Logical Reasoning

PT141 · S4 · Q15 Hine's emerald dragonflies are an

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Hine's emerald dragonflies are an endangered species that live in wetlands.

Given that ...

Hine's emerald dragonflies (HED) are endangered, live in wetlands, and have larvae that can survive only in the water, where they might be preyed upon by red devil crayfish (among others).

Why is it that ...

HED populations are more likely to remain healthy in areas where red devil crayfish are present than in areas w/o red devil crayfish.

Evaluate / Goal

Given that red devil crayfish are one of the species that eat dragonflies' babies, why is it a good thing for dragonfly populations when red devil crayfish are present?

Maybe the red devil crayfish, despite eating the dragonflies' babies, do other stuff that outweighs that downside (maybe they kill off other dragonfly predators or maybe they help dragonflies obtain food somehow).

Or, maybe red devil crayfish in general are bad for dragonflies, but they are a "marker" of a good habitat for dragonflies for some other reason (like the dragonfly needs bamboo to mate, and red devil crayfish are only found where bamboo is found).

15.

Which one of the following, if true, most helps to explain the surprising fact?

Correct
Red devil crayfish dig chambers
Why this is right
This explains a way in which red devil crayfish can do something positive for the dragonfly population, and thus why dragonflies tend to remain healthier in habitats where red devil crayfish are present. If the red devil crayfish dig these special water caves, then the dragonfly larvae have an aquatic nursery to grow in, even when the surrounding wetlands dry up. LSAC went to the trouble of telling us (for no apparent reason at the time) that the dragonfly larvae "can survive only in the water". When we hear details like that, and they don't seem to have any initial important or relevance, the correct answer often has plans to use them in some way. My stupid nickname for this is "crumbs", like LSAC sprinkled little crumbs of details into the stimulus so that they would have a detail they could make use of in a correct answer. There's no real actionable takeaway from this -- it's more about test savvy and spidey-sense. If you feel like you're hearing an extraneous detail, you might just register the suspicion of, "That phrase seems unnecessary. I wonder if they're planning to use that somehow later."
Skill tested: Paradox · how this choice captures the argument's function is the move to repeat next time.
70% picked this
Red devil crayfish present no
No Impact
This is neutral at best. But we need something that makes red devil crayfish (or the habitats where red devil crayfish are found) be a positive for the dragonflies. It needs to outweigh the negative fact that red devils are going to eat a bunch of dragonfly babies.
19% picked this
The varied diet of the
No Impact
This rules out one of the potential correct answer ideas we considered. We were thinking, "Okay, okay, red devil crayfish do a bad thing for dragonflies -- they prey on dragonfly babies. But maybe they also do a good thing, like prey on species that would otherwise be preying on dragonflies." But this answer says, "Nope. They don't do this helpful thing for dragonflies."
5% picked this
Red devil crayfish are found
No Impact
It doesn't make a difference to us whether red devil crayfish are found in more habitats, whether dragonflies are, or whether they're found in a similar number of habitats. We need a reason why the red devil habitats are better for dragonflies than the habitats without red devils.
2% picked this
Populations of red devil crayfish
No Impact
This answer is really saying nothing. It's saying "dragonflies do not have this effect on red devil crayfish populations". We're looking for an answer that says, "Red devil crayfish (or the habitats they're found in) do have this positive effect on dragonfly populations".
4% picked this

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