Logical Reasoning

PT139 · S1 · Q19 The goblin fern, which requires

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The goblin fern, which requires a thick layer of leaf litter on the forest floor, is disappearing from North American forests.

Conclusion

L. Rubellus is probably responsible for the goblin fern's disappearance.

Evidence

The goblin fern requires a thick layer of leaf litter. In spots where the fern has vanished, the litter is unusually thin and teeming with L. Rubellus, which eats leaf litter. L. Rubellus is not found in areas where the fern still thrives.

Evaluate

The author is trying to Solve the Causal Mystery of the disappearing goblin fern, so we can use our usual Curious Fact / Explanation template.

Curious Fact: Why is the goblin fern disappearing? Author's Story: L. Rubellus is eating the leaf litter until it's unusually thin, which they kills of the goblin fern, which needs thick leaf litter to survive.

We're supposed to now ask - how else could we explain the curious fact? - how plausible is the author's story?

Why else could the goblin fern be disappearing? The argument doesn't leave any breadcrumbs for possible alternate explanations, but changing climate / changing animal populations in the fern's habitat could be possible causes.

This author is assuming or .

The author did a pretty good job of establishing plausibility, since she showed covariation: in areas in which the fern is gone, the suspected culprit (L. Rubellus) is present. In areas where the fern still thrives, the suspected culprit is absent.

Goal

Hard to know exactly what we're looking for, but functionally the correct answer has to either - rule out a different explanation for the goblin fern's disappearance or - establish some idea that's essential for the plausibility of the causal story the author is telling.

19.

Which one of the following is an assumption on which the argument depends?

  1. Wherever there is a thick

    Too Strong: wherever Reversed Logic

    By saying that "goblin fern requires thick leaf litter", we'd get this conditional: goblin fern present → thick leaf litter This answer choice is just reversing that logic and saying, thick leaf litter → goblin fern

    5% picked this

  2. None of the earthworms that

    Too Strong: none Fake Opposite

    This is just taking a fact we were told a certain European earthworm eats leaf litter and then flipping both ideas oh, then non-European earthworms don't eat leaf litter? If we negated this answer, it might start to feel like an alternate explanation, "hey, author, some earthworms native to North America eat leaf litter!", but that's a very weak suggestion of an alternate cause. Are those earthworms also found in areas where goblin ferns have disappeared, but not in areas where goblin ferns still live? Until we know these other earthworms also have that relationship to goblin ferns, they're not going to be a very tempting alternative hypothesis.

    8% picked this

  3. Dead leaves from goblin ferns

    Too Specific: greater part

    Just because the goblin fern requires a thick layer of leaf litter doesn't imply that their dead leaves are the majority of the leaf litter in areas where they've vanished (or in areas where they presently live). Requiring thick leaf litter ≠ Being the primary contributor to leaf litter.

    5% picked this

  4. There are no spots in

    Too Strong: no spots

    The author said that there are no locations where goblin ferns are thriving that are also teeming with (full of) L. Rubellus. So, we'd never find both of them in great numbers in the same place, according to the author. But we still might find some numbers of each of them in the same place. We might say that there are no areas where live country music is thriving that are also teeming with fans of rap music. But there are still areas where both live country music and rap fans coexist.

    19% picked this

  5. Correct

    L. rubellus does not favor

    Why this is right

    This has the lovable "ruling-out" language we know is common to a lot of correct answers in Necessary Assumption: not "Not" answers are very easy to negate (just remove the "not"), and so we should test whether negating the answer creates an objection to the argument (if it does, then it's the correct answer). If we said to the author, "L. Rubellus favors habitats where the leaf litter is considerably thinner than what is required by goblin ferns", would that be an objection? Yes! That would pretty much ruin the plausibility of the author's hypothesis. She thought that goblin ferns were living in an area (thus, it had a thick layer of leaf litter), then L. Rubellus moved in, gobbled up leaf litter until it was unusually thin, and thus the goblin ferns were killed off. Negating this answer does serious harm to the plausibility of that story. L. Rubellus wouldn't be likely to move into an area where goblin ferns are, since the leaf litter would be considerably thicker than the habitats L. Rubellus favors. Given that we know L. Rubellus has moved into areas where goblin ferns used to live, this answer forces us to think that 1st - something thinned out the leaf litter (we still don't know what) 2nd - goblin ferns got killed off 3rd - L. Rubellus moved in, now that the leaf litter was to their liking

    Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.

    63% picked this

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