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.