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

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Complete Question Explanation

Assumption—CE. The correct answer choice is (E)

This stimulus deals with the goblin fern, a plant that requires a thick leaf litter layer on the forest floor. The fern is vanishing from the forests of North America, from areas where the leaf litter layer is unusually thin, and where there are many leaf-litter-eating European earthworms known as Lumbricus rubellus. The author concludes that the worms are probably the cause of the goblin fern’s disappearance.

As is the case with many causal arguments found on the LSAT, the author takes a correlation and jumps to a causal conclusion. In other words, the ferns have vanished from places where the earthworms propagate, and the author concludes that the worms must have been the cause.

The question that follows asks for an assumption required by the author’s argument.

Answer choice (A): The author’s argument does not rely on assuming that the ferns can be found in every thick leaf litter layer in North American forests. To confirm this, we can take away, or logically negate, this choice, and note the effect, if any, on the argument. The negated version of this choice would be as follows:

    Goblin ferns cannot necessarily be found wherever there is a thick layer of leaf litter in North American forests.

Since the negated version above does not weaken the author’s argument, this is not an assumption on which the author’s argument relies.

Answer choice (B): The author does not rely on this assumption, that no Native American earthworms eat leaf litter. To confirm that this choice is incorrect, the negated version is as follows:

    Some native North American earthworms eat leaf litter.

This does not weaken the author’s argument, that the culprits are the european earthworms known to be present where the goblin ferns have been vanishing. Since taking away the assumption presented in this choice does not weaken the argument presented in the stimulus, this confirms that the argument does not require this assumption, which should be ruled out of contention.

Answer choice (C): Based on the fact that the european earthworms are present where the fern has been vanishing, among unusually thin layers of leaf litter, the author concludes that the worms are probably the culprit. This does not require the assumption presented in this choice, which deals with the makeup of the layer of leaf litter, so this is not the right answer to this Assumption question.

Answer choice (D): The author’s argument does not rely on the assumption that no spot in the forest is home to both the fern and the L. rubellus. In fact, if the worm is to blame, that would seem to support the idea that they would both appear in many locations, so this cannot be an assumption on which the author’s argument relies.

Answer choice (E): This is the correct answer choice. The assumption presented here is that earth worms are not drawn to the thin layer of leaf litter. To confirm this choice as the right answer, we can apply the Assumption Negation Technique, by logically negating the assumption to see whether the author’s argument suffers as a result. This choice would be logically negated as follows:

    L. rubellus does favor habitats in which the leaf litter layer is much thinner than what is normally required by goblin ferns.

If this is the case, then it would seem that the thin layer of leaf litter could have been the cause of the appearance of the earth worms (as opposed to the earth worms causing the thin layer). Since the negation of this choice weakens the author’s argument, this is confirmed as the correct answer to this Assumption question.
lsatstudier
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Hi,

I'm sorry for all of the questions. Why is the answer E and not A? I'm not really sure how to diagram this question either. Is this section of the exam considered considerably more difficult compared to other tests?

Thank you!
Kristina Moen
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Hi lsatstudier,

We're glad you're using the Forum and asking questions!

There are really two reasons to diagram a stimulus. I don't consider underlining the conclusion, which I always do, to be "diagramming." Those reasons are when you see conditional reasoning and causal reasoning (aka "cause and effect").

Here, I see conditional reasoning in the first sentence with the word "required":
Fern -> thick layer of leaf litter (try saying that 10 times fast... :-D )

In the conclusion, I see cause and effect. The author concludes that the L. rubellis is probably the cause of the ferns disappearance. That's the only other part I'd diagram, so I'm clear on which part is the cause and which part is the effect. I would diagram it like this: L. rubellis => fern's disappearance. You can use a normal -> arrow too, as long as you don't get confused and think it's a conditional relationship.

The good news is that whenever you see an author concluding cause and effect, the argument is often weak and full of holes. It's very difficult to establish cause and effect in real life, so it's even harder when the test makers only have a few sentences to do it!

Here's we're told that 1. Fern -> thick layer of leaf litter. 2. In areas where it vanished, there's thin leaf litter and lots of worms. Conclusion: Worms cause ferns to vanish.

Lots of worms and no ferns are happening together, so one must cause the other, right? WRONG! That's correlation. What if there's an alternate cause of both the worms and thin leaf litter? What if the thin leaf litter causes there to be more worms, not the other way around? What is this argument assuming? Answer choice (E) is one assumption: That the worms are not favoring the habitat with the thin leaf litter. If you're still unsure, try the Assumption Negation Technique: "L. rubellus does favor habitats where the leaf litter layer is considerably thinner than what is required by goblin ferns." That kills the conclusion. The thin leaf litter layer was actually a factor in whether the worms moved there - not the other way around. So even if the ferns require thin leaf litter, the worms aren't really having any impact on that. The leaf litter is impacting the worms.

Answer choice (A) doesn't address the causal argument at all. The author is concluding that the worms cause the fern's disappearance. It's not important whether thick layer of leaf litter -> ferns or not. That's just flipping the first sentence, which was ferns -> thick layer of leaf litter.