Question 144·Hard·Central Ideas and Details
Ecologists studying the 2019 reintroduction of beavers into England’s River Otter were surprised by the animals’ effect on water quality. Over two years, the beavers’ dams increased wetland area by 27 percent and reduced agricultural runoff downstream by 45 percent. However, the data also showed that nitrogen levels directly behind the dams briefly spiked each spring as decomposing leaf litter accumulated. Lead researcher Lisa Schoenfeld notes that such seasonal fluctuations, though seemingly alarming, are transient; within weeks, microbial activity stabilizes nitrogen concentrations well below pre-beaver baselines. She cautions that evaluations conducted only in early spring could therefore misrepresent the dams as polluting rather than purifying the river system.
Which choice is most strongly supported by the text?
For central-idea/detail questions like this, first summarize the author’s overall point (here, that the dams improve water quality despite a brief seasonal spike). Then focus on the sentence that most directly addresses the question—in this case, the researcher’s caution in the last line. Evaluate each answer by asking, “Is this clearly stated or strongly implied?” and eliminate choices that (1) contradict specific details (like what happens to nitrogen after the spike), (2) exaggerate the importance of a minor/temporary effect, or (3) make absolute claims the passage doesn’t support.
Hints
Identify where the author gives an overall judgment
Skim for where the passage explains not just data, but how those data might lead people to form a wrong conclusion about the dams.
Pay attention to time frames
Notice how long the nitrogen spikes last compared to the two-year study period. How might the length of time you measure change what you think about the dams?
Watch for subtle exaggerations
Be wary of choices that slightly distort the passage’s claim about nitrogen (for example, suggesting it stays near baseline, dominates the overall effect, or doesn’t change at all).
Step-by-step Explanation
Grasp the main findings about the dams
First, summarize what the researchers found:
- Over two years, the dams increased wetland area and reduced agricultural runoff downstream.
- This means that, on the whole, the dams are beneficial for water quality.
- There is, however, a brief spring spike in nitrogen levels directly behind the dams.
Focus on the key warning in the last sentence
Now look closely at Lisa Schoenfeld’s comment:
- She says the nitrogen spikes are seasonal, transient, and that levels soon drop well below pre-beaver baselines.
- She then cautions that evaluations done only in early spring could misrepresent the dams as polluting rather than purifying.
- This sentence directly addresses how the dams might be judged incorrectly if measurements are taken at the wrong time.
Test each choice against the passage details
Compare each option with the evidence:
- (A) says nitrogen behind the dams returns only to about pre-beaver levels, but the passage says it stabilizes well below pre-beaver baselines.
- (B) claims the springtime nitrogen rise is the primary effect and that it offsets most downstream benefits, but the passage emphasizes the spike is brief and warns only that it can mislead short-term evaluations.
- (D) says the dams do not change nitrogen levels in the river, but the passage explicitly reports a brief spring spike behind the dams.
These choices either contradict or go beyond what the passage supports.
Match the remaining idea to the researcher’s caution
The remaining option says that short-term measurements may incorrectly imply that the dams worsen water quality. This directly matches Schoenfeld’s warning that evaluations done only in early spring could misrepresent the dams as polluting rather than purifying. Therefore, the correct answer is:
Short-term measurements may incorrectly imply that the dams worsen water quality.