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

[Administrator]

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]

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]

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... )

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.

[cornflakes]

Wanted to share my thoughts on this one since i just did PT 69 and blind review and still chose D for both. Feel free to correct me if I'm off base in my rationale- def a noob to this.

In my opinion, your success on this problem is entirely contingent on your ability to detect and prioritize the connection between Lumbricus rubellus (lets just call it "the worm") and the leaf litter itself, and not the actual goblin fern (lets call it GF). On my first 2 passes at it, I kept looking at it only through the lens of worm and GF and vice versa, almost picturing the worm eating GF. The details concerning thickness of the leaf layer itself seemed ancillary, and I believe the test makers made it that way intentionally. So, when the conclusion claimed "worm is probably responsible for GF's disappearance", I was only able to interpret that as the worm ate the fern.

Looking at it through this vantage point I have laid out, answer choice D seems more plausible, albeit not perfect.

D: "There are no spots in the forests of North America where both goblin ferns and earthworms of the species L. Rubellus can be found"

Negation of D: There are (at least) some spots (logical opposite of no spots) of North America where both goblin ferns and earthworms of the species L. Rubellus can be found.

If we were to accept the negation of D as a true, we know that worms and GF coexist in at least one location in NA. This tells us that there are some cases where worms do not cause the disappearance of the GF. The issue we run into, however, is that the conclusion is only asserting what is "probably responsible for" the disappearance of GF. By using this wording, it allows for the possibility for there to be instances where GF and worm coexist AND for worms to still be the most likely CAUSE in the instances that GF does, in fact, disappear. The only way that the reasoning above I have explained would work, and to my misfortune what I was wishing the conclusion of the stimulus actually said, is if the conclusion stated that worms make GF disappear, in every instance. I detected this flaw in real time actually - so I already had my doubts on the problem.

When I moved to E, the commentary on the leaf litter did not compute because I was viewing the causal conclusion strictly in terms of worms -> GF decrease. The negation of E - "worm does favor habitats where the leaf litter layers is considerably thinner than what its required by goblin ferns" - did not pop out to me as weakening the argument, because I had not contemplated the importance of the leaf layer thickness.

The correct way to visualize this problem, and what I somehow missed, is to picture the worm eating the dead leaves/leaf layer itself and not the live GF. Think of GF and leaf layer as separate entities - GF needs lot of leaf layer to live (required in fact). In turn, what the author is asserting here is that the worm is eating the leaf litter layer, taking it down to very thin depth and out of the thickness range that allows GF to survive, in turn making it a probable cause for GF's disappearance.

Now taking a look at E again (we've already highlighted D's flaws and they apply the same to both vantage points for the problem).

Original Conclusion: worms are probably responsible for (the cause of) the GF's disappearance (via eroding the leaf layer)

Negation: "worm does favor habitats where the leaf litter layer is considerably thinner than what its required by goblin ferns"

If the negation were to be the case, then it suggests that the worms would not have favored the leaf thick habitat that GF lives in, and in turn weakens the idea that they whittled the leaves down to the thin layer described because they would have had to begin in that thick layer habitat in order to do so. It lends more credence to the idea that the decreasing thickness actually caused the worms to appear in greater numbers due to their favoring of the thinner depth habitat. Furthermore, it becomes more apparent that there is an alternate, and unmentioned cause (severe weather, manmade problems, animal mating, etc) of the leaf layer decreasing. This would directly weaken the causal conclusion because it would imply an alternate cause producing the effect.

The problem to me did not make sense until I looked at it through my second vantage point. I'm curious to know if others had trouble visualizing it this way or if they followed a similar line of logic. More importantly, I'd be curious to know if I could have solved the problem correctly without making the connection between worms and leaves. Hope this helps someone and feel free to comment.

[KelseyWoods]

Hi cornflakes!

It's good that you're thoroughly breaking down questions like this--it helps you get the most out of your practice test reviews!

Assumption questions can be tricky so it's great that you're using that Assumption Negation Technique--it's really helpful!

One thing to be aware of with Assumption questions that involve causal arguments--typically you're looking for a Defender Assumption. Causal arguments leave themselves very open to attack and Defender Assumptions eliminate those potential sources of attack. Defender Assumptions can be tricky because sometimes they look like they are irrelevant--that's why you missed (E) here, you thought the leaf litter wasn't relevant to the causal argument between the L. rubellus and the goblin fern. But answer choice (E) eliminates an alternate cause. We know that the L. rubellus is found in areas where the leaf litter is thin. The author concludes that this is because the L. rubellus is causing the disappearance of the goblin fern. But really it could just be that the L. rubellus prefers areas with thinner leaf litter. Answer choice (E) defends the argument against this potential alternate cause attack.

Answer choice (D) is too strong. Even if there are some spots where we have both L. rubellus and goblin fern, that doesn't mean that the L. rubellus isn't still causing the disappearance of the goblin fern. I tend to cause the disappearance of cake. That doesn't mean that you're never going to find me and cake together. On the contrary--we're often found together, just not for long periods of time! It takes some amount of time for me to make the cake disappear. Likewise it may take the L. rubellus some amount of time to make the goblin fern disappear. So even if they are sometimes found together, it doesn't mean that the L. rubellus isn't working on making the goblin fern disappear.

Hope this helps!

Best,
Kelsey