#19 -The goblin fern, which requires a thick layer of leaf

[akanshalsat]

I dont understand why D is wrong and E is correct??

[Dave Killoran]

Hi Akansha,

Thanks for the question! In simple terms, in this Assumption question (D) is way too strong to be correct, whereas (E) is related to the causal nature of the conclusion. Let's look at each:

• Answer choice (D): Does the author assume that there are absolutely no spots at all where the ferns (thriving or not) and earthworms are found together? No, that's simply far beyond anything the author suggested or needs to have for the argument to make sense. If we negate this answer, we arrive at: "There are no spots in the forests of North America where both goblin ferns and earthworms of the species L. rubellus can be found." How does the author respond to that, or does it kill the argument? It definitely doesn't kill the argument, and the author would likely respond along the lines of, "Yes, they are found together, and in those areas eventually the earthworm will eat the leaf litter the fern needs, leading to its disappearance."
  
  Answer choice (E): The causal belief in this problem is that the earthworm is eating the thick layer of leaf litter the fern loves, which ultimately leads to the disappearance of the fern. It's a causal chain: earthworm reduced leaf litter disappearing fern. Inside that chain there has to be a belief that the earthworm actually inhabits those areas of thick leaf litter. But, what if the earthworm didn't like the places where the leaf litter was as thick as what the fern likes? That might very mean that the earthworm is hanging out in places with less leaf litter, which are also the kind of places that the ferns never thrived to begin with. If that's the case, maybe the earthworm isn't what's causing the problem. That would hurt the argument, and that's exactly what the negation of this answer does.

Please let me know if that helps. Thanks!

[katnyc]

Do you mind doing a break down with explanations. I understand what basic assumption is (necessary) I just dont really understand the wording with E. I guessed E since I did not like the other answers.

[Jeremy Press]

Hi Kat,

The stimulus contains a flawed causal argument (not surprising, since most causal arguments you see in LR questions are flawed!). We have enough information in the stimulus to say that, in places where leaf litter becomes too thin, the goblin fern will disappear. That's because the first premise tells us the goblin fern requires a thick layer of leaf litter.

But the conclusion that the earthworm is probably responsible for the goblin fern's disappearance is a step too far. All we know from the premise is that, in places where leaf litter has already become too thin to support the goblin fern, we see a lot of earthworms. In order for that conclusion to be valid, the author has to assume that the earthworm arrived when there was still a thick layer of leaf litter, and that the earthworm subsequently ate enough of that litter to thin out the leaf litter and cause the goblin fern's disappearance.

If you wanted to attack that conclusion, you could suggest that it's possible the earthworm actually prefers there to be a thin layer of leaf litter in places that it inhabits, so that the earthworm wouldn't be likely to come into a particular forest that had a thick layer of leaf. If that were true (if the worm weren't likely to arrive until the leaves were already thinned out), something else would be responsible for thinning out the leaf litter (and, ultimately, the goblin fern's disappearance). Since that's a viable attack on the conclusion of the argument, the author also has to assume that's not true, which is where answer choice E comes in. Answer choice E says the earthworm does not "favor" (i.e. does not prefer) a habitat where the leaf litter layer is already thin. The author has to assume that, because if the opposite were true, the earthworm would be unlikely to have come into the forest when the leaves were thick and thus would not be responsible for the goblin fern's disappearance.

Answer choice (A) is incorrect because it's a Mistaken Reversal of the given conditional relationship. We know that the goblin fern REQUIRES thick layers of leaves. This doesn't mean that everywhere there is a thick layer of leaves, you'll necessarily find a goblin fern.

Answer choice (B) is incorrect because the argument doesn't require any assumptions about North American earthworms. The conclusion references a European earthworm because that's the one we find where goblin ferns are disappearing. That doesn't mean there are no North American earthworms (perhaps in other locations) that eat leaf litter.

Answer choice (C) is incorrect because it's talking about an irrelevant issue. Nothing about the argument requires us to determine precisely what kinds of leaves the leaf litter in these forests is composed of.

Answer choice (D) is incorrect for the reasons Dave stated in his post above.

I hope this helps!