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Difficulty: HardIdentifying Underlying Assumptions

To control the invasive crown-of-thorns starfish population damaging a coastal barrier reef, marine biologists introduced a species of predatory snail that feeds exclusively on juvenile starfish. Five years after the introduction, surveys recorded a marked increase in healthy coral coverage across the reef. The biologists concluded that the predatory snails successfully suppressed the starfish population, thereby enabling the coral reef to recover.

Which of the following is an assumption on which the marine biologists' argument depends?

  1. Other environmental changes, such as a drop in ocean temperatures that reduced coral bleaching, were not the primary cause of the increased coral coverage during the five-year period.Answer
  2. B
    The population of the introduced predatory snail increased continuously in size throughout each of the five years following its release.
  3. C
    Crown-of-thorns starfish will eventually develop behavioral adaptations to evade predation by the introduced snail species.
  4. D
    Adult crown-of-thorns starfish cause significantly more damage to coral structures per individual than juvenile starfish do.
  5. E
    Prior to the introduction of the predatory snails, the crown-of-thorns starfish was the sole organism causing damage to the reef's coral.

Answer

Other environmental changes, such as a drop in ocean temperatures that reduced coral bleaching, were not the primary cause of the increased coral coverage during the five-year period.
The argument makes a causal claim: introducing predatory snails led to coral recovery by suppressing the starfish population. For this causal link to hold, the author must assume that no outside confounding variable was actually responsible for the coral recovery. The option stating that other environmental changes were not the primary cause eliminates an alternative explanation. Applying the Negation Test confirms its necessity: if outside environmental factors were indeed the primary cause of the coral growth, the conclusion that the snails caused the recovery is invalidated.

Step-by-Step Solution

1
Deconstruct the argument into premises and conclusion
Premise 1: Predatory snails that feed on juvenile starfish were introduced. Premise 2: Five years later, coral coverage increased. Conclusion: The snails caused the coral recovery by suppressing the starfish population.
Isolating the causal gap between the intervention and the observed outcome is essential for assumption analysis.
2
Identify the unstated assumption required for this causal claim
The author assumes that no alternative explanatory factor caused the observed increase in coral coverage over the same timeframe.
Causal conclusions depend on ruling out confounding variables that could independently account for the effect.
3
Apply the Negation Test to the correct option
Negated statement: 'Other environmental changes WERE the primary cause of the increased coral coverage.' If true, this completely undermines the conclusion that the snails drove the recovery.
A statement is a necessary assumption if its logical negation destroys the validity of the argument's conclusion.

Key Concept

Identifying Necessary Unstated Assumptions (Causal Reasoning and Negation Test)
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Topics

Identifying Underlying Assumptions

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