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Difficulty: HardExplaining Surprising Outcomes

In an effort to restore the declining population of native silver trout in Lake Mistic, conservationists completely removed a non-native predatory fish species, the spotted pike, which was known to feed on juvenile silver trout. Two years after the complete removal of the spotted pike, the population of juvenile silver trout has actually declined significantly below its previous levels, despite water quality and other environmental factors remaining optimal and unchanged.

Which of the following, if true, most helps to resolve the apparent discrepancy?

  1. A
    The lake's water quality, while considered optimal for adult silver trout, is actually slightly too alkaline for the ideal development of juvenile silver trout.
  2. B
    While the total number of juvenile silver trout has dropped, they now account for a larger proportion of the lake's overall fish population than they did previously.
  3. The spotted pike's primary food source was the redfin perch, a rapidly reproducing species whose diet consists almost entirely of the exact same aquatic insects that juvenile silver trout rely on for survival.Answer
  4. D
    In neighboring lakes, there is a strong historical correlation between the complete removal of spotted pike and unexpected, temporary declines in native prey fish.
  5. E
    Other species of predatory birds that are known to feed on silver trout migrate through the Lake Mistic region during the spring season.

Answer

The statement explaining that spotted pike primarily ate redfin perch, which compete directly with juvenile silver trout for food.
The correct answer resolves the paradox by introducing a secondary causal mechanism. By establishing that the spotted pike primarily preyed on redfin perch, and that redfin perch compete directly with juvenile silver trout for the exact same food source, it explains how removing the pike allowed the perch population to grow unchecked. The surging perch population then depleted the food supply, starving the juvenile trout.

Step-by-Step Solution

1
Analyze the stimulus to identify the apparent paradox.
The paradox is that removing a known predator of the silver trout resulted in a significant decline in the juvenile silver trout population, rather than the expected increase.
Understanding the conflicting facts is required before a resolution can be found.
2
Determine what characteristics a correct resolution must have.
The correct answer must provide a new piece of information that explains how the removal of the spotted pike actively caused or allowed a situation that harmed the juvenile silver trout, without contradicting the premise that the environment remained unchanged.
A valid resolution must bridge the two conflicting facts logically.
3
Evaluate the options against the required characteristics.
The correct option reveals an ecological chain reaction: removing the apex predator (pike) removed the check on a competitor species (perch), which then starved the juvenile trout by depleting their shared food source.
This indirect causal mechanism perfectly explains the surprising outcome while remaining consistent with all given premises.

Key Concept

Resolving a paradox by identifying an indirect causal mechanism or third variable that bridges seemingly contradictory premises.
Estimated Time:2m 0s
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Explaining Surprising Outcomes

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