PrepTest 139, Section 1, Question 15
To reduce the mosquito population in a resort(58 more words not shown)
Which one of the following,(10 more words not shown)
- AMost of the(16 more words not shown)
- BThe species of(21 more words not shown)
- CThe birds attracted(18 more words not shown)
- DSince the trees(18 more words not shown)
- EIncreases and decreases(9 more words not shown)
Reading the argument
Maybe the trees helped the mosquitoes more than the birds hurt them.
The plan sounds sensible: plant fruit trees, attract birds, and let the birds eat mosquitoes. The first steps worked. More birds arrived, and they ate many mosquitoes. Yet the mosquito population went up rather than down.
What else changed when the trees were planted? Perhaps they created useful mosquito habitat. If that benefit outweighed the loss to hungry birds, the same planting could attract mosquito predators and still leave more mosquitoes overall. That's a guess to investigate, not a fact we already know about these trees.
What we’re looking for
The question asks us to explain the increase. It's not enough to show that the plan was less effective than someone hoped. We need a reason planting the trees could actually backfire despite the birds eating many mosquitoes.
Working the answers
- A
Even if most of the bird species don't eat mosquitoes, other species did eat many of them. A helps explain why attracting more kinds of birds needn't produce a proportionate increase in mosquito eating. It doesn't explain why mosquito numbers rose instead of falling.
- B
The most numerous species might leave mosquitoes alone while other birds eat them. Again, that doesn't account for the increase. We need something helping mosquitoes enough to outweigh the birds that did eat them, not merely a group of birds that wasn't helping the plan.
- CCorrect
This supplies a different way the plan could backfire. The birds also ate insects that prey on mosquitoes, and they ate many more of those predators than mosquitoes. Removing mosquito predators can leave more mosquitoes alive later, potentially outweighing the birds' direct consumption of mosquitoes.
The birds can therefore be doing both things at once: eating mosquitoes and removing animals that would otherwise eat mosquitoes. That lost protection explains how bringing in more birds could produce the opposite of the intended result. We don't need the habitat guess once we have this alternative mechanism.
- D
Drier weather tends to keep mosquito populations down. That would give us another reason to expect a decrease, making the reported increase more puzzling. It doesn't explain how the trees caused the population to rise.
- E
A population can rise and fall in cycles, but E doesn't place this area in a rising part of a cycle or explain why planting the trees had the opposite of its intended effect. A general tendency to fluctuate doesn't connect this intervention with the result we're asked to explain.
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