#5 - During its caterpillar stage, the leopard magpie moth

[Amrita22]

Hello Powerscore team, thank you for this explanation. I think I may be overthinking this question, so any insight you can provide would be appreciated. First, my understanding of a necessary assumption was for the argument to be valid, the assumption HAS to be true. (Argument = Valid) (Assumption = True). But, I don't see how A HAS to be true. For example, let's just say Natal Grass Cycad is not the only means by which the leopard magpie can make itself unpalatable, it's just the easiest. Perhaps a different type of leaf also has the same effect, but it requires climbing a tree that often tires the caterpillar. Then, the leopard magpie can still be in danger of extinction. I get that it's not black and white. Answer A, if negated, still weakens the logical force of the argument. But doesn't B do that too, if negated?

[Beatrice Brown]

Hi Amrita! Thanks so much for this question.

Your understanding of an assumption is correct: an assumption is an unstated premise, and the Assumption Negation Technique can be used to determine whether an answer choice is actually necessary for the conclusion to be true.

Answer choice (B) is not necessary for the conclusion to be true. Be careful when negating this answer choice! The negation of answer choice (B) would read: the leopard magpie moth has the speed or agility to escape from some of its potential predators. This would still allow the conclusion to be true because maybe one of its potential predators would still be able to catch the moth and cause its extinction. Another way to think of this is how the author of the stimulus would respond if you offered the negation of answer choice (B) as a rebuttal to their argument. The author could reply to you that sure, the leopard magpie moth may still be able to outrunsome of its predators, but the major predator would still be able to catch it so without having the toxin, the moth is in danger of extinction from this major predator.

Answer choice (A), however, is necessary for the conclusion to be true. If we negate answer choice (A), we get the following: Feeding on the Natal grass cycad is not the only means by which the moth can make itself highly unpalatable to predators. If there are other ways for the moth to become unpalatable to predators, then they won't face extinction, weakening the force of the conclusion.

Here's another way to think of answer choice (A): Remember that the evidence that the author relies upon here is that the Natal grass cycad is facing extinction and that the moth needs the toxin from this grass to become unpalatable to predators. But there's also a gap in the author's reasoning: what if there are other ways for the moth to become unpalatable to predators, such that it may not face extinction? If we add answer choice (A) to the argument, this defends the argument, making answer choice (A) a Defender Assumption.

[alistotle]

Isn't a more fundamental assumption that the argument is making that the moth being unpalatable makes it less likely to be eaten by predators. The argument simply assumes that unpalatable = not a food source. But maybe predators eat it anyway, because food is food? In that case A does not seem to be correct, because even if there was a myriad of other plants that could make the caterpillar unpalatable, it would still face extinction because the predators would not care.

[Luke Haqq]

Hi alistotle!

You raise a fair point in thinking about the meaning unpalatable. Just because something isn't pleasant to taste doesn't on its own imply that predators will not under any circumstances eat it. In the end, though, we don't know one way or another whether the argument "simply assumes that unpalatable = not a food source," even if that seems likely.

We are given the following conditional reasoning in the stimulus:

NGC Toxin

We might more fully describe this conditional reasoning as:

NGC Toxin Deters predators

Because "the Natal grass cycad is now endangered and facing extinction," the author of the stimulus concludes that "the leopard magpie moth is also in danger of extinction." In other words, the author assumes:

NGC Deter predators

It's not clear why the elimination of the grass means a risk of extinction, however, unless the author is assuming that the grass is the only means by which the magpie develops the mentioned toxin.

To test answer choice (A), we can apply the Assumption Negation technique, negating the answer choice and plugging it back into the argument to see if the argument falls apart. Answer choice (A) states, "Feeding on the Natal grass cycad is the only means by which the leopard magpie moth can make itself highly unpalatable to predators." Negating this would result in: "Feeding on the Natal grass cycad is [not] the only means by which the leopard magpie moth can make itself highly unpalatable to predators." If this were true, then there is no necessary reason why the magpie would face extinction because of the absence of the Natal grass cycad. Thus this negated answer choice would make the argument fall apart, confirming that it is an assumption on which the argument depends.

