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Question

Difficulty: HardWeaken

Marine biologist: Over the last decade, the population of the blue-crested penguin on Solara Island has dropped by 30 percent. During this same ten-year period, the number of commercial fishing vessels operating in the island's surrounding waters has doubled. The nets used by these vessels are known to frequently entangle and drown deep-diving seabirds. Therefore, the increase in commercial fishing must be the primary cause of the decline in the blue-crested penguin population.

Which one of the following, if true, most seriously weakens the marine biologist's argument?

  1. The blue-crested penguin only dives for prey during daylight hours, while the commercial fishing vessels deploy their nets exclusively at night.Answer
  2. B
    The nets utilized by these commercial fishing vessels do not actually frequently entangle or drown deep-diving seabirds.
  3. C
    In marine sanctuaries where commercial fishing has been strictly prohibited, populations of deep-diving seabirds have increased significantly over the last decade.
  4. D
    The commercial fishing vessels primarily catch a species of fish that constitutes only a very minor portion of the blue-crested penguin's diet.
  5. E
    While the number of commercial fishing vessels around Solara Island has doubled, these vessels still represent less than one percent of the global commercial fishing fleet.

Answer

The statement that the penguins only dive during the day while the nets are deployed exclusively at night.
The correct answer destroys the causal mechanism proposed by the author. If the penguins are strictly diurnal and the nets are deployed exclusively at night, the penguins cannot possibly be getting caught in the nets. This completely undermines the conclusion that the nets are responsible for the population decline.

Step-by-Step Solution

1
Identify the argument's conclusion.
The increase in commercial fishing is the primary cause of the decline in the blue-crested penguin population.
Understanding the exact claim being made is necessary to find evidence that undermines it.
2
Identify the premises and the proposed causal mechanism.
Premises: Penguin population dropped 30%; fishing vessels doubled; the nets used frequently entangle and drown deep-diving seabirds.
Weaken questions often hinge on the specific mechanism linking the premises to the conclusion.
3
Identify the logical gap (assumption) in the argument.
The argument assumes that the blue-crested penguins on Solara Island are actually encountering these nets and getting entangled in them.
Finding the unstated assumption reveals the vulnerability of the argument.
4
Evaluate the options to find one that attacks this assumption.
The correct option shows that temporal separation (day vs. night) makes it impossible for the penguins to encounter the nets, entirely severing the causal link.
Severing the specific causal mechanism is one of the strongest ways to weaken a causal argument.

Key Concept

Weakening a Causal Argument by Severing the Mechanism
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Topics

Weaken

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