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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