Passage Summary: Goblin ferns need a lot of dead leaves on the ground to survive. In places where these ferns are dying out, the leaf piles are thin and full of a specific type of earthworm that eats leaves, so the worms are likely to blame.

Conclusion: The European earthworm is likely the cause of the goblin fern's decline in North American forests.

Reasoning: Goblin ferns depend on thick leaf litter, and in areas where the ferns have vanished, the litter is thin and contains earthworms that consume such litter.

Analysis: The argument observes a correlation between the presence of worms, the thinning of leaves, and the disappearance of ferns to conclude a causal relationship. To make this argument hold water, the author must assume there isn't a third factor—like a change in soil chemistry or a different predator—killing the ferns and thinning the leaves simultaneously. We need to bridge the gap between 'the worms are there' and 'the worms are the reason the ferns are gone.' Look for an answer that confirms the worms didn't simply arrive after the ferns were already dead.