PrepTest 129, Section 3, Question 7
Scientists removed all viruses from a seawater sample(26 more words not shown)
Which one of the following,(11 more words not shown)
- AViruses in seawater(18 more words not shown)
- BPlankton and viruses(9 more words not shown)
- CPlankton utilize the(10 more words not shown)
- DThe absence of(12 more words not shown)
- EAt any given(14 more words not shown)
Reading the argument
Removing viruses sounds as though it ought to help the plankton. Yet the plankton population shrank. What useful role might the viruses have been playing?
Maybe the viruses kill something that eats plankton. With the viruses gone, that predator could flourish and consume more plankton. Or perhaps the viruses limit a competitor for resources. Removing something harmful to one organism can have an indirect cost for another organism. It is also possible that the removal process takes away something else the plankton need, though we have no evidence of that yet.
What we’re looking for
The task is to explain the unexpected decline after the viruses were removed. My leading prediction is that the viruses had been keeping a threat to plankton under control. More broadly, I'd look for a benefit the plankton lose when the viruses disappear. We need the answer to connect the removal with the decline, rather than merely tell us something about plankton.
Working the answers
- A
To make this explain the decline, we'd need a further story: without viruses, plankton first multiply, use up their resources, and then crash. That is conceivable, but A does not state that this sequence occurred. Keeping a population below the maximum its resources can support does not by itself explain why removing that restraint makes the population smaller. The proposed initial increase and resource exhaustion are extra steps we'd be supplying.
- B
Removing a competitor for nutrients should free up resources for plankton. That would fit the scientists' expectation of growth, not explain why the population declined. B makes the unexpected result harder to understand.
- CCorrect
The viruses help supply food indirectly. When they kill organisms, nutrients are released, and plankton use those nutrients. Removing the viruses can therefore deprive plankton of a source of nourishment.
Imagine hyenas feeding on the remains of a gazelle killed by lions. Remove those lions, and the hyenas could lose that source of food. The viruses correspond to the lions, and the plankton correspond to the hyenas benefiting from what the lions leave behind. This is a different benefit from the predator-control possibility we predicted, but it directly explains the decline.
- D
This is tempting because it resembles the prediction: remove viruses, allow bacteria to flourish, and something gets harmed. But what gets harmed? “Other organisms” need not mean plankton. The bacteria might damage fish instead. The answer also says the absence of viruses can help bacteria flourish and that those bacteria sometimes cause damage; it does not establish that the relevant damage occurs here. C connects the viruses directly to nutrients used by plankton, while D needs those additional links.
- E
Existing infection tells us why scientists might expect virus removal to help. It does not identify a benefit the viruses provide or a harmful consequence of removing them. We still lack an explanation of the shrinking population.
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