Between 1980 and 2000 the sea otter population of the Aleutian Islands declined precipitously. There were no signs of disease...
GMAT Critical Reasoning : (CR) Questions
Between 1980 and 2000 the sea otter population of the Aleutian Islands declined precipitously. There were no signs of disease or malnutrition, so there was probably an increase in the number of otters being eaten by predators. Orcas will eat otters when seals, their normal prey, are unavailable, and the Aleutian Islands seal population declined dramatically in the 1980s. Therefore, orcas were most likely the immediate cause of the otter population decline.
Which of the following, if true, most strengthens the argument?
Passage Analysis:
| Text from Passage | Analysis |
| Between 1980 and 2000 the sea otter population of the Aleutian Islands declined precipitously. |
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| There were no signs of disease or malnutrition, so there was probably an increase in the number of otters being eaten by predators. |
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| Orcas will eat otters when seals, their normal prey, are unavailable, and the Aleutian Islands seal population declined dramatically in the 1980s. |
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| Therefore, orcas were most likely the immediate cause of the otter population decline. |
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Argument Flow:
The argument starts with a puzzle (otter decline), eliminates obvious causes (disease/starvation), then builds a case for predation by connecting orca feeding behavior to the timing of seal population decline.
Main Conclusion:
Orcas were most likely the immediate cause of the sea otter population decline between 1980 and 2000.
Logical Structure:
The argument uses elimination reasoning (ruling out disease/malnutrition) combined with circumstantial evidence (orca behavior + timing of seal decline) to support the conclusion that orcas caused the otter decline.
Prethinking:
Question type:
Strengthen - We need to find information that would make us more confident that orcas caused the otter population decline
Precision of Claims
The argument makes specific claims about timing (1980-2000 otter decline, 1980s seal decline), causation (orcas eating otters when seals unavailable), and elimination of other causes (no disease/malnutrition)
Strategy
To strengthen this argument, we need new information that either: 1) Provides more direct evidence linking orcas to otter deaths, 2) Eliminates alternative predators that could have caused the decline, or 3) Shows the timing relationship between seal decline and otter decline matches orca behavior patterns
The population of sea urchins, the main food of sea otters, has increased since the sea otter population declined. This tells us about the food chain effects after the otter decline but doesn't help us determine what caused the decline in the first place. We already know otters declined - this just shows a consequence of fewer otters eating sea urchins. It doesn't strengthen the case that orcas specifically were responsible.
Seals do not eat sea otters, nor do they compete with sea otters for food. This eliminates seals as a factor, but we weren't considering seals as predators anyway. The argument already established that seals are orca prey, not otter competitors or predators. This doesn't add meaningful support to the orca theory.
Most of the surviving sea otters live in a bay that is inaccessible to orcas. This is powerful evidence! If orcas were killing otters, we'd expect the surviving otters to be exactly where orcas can't reach them. This creates a clear pattern: where orcas can hunt = fewer otters survive, where orcas can't hunt = more otters survive. This directly supports the conclusion that orcas caused the decline.
The population of orcas in the Aleutian Islands has declined since the 1980s. This actually weakens the argument. If orca numbers were also declining, it makes it less likely that they could have caused such a dramatic otter population crash. We'd expect stable or increasing predator numbers to cause prey decline, not declining predator numbers.
An increase in commercial fishing near the Aleutian Islands in the 1980s caused a slight decline in the population of the fish that seals use for food. This provides an alternative explanation for why seal populations declined, which supports the chain of reasoning (less fish → fewer seals → orcas switch to otters). However, this is much weaker support than choice C because it's indirect evidence rather than direct evidence about otter survival patterns.