. To evaluate Palumbi’s contention, researchers Bolin and Creasey monitored 40 reef sites across three ocean basins during a severe, planet-wide heatwave. The sites were matched for coral species composition but differed markedly in their exposure to local stressors.
Which finding from Bolin and Creasey’s study, if true, would most directly weaken Palumbi’s contention?
For “weaken the claim” evidence questions, first underline the original claim and decide what the opposite or undermining evidence would look like. Then, pay close attention to the study conditions given in the passage (here, a global heatwave and varying local stressors) and favor answer choices that both (1) match those conditions and (2) directly address the specific outcome in question (here, coral bleaching). Quickly eliminate choices that change the outcome (e.g., disease instead of bleaching), change the time frame (non-heatwave vs. heatwave), or only show that one factor matters without comparing it against the other factor mentioned in the claim.
Hints
Clarify what it means to weaken a claim
First, restate Palumbi’s claim in your own words. Then ask yourself: what kind of result would make that claim look less likely to be true? You’re looking for evidence that points away from local stressors as the main cause of bleaching.
Focus on the conditions of Bolin and Creasey’s study
Notice that Bolin and Creasey collected data during a severe, planet-wide heatwave and that their sites differed in local stressors. Which answer choice uses this same heatwave context and directly compares reefs with different levels of local stressors?
Check what’s being measured
Make sure the choice you pick is about coral bleaching, not a different problem (like disease or general survival), and that it helps you compare the roles of local stressors and elevated temperatures in causing bleaching.
Step-by-step Explanation
Restate the claim and the task
Palumbi’s contention is that widespread coral bleaching is driven more by local stressors (nutrient runoff, sedimentation, etc.) than by elevated sea-surface temperatures.
To weaken this contention, we want a finding that suggests elevated temperatures, not local stressors, are the primary cause of bleaching, or that bleaching levels are similar regardless of local stressors when temperatures are high.
Use the study setup to know what’s relevant
Bolin and Creasey’s study is described as:
- Monitoring 40 reef sites
- Across three ocean basins
- During a severe, planet-wide heatwave
- Sites match in coral species but differ in exposure to local stressors
So, the most relevant weakening evidence will:
- Come from this heatwave context, and
- Compare bleaching on reefs with high vs. low local stressors under the same high-temperature conditions.
Eliminate answer choices that don’t directly challenge the claim in this context
Go through the options and ask: Does this directly compare bleaching under different local-stressor levels when temperatures are high?
- Choice A: Talks about reefs with sedimentation but not subjected to elevated temperatures, over five years. This is outside the heatwave context and does not compare bleaching at different stressor levels during high temperatures.
- Choice B: Talks about coral disease, not bleaching, and in non-heatwave years. It shows local stressors can cause disease, but does not address bleaching during high temperatures.
- Choice C: Lab experiment showing corals survive longer when water is filtered (fewer nutrients and sediments). This shows local stressors are harmful, but doesn’t compare them to the effect of high temperatures on bleaching, and it’s not field bleaching data from the heatwave.
None of these three directly undermines the idea that, in the real-world heatwave, local stressors are more important than elevated temperatures for bleaching.
Confirm the choice that most directly weakens Palumbi’s contention
Now look at the remaining option:
- Choice D: Says that during the heatwave, reefs with minimal local stressors had bleaching levels that were statistically indistinguishable from those of reefs with heavy local stressors.
This means that when temperatures were high everywhere, reefs with few local stressors bleached just as much as reefs with many local stressors. That directly contradicts Palumbi’s idea that local stressors are the main driver of widespread bleaching, and instead points to elevated temperature as the dominant factor.
Therefore, the correct answer is:
During the heatwave, reefs with minimal local stressors showed bleaching levels statistically indistinguishable from those of heavily stressed reefs.