Question

The global population of frogs has declined in recent years while the amount of ultraviolet radiation reaching the Earth has increased. Since the genetic material in frog eggs is harmed when exposed to ultraviolet radiation, and since the eggs themselves are not protected by shells or leathery coverings but are gelatinous, the frog population decline is probably due, at least in part, to the ultraviolet radiation increase.
Which of the following, if true, provides the strongest support for the argument?

• A. Even in those regions where there has been no significant increase in ultraviolet radiation, only a small proportion of the frog eggs that are laid ever hatch.
• B. In areas where there has been the least decline in frog populations, populations of species of insects that frogs eat have decreased.
• C. The eggs of frog species whose populations are declining tend to have higher concentrations of damaging pesticides than do the eggs of frog species whose populations have not declined.
• D. In many places where turtles, which lay eggs with tough, leathery coverings, share habitats with frogs, turtle populations are also in decline.
• E. Populations of frog species that hide their eggs beneath rocks or under sand have declined considerably less than have populations of frog species that do not cover their eggs.

Hint:

When strengthening a causal argument, look for an answer that isolates the proposed cause. The best answers often create a comparison (a control group) that shows a difference in the effect that corresponds to a difference in the presence or absence of the cause.

Answer 1

Answer: (E)

Step 1: Understanding the Concept:
This question asks us to find the piece of evidence that would most strengthen a causal argument. The argument claims that increased ultraviolet (UV) radiation is a cause of the global decline in frog populations.
Step 2: Key Formula or Approach:
A causal argument (X causes Y) is strengthened by evidence that shows a strong correlation between the proposed cause and the effect. One of the strongest forms of evidence is a "control group" or a "natural experiment": showing that where the cause is absent or mitigated, the effect is also absent or lessened.
Step 3: Detailed Explanation:
Argument Analysis:
- Observation 1: Frog populations are declining.
- Observation 2: UV radiation is increasing.
- Mechanism: UV harms unprotected, gelatinous frog eggs.
- Conclusion: Increased UV is causing the frog decline.
Evaluating the Options:
- (A) This weakens the argument by suggesting that frog eggs have a low hatch rate for reasons other than UV radiation.
- (B) This weakens the argument by introducing a plausible alternative cause for the decline: a decrease in the food supply.
- (C) This weakens the argument by introducing another strong alternative cause for the decline: pesticides.
- (D) This is irrelevant. The decline of turtles, which have protected eggs, doesn't support the specific mechanism proposed for frogs. It might suggest a different, broader environmental problem affecting both.
- (E) This provides a perfect "natural experiment." It compares two groups of frogs: those whose eggs are exposed to UV and those whose eggs are protected from UV (by being hidden under rocks/sand). The observation that the protected group has declined "considerably less" creates a strong correlation: less UV exposure leads to less population decline. This directly supports the hypothesis that UV exposure is a significant cause of the decline.
Step 4: Final Answer:
By comparing two different groups of frogs and showing that the group naturally protected from UV radiation is doing better, this option provides strong evidence that UV radiation is indeed a key factor in the population decline.