Question 18·Hard·Inferences
Ecologists have long attributed the decline of the coastal marsh sparrow primarily to the reduction of available nesting habitat caused by sea-level rise. Yet a recent longitudinal study comparing marsh sites with similar rates of sea-level rise found that sparrow numbers plummeted only in those areas adjacent to intensive agricultural lands, where high concentrations of neonicotinoid pesticides were detected in marsh water. Based on this evidence, the lead researcher inferred that _____
Which choice most logically completes the text?
For inference questions, identify the earlier explanation and then isolate what the new study controls (held constant) versus what changes across cases. The best inference usually adds a supported factor with cautious language (e.g., "likely," "contributing") rather than making absolute claims that dismiss prior causes or assert a single definitive cause.
Hints
Focus on the contrast word
Look closely at the word "Yet" near the start of the second sentence. How does the new study’s evidence challenge or add to the earlier belief about sea-level rise and habitat loss?
Notice what is the same and what is different across sites
All the marsh sites in the study have similar rates of sea-level rise. What key condition is present only in the sites where sparrow numbers plummeted?
Think about strength and scope of the conclusion
Is the researcher likely to completely dismiss sea-level rise, or to say that a new factor also seems important? Avoid answers that make absolute claims not supported by the passage, such as ruling out earlier explanations or introducing new causes not mentioned.
Eliminate answers that go beyond the evidence
Ask yourself for each option: does the passage actually give evidence for this, or would I have to assume extra information (for example, about habitat change unrelated to sea-level rise or about pesticides being the only cause)?
Step-by-step Explanation
Clarify the old explanation and the new evidence
First, note the contrast signaled by "Yet." Ecologists used to think the sparrow’s decline was primarily due to loss of nesting habitat from sea-level rise. The new study adds evidence that may modify or refine this view.
Understand what the study controlled and what varied
The study compares marsh sites with similar rates of sea-level rise. That means sea-level rise is roughly the same in all the sites and cannot explain differences between them. The key difference is that sparrow numbers "plummeted only" in areas adjacent to intensive agricultural lands, where high concentrations of neonicotinoid pesticides were found in the water.
Figure out the most reasonable inference
Because sea-level rise is similar across sites, but sharp declines occur only where neonicotinoid pesticides are present, the researcher can reasonably infer that these pesticides are linked to or contributing to the declines. However, the passage never says sea-level rise is unimportant or that pesticides are the only cause, so a careful inference must treat pesticides as an additional, significant factor rather than the sole explanation.
Match the inference to the best answer choice
The correct completion must say that pesticide contamination is a likely, important contributor to sparrow decline in addition to habitat loss from sea-level rise. Therefore, the best choice is: "neonicotinoid pesticide contamination is likely a significant factor contributing to the sparrow’s population decline alongside habitat loss."