PrepTest 153, Section 3, Question 14
Guam has 40 times more spiders than nearby(59 more words not shown)
Which one of the following,(7 more words not shown)
- ABirds compete with(4 more words not shown)
- BThe biologists counted(9 more words not shown)
- CSpiderwebs are more(7 more words not shown)
- DThe two bird(14 more words not shown)
- EBrown tree snakes(7 more words not shown)
Reading the argument
Forty times as many spiders is a striking difference. How could introducing a snake produce that? I can picture an indirect effect: snakes eat birds, fewer birds eat spiders, and the spiders have a better chance to survive. The passage adds another pressure from birds: some take spiderwebs for their nests.
That chain is plausible, but let's slow down at 'ten of twelve species.' Losing species isn't necessarily the same as losing most individual birds. Imagine 1,000 birds originally spread across twelve species. If the two surviving species expanded, there could still be 1,000 birds afterward, now belonging to only two species. Depending on what those birds eat and how they use webs, substantial pressure on the spiders could remain.
Those are hypothetical numbers, not information about Guam. They show why I want to know what actually happened to the birds and their effects on spiders. We could also ask whether another change, rather than the snakes, helped the spiders multiply.
What we’re looking for
For the strengthener, we need evidence for the proposed chain from snakes through birds to spiders. The explanation already makes sense as a possibility; what would give us another reason to think removing birds really helped the spider population?
Working the answers
- ACorrect
The birds also eat the insects that spiders need. That supplies an additional effect of removing birds: less competition for the spiders' food. We initially focused on birds eating spiders and taking webs. Now the answer gives us another way birds can hold spiders back.
Imagine insects that either a bird or a spider could catch. When fewer birds compete for them, more may be available to spiders. That makes an increase in spiders more understandable after the loss of birds. A strengthens the causal chain without needing to settle every question about the surviving species.
- B
This tells us how the researchers estimated the number of spiders. The issue is why the spider population increased. Merely naming the counting method neither connects the increase to snakes nor explains the effect of removing bird species. We would need additional information about that method to draw anything further.
- C
More webs fits the observation that Guam has many more spiders. It doesn't identify why the difference exists. Confirming the pattern we are trying to explain is different from supporting this particular explanation of it.
- D
This resembles the concern raised in our 1,000-bird example. If the remaining species multiplied, bird pressure on spiders might not have fallen as much as losing ten species suggests. We don't know that their growth fully replaced the lost birds or their effects, so D doesn't disprove the explanation. But it raises a concern rather than providing the requested support.
- E
Difficulty removing the snakes may explain why they remain on Guam. It doesn't tell us why the spiders multiplied or connect the snakes to that multiplication. A provides a further bird-to-spider mechanism; E only gives us a problem with getting rid of the snakes.
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