The stimulus tells us that the fern requires a thick layer of leaf litter. But that doesn’t imply that anywhere there’s a thick layer of leaf litter, the fern will exist. Even if the fern sometimes doesn’t exist when there’s a thick layer, that doesn’t undermine the author’s reasoning.

Not necessary, because other earthworms can eat leaf litter. What matters is that we’ve seen a correlation between fern disappearance and the L. Rubellus. The author does have to assume that there isn’t a similar correlation observed with other worms; but that doesn’t mean the author thinks no other worms eat leaf litter.

Not necessary, because even if dead leaves from the ferns are less than half of the leaf litter layer, that doesn’t undermine the author’s reasoning. Perhaps there are many other plants in the area that also make leaf litter, and that’s why the leaf litter could be mostly other plants, not the fern.

Not necessary, because the fern could be in the process of disappearing in some places. If it turns out that there are some spots where we find both the ferns and the worms, it could just mean that the worms are in the process of eating leaf litter and the ferns will die soon. This is completely consistent with the author’s reasoning.

Necessary, because if it were not true — if the worm DOES favor habitats where the leaf litter is considerably thinner than that required by goblin ferns — that could be the true explanation for the presence of the worm in areas where the fern has disappeared. It’s not that the worm eats the leaf litter, which then causes the ferns to die — it could be that the leaf litter is already thin, which causes the ferns to die, and the worms show up after the leaf litter is thin.