Posted by: Chris | January 21, 2012

LSAT v. Newton

In the spirit of one of our earliest posts, here is an irritating question from an old LSAT I was recently reviewing:

Amphibian populations are declining in numbers worldwide.  Not coincidentally, the earth’s ozone layer has been continuously depleted throughout the last fifty years.  Atmospheric ozone blocks UV-B radiation, a type of ultraviolet radiation that is continuously produced by the sun, and which can damage genes.  Because amphibians lack hair, hide, or feathers to shield them, they are particularly vulnerable to UV-B radiation.  In addition, their gelatinous eggs lack the protection of leathery or hard shells.  Thus, the primary cause of the declining amphibian population is the depletion of the ozone layer.

Each of the following, if true, would strengthen the argument EXCEPT:

A) Of the various types of radiation blocked by atmospheric ozone, UV-B is the only type that can damage genes.

B) Amphibian populations are declining far more rapidly than are the populations of nonamphibian species whose tissues and eggs have more natural protection from UV-B.

C) Atmospheric ozone has been significantly depleted above all the areas of the world in which amphibian populations are declining.

D) The natural habitat of amphibians has not become smaller over the past century.

E) Amphibian populations have declined continuously for the past 50 years.

The LSAC answer and the one that Tai, amongst others, would affirm to be correct, beyond the jump.

The LSAC says the answer is A.

Leaving aside the correlation-equals-causation fallacy that the author employs, the clear correct answer is E).   A) is, at worst, neutral, to the strength of the argument; E) disconfirms it, albeit weakly.  If there is, as the author suggests, a constant dimunition of the ozone layer over the past 50 years, then you would expect a constant increase in the amount of UV-B radiation reaching the Earth’s surface.  Again leaning on the author’s logic, if you assume that UV-B radiation is harmful to the survival of amphibians, you would expect this constant increase of UV-B exposure would lead to a constant increase in the death rate of the amphibian population (assuming no selection takes place).  

However, answer E) does not measure the rate of death, but the overall population.  With a constantly increasing  death rate, one would expect the size of the population to diminish exponentially (integral of the death rate), rather than concomitantly with the shrinking of the ozone layer.  Thus, an observation like E) would serve to actively weaken the author’s argument and is the best answer for the question.

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Responses

  1. A is the better answer because E doesn’t preclude exponential population decline. Continuous change stands opposed to discrete change, not exponential change. That is: continuity involves the way change manifests as opposed to the rate of change. So E says is that the relevant populations have been declining smoothly, without discrete changes, over the same period that the ozone layer has been continuously depleting, which (weakly) suggests the absence of an alternative cause of the populations’ diminution. By contrast, sudden, non-continuous, drops in population, while still compatible with ozone depletion being the primary cause of population decline, also raise the possibility of some other more discretely devastating source of the population’s decline [flood or something like that].

    Also, A is not at worst neutral — it somewhat weakens the argument by implying that ozone depletion lets in fewer possible causes of population decline than, absent A, we have reason to think it does.

    • CF:

      I think you are right. I read the “continuous” in E) to be referring back to the “continuous” in the text and inferred a link between the rates. But it is true that this is not necessarily the case.

  2. Exponential decline is not necessarily a given. To the extent that they fill a distinct niche that cannot be co-opted by non-amphibians, a higher death rate due to increased UV-B would relieve population pressure and increase availability of food, habitat, and such – which is to say the natural equilibrium population would simply be shifted downwards with an increase in UV-B. Probably not a linear downwards shift, but also probably not an exponential downward shift (at least for small changes in UV-B; for large changes, amphibians would indeed go extinct at basically exponential rates).


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