Ecologists studying the 2019 reintroduction of beavers into England’s River Otter found that the animals’ dams increased wetland area by 27 percent and reduced agricultural runoff downstream by 45 percent. Sensors directly behind the dams, however, registered sharp nitrogen increases each April as accumulated leaf litter decomposed. By late May, the nitrogen readings had fallen below the pre-reintroduction baseline and remained there for most of the year. Reports based only on April samples characterized the dams as pollution sources, whereas reports using observations from throughout the year characterized them as improving water quality. Lead researcher Lisa Schoenfeld says neither dataset is faulty: April samples capture a recurring event, while annual observations establish that the event is not typical. Yet she notes that annual averages can conceal the fact that April concentrations briefly exceed a level associated with stress in some aquatic species. Her team consequently publishes seasonal ranges alongside annual averages.
Which choice best states the main idea of the text?
For a difficult main-idea question, identify not only the passage’s broad topic but also the qualification that determines its emphasis. Compare close choices for whether they preserve every important part of the author’s reasoning, especially evidence that prevents the passage from fully endorsing either of two competing interpretations.
Hints
Separate accuracy from completeness
Determine what the April samples establish and what the observations from the rest of the year establish. An accurate observation is not necessarily a complete evaluation.
Consider the ecological threshold
Ask why the possible stress to aquatic species matters even though the nitrogen level is lower during most of the year.
Examine the reporting decision
Consider what can be inferred from the team’s decision to publish seasonal ranges together with annual averages rather than choosing only one kind of summary.
Step-by-step Explanation
Distinguish the temporal patterns
Nitrogen concentrations rise sharply each April but fall below the earlier baseline by late May and remain there for most of the year. The dams therefore have different apparent effects depending on the period examined.
Evaluate both kinds of evidence
Schoenfeld does not reject either dataset. April samples accurately capture a recurring event, while year-round observations establish that the increase is not representative of most of the year.
Interpret the ecological qualification
The annual pattern supports an overall water-quality benefit, but an annual average can hide the fact that April concentrations may briefly reach levels associated with aquatic stress. Thus, the short-term increase cannot simply be dismissed as unimportant.
Use the team’s reporting method
The team publishes seasonal ranges alongside annual averages, indicating that neither time scale should replace the other. The third choice best captures this balance between a recurring short-term risk and a sustained benefit.