Because the question stem says "Which one of the following is an assumption on which the argument depends," This is a Necessary Assumption question. What we're doing here is assuming that the conclusion and the premises are true and, if they are true, figuring out one additional thing (the correct answer) that must also be true.
The argument starts with a troubling fact about the world that warrants explanation. Then we get another fact in the second sentence that correlates the disappearance of the fern in the first sentence with the presence of earthworms. The last sentence says, in essence, that since the two things are correlated, they are causally related. We need to find a necessary assumption that the stimulus is making by jumping from correlation to causation in this particular case. Before I look at the answer choices, I'm predicting that the correct answer could be something that connects the correlation to the causal explanation, or the correct answer could rule out an alternative explanation.
A. Way too strong, and we don't need to assume this. This doesn't even support our conclusion at all.
B. We don't need to assume this to be true. Even if this were false, the conclusion could still be true.
C. This doesn't link the earthworms to the goblin ferns at all and is irrelevant.
D. Way too strong. We don't need to assume there are no spots where both could be found just to say that one causes the other to disappear.
E. We definitely need to assume this for the conclusion to be correct. This answer rules out a possible alternate explanation for the fact that the ferns' absence is correlated with the earthworms presence. Specifically, this answer rules out the reversal of cause and effect. If E is not true, then maybe the L. rubellus is there because of the thin leaf litter layer, rather than causing the thin leaf litter layer (and thereby hurting the goblin fern.) If E's not true, it casts serious doubt on the given argument. Therefore this is the correct answer.
About Larkin: Larkin Robson is an LSAT Tutor and Attorney based out of New York City. He has a JD from NYU School of Law, has taught at West Virginia University School of Law and John Jay College in Manhattan. He has been teaching the LSAT for 8 years and founded an independent boutique LSAT company called 180 Degrees LSAT. Like Nathan, he scored a 179 on the actual test. His website is 180degreeslsat.com Tweet him @180degreeslsat.
From Larkin: I like the LSAT because there is actually a reason for the test. It is testing critical thinking and reading comprehension, both skills that are useful and valuable in the real world. I am a die hard philosophy buff and I think that the humanities and critical thinking are vastly undervalued in today's society, and the LSAT is essentially testing those skills. It means that studying for the test does not mean you are just studying for a stupid test, you are actually learning something useful.
Please ask questions and/or suggest corrections to anything that seems confusing... we want to make this the best resource we can for LSAT students. We'll have all the June 2013 explanations up as quickly as possible. Thanks for reading. Tell your friends! --nathan