PrepTest 139, Section 1, Question 19
The goblin fern, which requires a thick layer(58 more words not shown)
Which one of the following(8 more words not shown)
- AWherever there is(15 more words not shown)
- BNone of the(10 more words not shown)
- CDead leaves from(24 more words not shown)
- DThere are no(21 more words not shown)
- EL. rubellus does(18 more words not shown)
Reading the argument
The worm eats the leaves the fern needs. That is a plausible explanation—but did the worms arrive before or after the leaves disappeared?
The proposed story is that L. rubellus moves in, eats the leaf litter, and leaves too little for goblin ferns to survive. Yet the observations could fit another order: the litter becomes thin for some other reason, the ferns disappear, and the worms move into the newly suitable habitat. We'd then find worms where ferns have vanished without the worms having caused that disappearance.
We could also ask about a broader environmental change. Perhaps a change in temperature or rainfall harms the ferns while favoring the worms. Again, that's a possibility to investigate, not an established fact about either species.
What we’re looking for
For the required assumption, look for a defense against an alternative explanation of the association. The author needs the worms' presence to support blaming them for the loss of litter and ferns, rather than simply reflect conditions that already existed when they arrived.
Working the answers
- A
Needing thick leaf litter doesn't mean ferns grow everywhere that has it. Other requirements could be missing. The author can blame disappearing litter for a fern's loss without assuming that litter alone guarantees ferns in every location.
- B
Native earthworms could eat some leaf litter without accounting for the unusual loss in these spots. For example, they might consume much less than L. rubellus. The argument doesn't require every native earthworm to leave every leaf untouched in order to attribute the problem to this particular species.
- C
It doesn't matter whether most of the litter came from goblin ferns or from other plants. The argument concerns the amount of litter available to sustain the ferns and what removed it. It doesn't rely on the ferns supplying most of their own necessary litter.
- D
The fern and the worm could coexist somewhere with enough leaf litter to support the fern despite some consumption by worms. That wouldn't rule out worms depleting the litter in the particular places described. Blaming this species for the disappearance doesn't require total incompatibility everywhere in North America.
- ECorrect
If L. rubellus favored places where the litter was already too thin for goblin ferns, its presence could be a result of that thin litter rather than the cause. The same condition would explain both the ferns' disappearance and the worms' arrival.
E denies that habitat preference, protecting the argument from this alternative account. It doesn't prove the worms caused the disappearance or eliminate every other explanation. It addresses why finding these worms in fern-free, low-litter spots can be used as evidence against them.
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