PrepTest 149, Section 3, Question 8
Over the last thousand years, plant species native(60 more words not shown)
Which one of the following,(10 more words not shown)
- AMost of the(16 more words not shown)
- BMany plant species(16 more words not shown)
- CCommercial development on(12 more words not shown)
- DThe rate of(15 more words not shown)
- ELarge land mammals(17 more words not shown)
Reading the argument
Why would island plants be disappearing faster? My first thought might be rising sea levels: losing some land could matter much more to a small island than to a mainland region. That's a possible explanation to investigate, not something the passage establishes.
The biologists offer a different explanation. Imagine people arriving with large grazing animals, such as goats, on an island whose plants haven't developed defenses against them. The animals could eat plants that aren't equipped to withstand that new pressure. That makes sense as a possible account of the losses.
What we’re looking for
But a sensible explanation still needs evidence. The passage connects the arrival of large mammals with human colonization. For a strengthener, I'd want evidence that these animals really are connected to the plants' disappearance, or something that makes a competing explanation less convincing. We don't need to prove that this is the only threat island plants face.
Working the answers
- A
A count of surviving species doesn't establish the reason for the difference in extinction rates. There could be more surviving mainland species for several reasons, including how many species there were to begin with. A doesn't connect island losses to a lack of defenses against large mammals.
- B
Nonnative plants becoming established on islands doesn't show that mammals caused the native plants to disappear. We aren't told whether these arrivals replace native plants, resist the animals, or have some other advantage. Their success alone doesn't supply the connection in the biologists' explanation.
- C
Habitat loss offers another possible reason native plants disappear. That could compete with the explanation about large mammals; it doesn't support that particular explanation. Both causes might operate, but evidence for commercial development's harm isn't evidence that a lack of defenses against mammals accounts for the pattern.
- DCorrect
This lines up the extinction increase with human arrival, which the passage connects to the establishment of large mammals. Before colonization, those mammals ordinarily haven't become established; afterward, plant extinctions rise dramatically. That is the timing we would expect if newly arriving mammals were harming undefended plants. Other changes accompanying colonization could still matter, so D doesn't prove the mechanism. It gives the proposed explanation a relevant pattern to account for, rather than merely a plausible story.
- E
Preferring mainland plants doesn't give us support for mammals devastating native island plants. It points their preference toward a different food source. That doesn't mean they never eat island plants, but the answer supplies no increased feeding pressure on the plants whose losses we're trying to explain.
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