Q21 - Several years ago, as

[ebrickm2]

Conclusion: decline attributable to poisonous fungus
Premise: population significantly declined in area where poisonous fungus introduced.

Stem asks us what strengthens the conclusion.

So we have correlation and a causation conclusion, fine.

What would strengthen this...a lot of things potentially.

A, I suppose, states that in the absence of experiencing a decline, we can know that it was ineffectual on that population.

I may be wrong in my understanding of this, b/c I hate A as an answer choice.

Though we can't necessarily assume that the assumption that B makes is correct, it does eliminate a possible alternate cause potentially, and this is an effective means for strengthening an argument that is made on causal grounds.

Halp!

[ManhattanPrepLSAT1]

You're right on, with regard to the argument core and our task.

The argument's conclusion does mistake a correlation with causal relationship. It concludes that the fungus reduced the gypsy moth population.

We're asked to strengthen this argument, so eliminating a possible alternative cause would definitely strengthen this argument.

(A) strengthens the conclusion. If a strain that is unaffected by the fungus has increased its share of the total gypsy moth population, then whatever is reducing the gypsy moth population is not affecting this unaffected strain of gypsy moths.
(B) is irrelevant. Whether or not this fungus is poisonous to other animals does not support that the gypsy moth population has been reduced by the fungus.
(C) weakens the argument because it suggests that drops in gypsy moth populations could be the result of some species who prey on moths.
(D) weakens the argument because it suggests that acid rain could be responsible for the gypsy moth decline.
(E) weakens the argument because it says that the moths did not encounter a decline greater than other gypsy moth populations not exposed to the fungus. Why? Could there be something other than the fungus causing the common decline?

Good question! Does help clear things up?

[ebrickm2]

I don't think that B is necessarily irrelevant. Consider the following:

We want to strengthen the causal conclusion that the decline is the effect of the fungus use and nothing else.

A way a causal relationship can be strengthened is by eliminating potential alternative causes, as you noted above.

If we know that the population of other insect species is not being affected by the fungus, then we know than we can cross off our list of potential alternative causes a changed interaction between the insect species population and our caterpillars.

See where I am coming from?

[ManhattanPrepLSAT1]

Interesting point, and actually I do see where you're coming from. Correct me if I'm wrong, but I think you're saying that because the fungus is poisonous to few insect species other than the gypsy moth, that the there isn't some radical shift in the balance between the gypsy moth and those other insects that could be the explanation for the decline in the gypsy moth population.

Unfortunately, I just don't feel that the connection is close enough to be eliminating a possible alternative cause. If we knew that those other insects were predators of the gypsy moth, then maybe. But unless we knew that those other insects were predators of the gypsy moth, I think we can't say that this is really eliminating a possible alternative cause.

Good thinking! Just be careful to keep the assumptions to a minimum, unless you're being asked to find the assumption!

[cyruswhittaker]

This is a very good point about choice B. While doing this question, I narrowed down the choices to A and B. I was also reasoning similair to ebrickm2 regarding choice B, but the problem with this choice is that it requires more assumptions.

The question stem asks which "most strongly supports," and choice A gives a clear example of a strain of gypsy moth that differs from that under analysis (the specified causal relationship doesn't apply) in which the effect also does not occur.

For choice B to strengthen the argument, it would need more information about how the population dynamic between the gypsy moth and other insect species is inter-related (such as a predator-prey scenario), but nowhere in the argument is such a dynamic alluded to.

I've answered many questions incorrectly due to going beyond what the argument says, and I think this is a very important point: make sure that the answer choice is within the explicitly stated details of the argument.

[opulence2001]

I have been staring at this question so long my head was starting to hurt...but I finally get it.

So.. to my understanding B is incorrect because it only tells us that few other insects besides the gypsy moth are fatally affected by the fungus. It is only telling us about the few other insects and nothing about the moths in question.

I think I was assuming that B is saying that no other insects are fatally affected but gypsy moth. And it is isn't. This is the assumption you'd have to make for B to work, and this info is not given.

A is correct because it is saying the only gypsy moths surviving and increasing in population are those of a different strain altogether?

Am I right? Please say yes...

[ManhattanPrepLSAT1]

Not exactly. Answer choice (A) supports the argument that the decline is attributable to the presence of the poisonous fungus because the gypsy moths that are unaffected by the poisonous fungus are increasing as a share of the total gypsy moth population. Something would need to explain why, and the poisonous fungus would be a natural suspect since it's directly related to the immunity that is growing within the gypsy moth population.

Suppose the decline in the gypsy moth population was due to something other than the poisonous fungus. Well then, we would expect the unaffected (immune) gypsy moths to suffer equally with all the rest of the gypsy moths. If however, the immune gypsy moths are increasing as a share of the total gypsy moth population, that seems to imply that the factor diminishing the gypsy moth population is somehow related to the immunity of some gypsy moths to the poisonous fungus. If it didn't, what would account for that immunity to be growing as a percentage of the total gypsy moth population?

Answer choice (B) can be eliminated simply because it is out of scope. What happens to other insects besides the gypsy moth, doesn't play a role in helping us discover what has led to the decline in the gypsy moth population. Even if we were to say that fungus is not poisonous to any other insect other than the gypsy moth, that wouldn't necessarily support the conclusion that the poisonous fungus is responsible for the decline. That would potentially undermine a possible conclusion about whether the fungus is causing a decline in other insects, but "other insects" simply aren't part of the argument's conclusion.

Does that help clear this up?

[WaltGrace1983]

(A) is actually a typical case of ~cause --> ~effect in disguise. The caterpillars are unaffected: ~cause. These unaffected caterpillars are increasing their share, staying roughly the same population or perhaps even increasing population: ~effect.

How do we know this? We know that the population of caterpillars and adult gypsy moths have "significantly declined." If these unaffected caterpillars are increasing their share among the whole population then they are most likely not having much of a population decrease.

As for (B), it is simply out of scope. We don't care about other insect species unless we know that they directly relate to the gypsy moths in some way. The core was (gypsy population declining --> probably caused by poisonous fungus). If all we know from the answer choice is that there are a few other insect species that are poisoned by that fungus too then so what?