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Sea Otters population decline

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Sea Otters population decline

by LulaBrazilia » Tue Jan 28, 2014 7:10 pm
In the late 1980s, the population of sea otters in the North Pacific Ocean began to decline. Of the two plausible explanations for the decline - increased predation by killer whales or disease - disease is the more likely. After all, a concurrent sharp decline in the populations of seals and sea lions was almost certainly caused by a pollution-related disease, which could have spread to sea otters, whereas the population of killer whales did not change noticeably.

Which of the following, if true, most seriously weakens the reasoning?

(A) Killer whales in the North Pacific usually prey on seals and sea lions but will, when this food source is scarce, seek out other prey.

(B) There is no indication that substantial numbers of sea otters migrated to other locations from the North Pacific in the 1980s.

(C) Along the Pacific coast of North America in the 1980s, sea otters were absent from many locations where they had been relatively common in former times.

(D) Following the decline in the population of the sea otters, there was an increase in the population of sea urchins, which are sea otters' main food source.

(E) The North Pacific populations of seals and sea lions cover a wider geographic area than does the population of sea otters.
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by Patrick_GMATFix » Tue Jan 28, 2014 7:40 pm
There are 3 basic ways to weaken an argument:

(1) add a counter premise
(2) refute an assumption
(3) draw a different conclusion from the given premises

Here the right answer should suggest that increased predation by killer whales is the reason for the population decline. The full solution below is taken from the GMATFix App.

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by ceilidh.erickson » Mon Feb 03, 2014 9:44 am
Two weaken an argument, we want to find a flaw in the reasoning.

Premises:
- 2 plausible explanations are predation by killer whales and disease
- disease affected seals and sea lions, and could have spread to sea otters
- the population of killer whales did not increase

Conclusion:
- disease is the more likely explanation

Flaw:
- what if predation increased for some reason, even if the population of killer whales did not increase?
- just because the disease can spread from seals to sea otters, does that mean it did?

We want an answer that suggests that predation by killer whales may have increased.

(A) Killer whales in the North Pacific usually prey on seals and sea lions but will, when this food source is scarce, seek out other prey.
If this is true, then killer whales may have sought out more sea otters when seals and sea lions fell prey to disease. Correct!

(B) There is no indication that substantial numbers of sea otters migrated to other locations from the North Pacific in the 1980s.
This is eliminating an alternative explanation. This wouldn't weaken, it would strengthen.

(C) Along the Pacific coast of North America in the 1980s, sea otters were absent from many locations where they had been relatively common in former times.
This supports the idea that the population declined, but we already knew that. It doesn't weaken the fact that disease is responsible.

(D) Following the decline in the population of the sea otters, there was an increase in the population of sea urchins, which are sea otters' main food source.
This is irrelevant to explaining WHY sea otters declined.

(E) The North Pacific populations of seals and sea lions cover a wider geographic area than does the population of sea otters.
This is irrelevant to explaining WHY sea otters declined.

The correct answer is A.
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education
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