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

Christine on September 21, 2015


I'm having trouble understanding the reasoning behind the correct answer choice. If the worm did not prefer thin foliage layers, then why is it found in places with the thin foliage layers? Is it implying then that the worm originally prefers the thick layer required for the fern and ends up making it a thinner layer?

1 Reply

Melody on September 24, 2015

Here we have a strengthen with necessary premise question. Remember that a premise is necessary for a conclusion if the falsity of the premise guarantees or brings about the falsity of the conclusion. First we check to see if the answer choice strengthens the passage, and then, if it does strengthen, we negate the answer choice to see if its negation makes the argument fall apart. If the answer choice does both those things then it is our correct answer.

Conclusion: L. rubellus is thus probably responsible for the fern's disappearance.

Why? We are told that the goblin fern requires a thick layer of leaf litter. We know that in spots where it has recently vanished, the leaf litter is unusually thin and, unlike places where this fern is still thriving, the leaf litter is teeming with the L. rubellus, which eats leaf litter.

Our reasoning here is that L. rubellus is causing the leaf litter to be thinner since it eats the leaf litter.

So, when we have a cause and effect argument, we should always think about whether X caused Y, or Y caused X or if a third factor Z caused both X and Y.

Answer choice (E) states: "L. rubellus does not favor habitats where the leaf litter layer is considerably thinner than what is required by goblin ferns."

Does this strengthen the argument? Yes. We are explaining that it is not the thin leaf litter area tat is causing a large presence of L. rubellus. We are, therefore, eliminating Y causes X in order to support our theory that X (L. rubellus) causes Y (the thin leaf litter).

Negation: L. rubellus does favor habitats where the leaf litter layer is considerably thinner than what is required by boglin ferns.

Does this make the argument fall apart? Yes.

Here we are explaining that it is actually the thin leaf layer that causes there to be a lot of L. rubellus and not the other way around. Thus, L. rubellus is no causing the fern's disappearance, because in areas where L. rubellus is prominent, there was already thin leaf litter to begin with.

Hope that helps clear things up! Please let us know if you have any other questions.