Question 87·Hard·Inferences
In a recent study, scientists cultivated common milkweed plants () in controlled chambers set to either ambient atmospheric conditions or to concentrations of carbon dioxide comparable to those measured beside major highways. Analyses showed that milkweed grown under elevated developed thicker leaves but contained markedly lower levels of cardenolides—chemical compounds that protect monarch butterfly () larvae from predators. When monarch larvae were reared on the two milkweed types, those feeding on the high‐ plants gained mass more rapidly during the larval stage, yet fewer of the resulting adults survived a laboratory protocol that mimicked the energetic demands of long-distance migration. These findings suggest that ______
Which choice most logically completes the text?
For "Which choice most logically completes the text?" questions, first underline the key findings, especially contrasts (like short-term benefit versus long-term harm). Then, before looking at the choices, briefly state in your own words what those findings imply overall. Next, eliminate choices that (1) introduce new ideas not mentioned in the passage, (2) contradict the data, or (3) focus on only part of the evidence. Pick the option that accurately captures all major results and matches the passage’s scope and level of certainty (words like "may" or "likely" are often more appropriate than absolute claims).
Hints
Focus on the last sentence
Reread the final sentence that ends with "These findings suggest that ____." Your answer should summarize what the overall pattern of results implies, not restate just one detail.
Compare short-term and long-term effects
Think about what happened during the larval stage versus what happened during the migration-like test. Are the effects of high- milkweed all positive, all negative, or mixed?
Avoid unsupported cause-and-effect claims
Be careful with choices that claim the study proved why larvae grew faster (for example, blaming leaf thickness alone). The passage reports results but doesn’t isolate a single cause.
Step-by-step Explanation
Restate what the experiment changed
First, identify the key difference between the two groups of plants.
Scientists grew common milkweed under two conditions:
- normal (ambient) atmospheric conditions
- elevated levels similar to those near highways
Under elevated , the milkweed:
- had thicker leaves
- had much lower levels of cardenolides (chemicals that protect monarch larvae from predators).
Connect plant differences to monarch outcomes
Next, see what happened to monarch larvae and adults on each type of plant.
Larvae feeding on high- milkweed:
- gained mass more rapidly during the larval stage (short-term benefit)
- but fewer of the resulting adults survived a lab test mimicking long-distance migration (long-term harm)
So the findings show a trade-off: faster early growth but worse survival later.
Infer the big-picture consequence
Now ask: If larvae grow faster but fewer adults survive migration-like conditions, what might this mean for monarch populations over time?
Because migration is crucial for monarch butterflies, lower adult survival during migration-like conditions could reduce the number of monarchs that successfully complete migration and reproduce. Even though high- milkweed seems good for larval growth, the long-term effect could be negative for overall monarch numbers.
Match that inference to the best choice
The correct completion must:
- be based only on what the study actually shows
- reflect both the initial benefit (faster larval growth) and the later cost (reduced adult survival)
The only choice that fits all of this is: exposure to elevated during milkweed growth may ultimately decrease monarch population sizes even if it initially seems beneficial for larval growth.