[mtdaniel]

Although one of the stated premises is that "toxin makes magpie unpalatable to predators", the argument must then make the assumption that "being unpalatable is the only way to defend against predators", correct?

If this is the case, then isn't (B) also a required assumption? That is, saying that "being unpalatable is the only way to defend itself" compatible with "does not have speed or agility to defend itself"?

I understand that (A) is a better choice based on the assumption negation test, but maybe I confused myself by assuming "being unpalatable is the only defense".

[Jeff Wren]

Hi mtdaniel,

There have been several explanations above on Answer B, so I will only add one point that may not have been clear.

The argument begins "during its caterpillar stage, the leopard magpie moth feeds on a plant...." The implication is that the moth only needs to be unpalatable during this stage, while it is presumably vulnerable. Not to stereotype caterpillars, but they generally aren't super fast. Later, once the moth is actually a moth that can fly around, it may very well have the speed and agility to avoid predators.

The problem is that it won't make it to adulthood if it's eaten while still a caterpillar. By not specifying which stage they are talking about, Answer B seems to be implying adult moths. It would be like if I made a general statement that "cheetahs are very fast," but technically newborn cheetahs aren't very fast yet.

[sxzhao]

Hi I'm having big problem wtih Choice A being correct because, according to my reasoning, its negation does not break the argument.

The argument says grass facing extinction causes moth to be in danger of extinction. I noticed the phrase "to face extinction" and " to be in danger of extinction" - which should refer to the fact that the species are now scarce as opposed to extinct already.

If we negate choice A, we enter a world where moth gets its protection from food sources other species, but here moth can still face extinction danger as long as natal grass is its MAIN source of food, say consisting 90% of their diet. Natal grass does not have to the their only protection, it just needs to take a share that's big enough to make survial on other sources impossible. To me this is a sufficient but not necessary assumption.

In contrast, choice C describes a world where "abundant natal grass" is required to locate them, for which even the status of "facing extinction" serves as abundance's negation. If we negate chocie C, we say that moth is able to locate grass even if the latter is facing extinction. Then natal grass going extinct has zero effect on moth's locating this food source.

I acknowledge that there's a difference between "locating" vs. " having enough to feed" but choice C was the best answer after eliminating choice A, and I don't see how A can be correct here.

[Adam Tyson]

If the Natal grass cycad is not the only source of protection for the moths, then there is no reason to believe the conclusion of the argument, sxzhao.

I eat citrus fruits, which provide me with vitamin C, which is essential to my good health. If citrus fruits are not available to me for some reason, could I conclude that I will face increased illness due to a lack of vitamin C? Only if I assume that I have no other sources of vitamin C available. If I do have other sources, like vitamin pills, then there is no reason to believe I will be troubled by a vitamin C deficiency. Could I still face that deficiency? Sure, but the lack of citrus fruits is no longer a reason to think so.

My argument is analogous to the one in the stimulus. The author tells us that the moth gets protection from this thing that it eats, and then concludes that if that thing becomes hard or impossible to get, the moth will be inadequately protected. But if there are other sources of protection, there is no reason to believe they will be threatened. Could they be? Yes, but the argument gives no reason to think so.

You said "If we negate choice A, we enter a world where moth gets its protection from food sources other species, but here moth can still face extinction danger... ." That's true, but the argument would no longer lead to that conclusion. To get to that conclusion, you would have to do as you did - add more assumptions, such as that the Natal grass cycad is such a substantial portion of their diet that without it, even if they have some other sources, they will be in trouble. Instead of making additional assumptions to show that the conclusion could still be true, you should ask yourself whether the conclusion follows just from the evidence that was provided, even in the face of the negation of that answer choice.

When we negate answer D, we get that the moths can find the cycads even when it is scarce, but that doesn't ruin the reasoning in the argument, because there still might not be enough of it to support the moth population, and once the grass becomes extinct, there will be none left to find. In other words, there is still a reason to be concerned about the moth if their source of protection from predators is dwindling. The author doesn't have to believe that's the only way the moths can find their food; they only have to believe that when the cycad supply gets very small or disappears altogether, the moth population will suffer.