# 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

Question

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?

6 Replies

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.

Filippo on June 6, 2020

I am not quite sure I understand this explanation. Could anybody please try and explain again why E is the correct answer? Thank you!

Victoria on June 16, 2020

Hi @filozinni,

I'll give it a shot!

The passage concludes that the earthworm is probably responsible for the fern's disappearance.

Why? The fern requires a thick layer of leaf litter. In spots where the fern has disappeared, the litter is unusually thin and is teeming with the earthworm which likes to eat leaf litter.

Therefore, the passage concludes that the earthworm likely eats the leaf litter, lowering it to a point where the fern can no longer survive.

We are looking for the answer choice which, if negated, no longer allows our conclusion to be properly drawn.

What is the negation of answer choice (E)? The earthworm favours habitats where the leaf litter is thinner than what is required by the ferns.

This fact would no longer allow us to properly draw the conclusion. If the earthworm prefers habitats where the litter is thinner than what is required by the ferns, then it likely did not cause the fern's disappearance.

Why? The leaf litter would become too thin for the fern to survive prior to the arrival of the earthworms. Therefore, it is not possible for the earthworm to have eaten the leaf litter down to a point where the fern could not survive as it was already at that point prior to the earthworms' arrival.

If the earthworm does not favour these habitats, then we can draw our conclusion properly. If the earthworm favours habitats where the leaf litter is thicker or as thick as necessary for the survival of the fern, then it makes sense that they could have caused the fern's disappearance.

The earthworm would have arrived in its preferred habitat when the fern was still thriving in the thick leaf litter. We know that places where the fern has vanished have unusually thin leaf litter and many earthworms. Therefore, it is logical to conclude that the earthworms were the cause of the thinning of the leaf litter as the litter would have thinned after their arrival and we know that they like to eat leaf litter.

Hope this helps clear things up a bit! Please let us know if you have any further questions.

Filippo on June 16, 2020

It is more clear now, thank you so much for explaining this so thoroughly @Victoria!

Filippo on August 25, 2020

By relooking at it now, I do understand why E is the correct answer, but not really why D is wrong. Could @Victoria or anyone else please explain what makes D not a necessary assumption as well? thank you!

Victoria on October 14, 2020

Hi @filozinni,

Happy to help!

Answer choice (D) is not a necessary assumption because its negation does not cause the argument to fall apart. It is entirely possible that there are at least some spots in the forests of North America where both goblin ferns and earthworms of the species L rubellus are found. Maybe there are too few worms at these locations to have any sort of impact on the ferns. Maybe the worms just arrived there and are in the process of eating the leaf litter that the ferns require.

In this way, the author can still conclude that L rubellus is probably responsible for the fern's disappearance even if there are spots in North American forests where the two co-exist.

Hope this helps! Please let us know if you have any further questions.