PrepTest 117, Section 2, Question 4
The top 50 centimeters of soil on Tiliga(70 more words not shown)
Which one of the following(8 more words not shown)
- AThe bird species(14 more words not shown)
- BMany of the(17 more words not shown)
- CThe arrival of(21 more words not shown)
- DBones from bird(18 more words not shown)
- EThe birds that(14 more words not shown)
Reading the argument
Fewer birds after people arrive certainly makes people look suspicious. It doesn't establish that people caused the decline.
First, follow the evidence. The upper layer contains bones from birds eaten since humans arrived about 3,000 years ago. Comparing that layer with the older layer reveals that the earlier bird population was larger and more varied. The author uses that difference to blame the arrival of humans for the loss.
One question is how well bones in soil represent the whole population. But even if we accept what the comparison tells us about the decline, we can still question its cause. What if the weather changed around the same time? What if a disease arrived and killed many birds? The birds could decline after people arrived without the people being responsible for all that change.
What we’re looking for
We're asked to weaken the argument. I'd look for an alternative cause that fits the timing. We don't have to show that people never killed birds—the passage expressly says they ate them. We need a reason to doubt that their arrival explains the dramatic loss in population and diversity.
Working the answers
- A
Having few natural predators doesn't supply another explanation for the decline. If anything, it makes predation by other animals a less promising alternative to the human explanation. Few predators isn't none, so A doesn't prove that humans caused the loss; it simply doesn't give us the challenge we're looking for.
- B
This makes humans look more suspicious. Similar islands without people kept many of the bird species until much later, while Tiliga lost them earlier. That comparison fits the thought that human arrival made a difference. It doesn't undermine the author's explanation.
- CCorrect
A microbe deadly to many birds arrived immediately before the first humans. Now we have another possible cause in the right time period. Birds could carry it onto the island, and it could kill other birds without human involvement.
This doesn't prove that the microbe caused the whole decline or that humans had no effect. It does mean the decline can no longer be confidently attributed to people just because it accompanied their arrival. The evidence needs to distinguish between those possible causes.
- D
Some species eaten by people were present before people arrived. That is consistent with a decline. Imagine the older island had species 1, 2, 3, and 4, and species 1 remained available for people to eat after the other three disappeared. Finding species 1 in both layers wouldn't undo the loss of the others or explain what caused it.
- E
Poor flight doesn't tell us how many birds there were or what caused their numbers to fall. It might even have made the birds easier for arriving people to catch. Without a connection that points away from the human explanation, this feature of the birds doesn't weaken the argument.
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