Logical reasoning PrepTest 153 · Section 3 · Question 14

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen

Stimulus Summary:
Terrifying stimulus here. Guam has 40x more spiders than neighboring islands have. Biologists say this happened because an invasive snake killed off 10/12 of Guam's native bird species. They support this theory by pointing out that not only do many birds eat spiders, some birds steal spiders' webs to use in their nests.

Answer Anticipation:
First thing to notice here is that the conclusion is causal, so we're looking to strengthen causation. We'd ideally like to see some kind of additional support for the bird decline causing a spider explosion (so sorry about that mental image). For example, if there was another island of similar size where elimination of birds had the effect of causing spider populations to skyrocket, that would strengthen this argument. Or, what if there's an additional piece of evidence that birds can keep the spider population down? Maybe birds also eat spider eggs? Either way, good luck sleeping tonight, LSAT students. (Except those of you with ice water in your veins and a fondness for massive quantities of spiders. That's what we call biglaw potential.)

Answer Explanation:
If birds were eating a bunch of bugs before, and now the birds are gone, there are more bugs for the spiders to eat. So theoretically the island can support more spiders, which might explain why it is, in fact, supporting more spiders. This strengthens, and is correct.

Key Takeaway:
If you have a common phobia, expect that the LSAT might throw it into a stimulus just to weird you out. Beyond that minor insight, keep in mind that strengthening a causal conclusion can take the form of giving an additional mechanism for the causal relationship. In this case, it's competition for prey, on top of the mechanisms of predation and web thievery already noted in the stimulus.

Incorrect. Neutral additional factoid, a common form of trap answer on strengthens. This has no effect on the argument one way or the other.

Incorrect. This is likely true, but doesn't strengthen the conclusion. Would make a decent most strongly supported answer, but not correct in this context.

Incorrect. This answer choice weakens the argument. If there are fewer species of birds but the remaining species have grown significantly in population, that might suggest a lack of birds isn't the cause of the increase in spiders.

Incorrect. Another neutral additional fact. It doesn't matter if they're tough to eradicate or not. Even if they've been totally eradicated, the birds are already gone and we've established that the snakes caused the birds to disappear.