Question 128·Medium·Command of Evidence
Ecologists studying river restoration have proposed reintroducing beavers to degraded streams. Beaver dams, they note, create ponds and slow water flow. Dr. Leena Rathi argues that because beaver dams slow the current, suspended nutrients settle to the bottom, enriching the soil, which in turn leads to greater plant growth and ultimately supports a wider variety of animal species.
Which finding, if true, would most directly support Rathi’s explanation in the bolded sentence?
For “Which finding would most directly support the explanation?” questions, first paraphrase the explanation into a short cause-and-effect chain. Then look for an answer that (1) compares situations where the proposed cause is present versus absent, and (2) measures the specific outcomes mentioned in the explanation (here, sediment-bound nutrients and plant/animal diversity). Quickly eliminate choices that are just background facts, examples that don’t involve the proposed cause, or statements that talk about related topics without directly testing the prediction.
Hints
Identify the key cause-and-effect chain
Reread the bolded sentence and list out, in your own words, what Rathi is saying beaver dams do to the stream and to the living things there.
Think about what evidence would test that chain
If a scientist wanted to check whether Rathi is right, what would they compare, and what specific things (nutrient levels, plant diversity, etc.) would they measure?
Compare with vs. without beavers
Look for an option that compares stream areas with beaver dams to similar areas without dams, and that mentions both nutrients in the water/soil and plant variety.
Beware of related but indirect facts
Some choices mention beavers, wetlands, or other animals but don’t actually confirm the nutrient → soil → plant growth part of Rathi’s explanation. Make sure the choice you pick gives evidence for that chain, not just a general fact about ecosystems.
Step-by-step Explanation
Understand what Rathi is claiming
Focus on the bolded sentence. Rathi makes a cause-and-effect chain:
- Beaver dams slow the current.
- Because the water slows, suspended nutrients settle to the bottom.
- Those nutrients enrich the soil.
- Richer soil leads to greater plant growth.
- More plants support a wider variety of animal species.
So Rathi’s explanation is: beaver dams → more settled nutrients → richer soil → more plant species (and then more animal diversity).
Decide what “most directly support” means here
"Most directly support" means we want evidence that tests or confirms this specific chain.
The strongest support would be a finding that compares areas with beaver dams to similar areas without them and shows:
- More sediment-bound (settled) nutrients where dams exist, and
- More plant species (greater plant diversity) where dams exist.
Evidence that is only about beavers in general, or about wetlands in general, or about other animals, is less direct.
Check each option against Rathi’s chain
Now compare each answer choice to Rathi’s explanation:
- Choice A talks about beaver population decline in the nineteenth century. That is background history; it doesn’t say anything about nutrients, soil enrichment, or plant diversity.
- Choice C talks about fish that live in fast-moving streams with no wetlands. That shows that some animals don’t need beaver-created wetlands; it doesn’t give evidence that beaver dams do increase nutrients or plant diversity.
- Choice D says seasonal flooding can create temporary wetlands without beavers. That suggests wetlands can form in other ways; again, it doesn’t test whether beaver dams specifically cause more settled nutrients and more plant growth.
- Choice B describes a comparative study that directly measures nutrient levels and plant species in beaver-dammed vs. free-flowing stream sections.
Only one of these directly lines up with Rathi’s claim that beaver dams increase sediment-bound nutrients and plant diversity.
Select the answer that best matches the explanation
The finding that “beaver-dammed stream sections contain significantly higher concentrations of sediment-bound nutrients and nearly twice as many plant species as adjacent free-flowing sections” (Choice B) most directly supports Rathi’s explanation, because it shows exactly the pattern she predicts: where there are beaver dams, there is more settled nutrient in the sediment and more plant diversity.