LR Question 19 Explanation, by LSATHacks
QUESTION TEXT: The goblin fern, which requires a thick layer of leaf…
QUESTION TYPE: Necessary Assumption
CONCLUSION: L. rubellus is probably the reason the goblin fern is disappearing.
REASONING: The goblin fern needs leaf litter. Leaf litter is thin where the fern has vanished. The L. rubellus worm is often found where the fern has vanished, and the worm eats leaf litter.
ANALYSIS: This sounds persuasive. But really, the question has just given us a correlation between the worm and the decline in the goblin fern.
Correlations never prove anything. It could be true that worms cause thin leaf litter. However, the right answer raises the possibility that thin leaf litter attracts L. rubellus.
The worm only comes after the leaf litter and fern are already in decline. It is an effect, not a cause.
___________
- It doesn’t matter if goblin ferns are everywhere there is leaf litter. We just need to know that leaf litter is everywhere there are goblin ferns.
- It doesn’t matter if other worms eat leaf litter. Other worms could add to the problem, but it wouldn’t change the fact that L. rubellus appears to be harming ferns.
- This just tells us something about what leaf litter is like after the fern has vanished. We only care what happens to leaf litter when the fern is still there.
- The argument might be stronger if this weren’t true. If L. rubellus is killing the fern, then presumably the worm sometimes appears in the same area as the fern before the fern dies.
- CORRECT. This implies that something else causes the thin leaf litter. Then once the leaf litter is thin, the fern dies and the worm appears. The worm didn’t cause the fern’s death, it was an effect of the thin leaf litter.
Negation: L. rubellus favors habitats where the leaf litter is thinner than what is required by goblin ferns.
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Hi Graeme, I don’t believe your explanation below is correct for this question.
“This implies that something else causes the thin leaf litter. Then once the leaf litter is thin, the fern dies and the worm appears. The worm didn’t cause the fern’s death, it was an effect of the thin leaf litter.”
We are looking for an answer choice that strengthens the argument that the worms are “probably responsible” Your explanation above implies that E) rules out the worm as the cause of the lack of ferns. I believe it’s actually the opposite, E) rules out the possibility that the leaf litter layer was low before the worms arrived- indicating that the worms are “responsible”. So they did in fact “cause” the death of the ferns, whether or not the worms were the direct cause is out of scope.
This is a necessary assumption question (“which one of the following is an assumption on which the argument depends.”) So, we’re looking for the answer that’s necessary in order for the conclusion to follow from the premises; we’re not just looking to strengthen the connection between the premises and conclusion.
The explanation is indicating that (E) rules out a reason that the worms might not be responsible for the death of L. rubellus. The explanation for (E) is worded the way that it is to indicate that if (E) actually said “L. Rubellus does favor habitats where the leaf litter layer is considerably thinner…” then (E) would imply that something else causes the thin leaf litter layer.
Here’s an edit of what I sent in prior****
Okay, firstly thanks for posting this…I have a few questions…So because this is a necessary assumption question, we aren’t supposed to guarantee the author’s conclusion? Period? We’re only supposed to prove that the worm was at the location?
I may have miss understood the actual argument. The author says, “In spots where it has recently vanished, the leaf litter is unusually thin and, unlike those places where this fern still thrives, is teaming with the European earthworm Lymbricus rubellus.
When I worked this problem out I interpreted the above mentioned stimulus as meaning – that where in spots that it has recently vanished the leaf litter is unusually thin as well as that there was much of the earthworm Lymbricus rubellus. So where in North American forests where the goblin fern has disappeared, that leaf litter is thin and that the area is teaming with the earthworm LR.
So, I went with C, which only made sense in my above interpretation. Because If what I interpreted the argument as was correct, E wouldn’t make any sense because that particular earthworm does wouldn’t favor a thin layer of leaf litter.
I think the author of this question knew of this very vague way of putting together the comparison and designed the answers to represent this.
I’m just looking for some validation of my self critique as to where I went wrong.
Thanks.
You’re right about necessary assumption questions. We’re not looking for something that will guarantee the truth of the conclusion. We’re just looking for something that is unstated and that is required for the conclusion to follow from the current set of premises. So, without that necessary assumption, the conclusion would not follow from the premises.
Here’s a breakdown of the stimulus:
(1) The goblin fern requires a thick layer of leaf litter on the forest floor
(2) In spots where it’s vanished, the leaf litter is unusually thin, and populated with Lumbricus rubellus which eats leaf litter
Conclusion: Lumbricus rubellus probably caused the fern’s disappearance
So, yes, part of your interpretation is correct: where the leaf litter is thin, there’s much of the earthworm, and the fern has disappeared. But the relationship between the earthworm’s presence and the fern’s disappearance might just be correlative and not causation. (E) is necessary because it shows that that the relationship isn’t just correlative. When we negate (E) and add the negation to the argument, the argument falls apart. Here’s the negation: “L. Rubellus does favor habitats where leaf litter is considerably thinner than what is required by goblin ferns”. If that’s the case, then there’s much less reason to believe that L. Rubellus is the probable cause of the disappearance via the argument’s line of reasoning.
I’m having a hard time wrapping my head around this. In your analysis, you say that the worm is attracted to thin leaf litter… But E specifically says that the worm does *not* favor habitats where there is thin leaf litter… I’m so confused.
I meant to write “the *negation* of the right answer shows that worms favor habitats where leaf litter is thin”. I edited this for clarity.