LSAT 16 - Section 2 - Question 21

This is a strengthening question, which we know because the question stem asks: Which one of the following, if true, most strongly supports the conclusion drawn by the entomologists?

The stimulus tells us that a few years ago some entomologists (insect scientists) introduced a fungus to oak forests to get rid of gypsy moths which eat oak leaves. The fungus is poisonous specifically to the caterpillars of these moths, and in the years since the population of both the caterpillars and moths has declined where the fungus was introduced. The entomologists conclude with a hypothesis that it was the fungus which caused the moth decline.

Like many strengthening questions this one involves a causal hypothesis; the passage gives us a correlation between the introduction of fungus and the decline of the moths and caterpillars, and we are supposed to find the answer which provides more justification for the entomologists’ hypothesis that this correlation really is a causal relation. With that in mind, let’s go about eliminating some of these answers:

Correct Answer Choice (A) This answer gives us a subset of gypsy moths with which to compare the effects on the other susceptible moths. The only difference between the rest of the gypsy moths and this strain which we are told about is that the subset is immune to the poisonous fungus. This makes them a really good control population to test our hypothesis. Since the only known factor by which the two groups differs is whether they are affected by the fungus, and the resistant group is growing in size while the moths as a whole decline, this is good evidence that it is the fungus which has caused the overall decline of the moths and caterpillars.

Answer Choice (B) This question introduces other insects to the equation, and you should immediately recognize that no mention is made of them in the entomologists’ hypothesis. Other insect groups are irrelevant to the phenomenon of gypsy moth decline.

Answer Choice (C) This answer introduces another possible cause of the gypsy moth decline, predation. Considering we are trying to support the fungus hypothesis, this answer is if anything weakens what we want to strengthen, and an increase of moths and caterpillars following a drop in predation does nothing to explain the overall significant decline which we are interested in.

Answer Choice (D) This answer introduces an entirely new phenomenon, a decline in oak tree populations caused by air pollution and acid rain. This might be a good answer if this were a weakening question, since the moths feed on oak trees and therefore the decline in oak trees would be an alternative explanation for the decline of moths. But since this is a strengthening question this answer does the opposite of what we are looking for, and is consequently incorrect.

Answer Choice (E) Just as A introduced a control population which was immune to the fungus, E introduces a control population where the fungus has not been introduced. The problem is that where A suggested the fungus as a cause because relative to the general moth population the fungus-immune moths seemed to be doing well, here the moths seem to be declining regardless of whether the fungus is present. This shows that the correlation between population and fungus decline is very weak, and therefore weakens the fungus hypothesis which we want to strengthen.