#25 - The populations of certain species of amphibians have

[Administrator]

Question #25: Assumption—CE. The correct answer choice is (A).

The argument challenges the scientists’ claim that industrial pollution is responsible for the decline in certain species of amphibians. To do so, the author proposes an alternative cause: natural variations in the weather could just as easily be the cause, because such variations cause most amphibian species’ populations to vary greatly from year to year.

• Scientists: Industrial pollution (cause) Decline in population of species A (effect)
  
  Author: Variations in weather (cause) Population fluctuation of most species (effect)

The author is certain that the alternative cause kills the scientists’ argument, making it impossible to prove the real cause for the decline. (A very similar argument is made by those who deny that fossil fuels are responsible for global warming).

At first glance, the argument makes sense: alternative causes can, and do, undermine claims of causation. Read a bit more closely, however, and you’ll realize that the author’s counterpremise applies to most, but not necessarily all, amphibian species. The scientists, by contrast, are only worried about certain specific species of amphibians. Are these among the species whose populations vary greatly as a result of natural variations in the weather? Hard to say. This is the real issue with this argument—if you spotted it, answering this question would be relatively straightforward.

Because this is an assumption question, the answer you select must contain a statement upon which the argument depends, i.e. a statement that is necessary for the conclusion to be true. Remember to apply the Assumption Negation Technique when examining your contenders: the correct answer choice, when logically negated, must definitively disprove the conclusion.

Answer choice (A): This is the correct answer choice. Due to the convoluted nature of this statement, your best—and perhaps only—approach is to negate the statement and ask yourself if the negation would weaken the author’s conclusion:

The amphibian species that scientists are worried about are among those species whose populations do not vary greatly as a result of natural variations in the weather.

If true, then the author’s counterpoint does not apply to the species in question, and the natural variations in the weather could not be responsible for their decline. Since the logical opposite of answer choice (A) weakens the author’s conclusion, answer choice (A) contains an assumption necessary for the conclusion to be true.

Answer choice (B): The author makes no assumptions about the relative impact of the two causes being discussed. If declines due to weather were just as large as those due to industrial pollution, that would actually strengthen, not weaken, the conclusion of the argument.

Answer choice (C): This is an attractive, but incorrect, answer choice. The author concludes that we cannot be sure which is the correct cause, not that there is only one possible cause. The causes need not be exclusive. Indeed, even if both factors—pollution and the weather—contributed to the amphibian population’s decline, that would still be consistent with the views expressed in the conclusion.

Answer choice (D): This answer choice can easily be eliminated on the basis of its language alone. The argument makes no assumptions about reversing the decline in the future. The issue at stake is what caused the populations of amphibians to decline in the past.

Answer choice (E): Whether one of the two possible causes can actually exacerbate the other is impossible to prove, and is hardly necessary for the argument’s conclusion to be logically valid.

[mpoulson]

Hello,

Can you explain how to apply the negation test for Answer A? Since it is already has two negatives "not known" and "do not vary", how do you know which on one to negate. I understand now why this is the answer, but in the explanation provided they only seem to apply the negation rule to part of the sentence (not known to known) and did not apply it to (do not vary). Thank you.

Respectfully,

Micah

[David Boyle]

Hello,

Can you explain how to apply the negation test for Answer A? Since it is already has two negatives "not known" and "do not vary", how do you know which on one to negate. I understand now why this is the answer, but in the explanation provided they only seem to apply the negation rule to part of the sentence (not known to known) and did not apply it to (do not vary). Thank you.

Respectfully,

Micah

Hello,

You negate only once, since if you negated twice, that might be a "double negative", and you would not really be negating. So only the primary verb in the sentence, "known", receives the negation treatment, by crossing out the "not" in front of it.

David

[Nikki Siclunov]

Micah,

When negating complex sentences (sentences that contain one independent clause and at least one dependent clause), always negate the verb in the independent (i.e. main) clause only. The dependent clause does not bear the weight of the negation, and should be left intact.

For instance:

Though Frank was rich, he was unhappy
NEGATES TO:
Frank was happy (whether or not he was rich)

Wherever you go, you can always find beauty
NEGATES TO:
You cannot always find beauty wherever you go.

The play was fascinating, as we expected.
NEGATES TO:
The play was not fascinating, contrary to expectations.

Vegetarians are less likely to develop arthritis than are non-vegetarians.
NEGATES TO:
Vegetarians are no less likely to develop arthritis than are non-vegetarians (i.e., vegetarians are at least as likely to develop arthritis as non-vegetarians).

The cities whose population declines have been attributed by experts to unemployment are not known to be among those cities whose population has been affected by lower birth rates.
NEGATES TO:
The cities whose population declines have been attributed by experts to unemployment are known to be among those cities whose population has been affected by lower birth rates.

Note: In the last example, it was especially critical to identify the main, independent clause and negate it (known not known). That said, it is just as important to understand the logical implications of your negation: If the cities whose population declines have been attributed to unemployment are among those cities whose population has been affected by lower birth rates, this would suggest that there are two competing causes (unemployment vs. lower birth rates) for the same effect (population decline). This would serve to weaken an argument positing either cause as the only plausible cause for the observed effect.

Hope this clears things up!

[mpoulson]

Great Response. Thank You.

[graceli17]

Hi,

Can you explain why "It is therefore impossible to be sure that the recent decline in those amphibian populations is due to industrial pollution." isn't the conclusion in this stimulus?

Thanks!

[Jonathan Evans]

Hi, Graceli,

This actually is the conclusion of the stimulus!

In our explanation above, we note that the author of this argument challenges the view of scientists who attribute population drops among amphibians to industrial pollution.

Instead, in support of this conclusion, the author posits that weather variations could be responsible for these fluctuations (this is the causal structure illustrated in the answer explanation above).

In our discussion of this argument, we focus on the assumption that these weather variations could in fact be responsible for the population decline. Since this is the locus of the flawed reasoning, this is where we have focused our explanation. However, you are absolutely correct that the last line expresses the main conclusion of the argument. The assumption here involves how applicable certain evidence (change in population of some amphibian species based on natural weather variations) is to the conclusion.

Does this help clarify this question? Thanks!

[glemon]

When I negate C, I get, "Both pollution and natural variations in weather caused the population declines in the amphibian population." If you know that to be true, it seems like you're sure that the declines were caused at least in part by the pollution (which goes against the conclusion). But I guess it's still not the right answer because the focus is on "being sure"? Like, even if the question stem writer knows that both pollution and weather are causes of the decline, they would still struggle to establish from the declines alone that pollution in part was a factor. Is that right?

[Rachael Wilkenfeld]

Hi Glemon,

Your negation was a bit off here, which might be why you are struggling. You need to have your negation say that it's either BOTH cause it or NEITHER do. That's the correct negation here.

Remember with negations we are looking to split the world into two---our answer choice, and the rest of the world. Our answer choice for answer choice (C) covers the idea that exactly one of pollution or weather caused the decline. Our negation, therefore, needs to cover the idea that both caused the decline or neither caused the decline. For that answer choice, the negation is consistent with the stimulus that we can't know if it was the pollution or not. Since the negation is consistent with the stimulus, it's an incorrect answer.

Hope that helps!