PrepTest 106, Section 1, Question 5
The number of codfish in the North Atlantic(59 more words not shown)
Which one of the following,(7 more words not shown)
- APeople who fish(17 more words not shown)
- BWater pollution poses(11 more words not shown)
- CThe harp seal(14 more words not shown)
- DCod feed almost(14 more words not shown)
- EThe cod population(14 more words not shown)
Reading the argument
More seals and fewer cod don't by themselves tell us what caused the decline. Perhaps people are catching more cod. Perhaps another predator is eating them. We shouldn't automatically blame the seals just because the two populations moved in opposite directions.
But the author goes further and largely clears the seals. Cod are only a tiny part of their diet, so the growing seal population probably hasn't contributed much to the decline. Does that evidence rule out a substantial effect?
There are more than three million seals. Even a small part of that enormous group's diet could matter to a much smaller cod population. Imagine only 100,000 cod were present. Losing a significant fraction of those fish might still make cod a tiny part of everything the seals eat. We need the amount of cod eaten relative to the cod population, not just its share of the seals' meals.
There could also be indirect effects. Perhaps more seals support more predators that also eat cod. Or what if the seals and cod compete for the same food? Call it fish Q. If the growing seal population consumes more of fish Q, cod could go hungry even though seals rarely eat cod themselves.
What we’re looking for
That's the possibility I'd especially look for in the weaken question: a shared food source. We can accept the statement about the seals' diet and still find a way the seals could be contributing to the decline.
Working the answers
- A
Inconveniencing fishermen doesn't establish harm to cod. If the seals make it harder to catch cod, they might even reduce fishing pressure and help the cod population. The answer doesn't say how much fishing changes, so we shouldn't assume that benefit is proven. But it offers no mechanism showing that the seals contribute to the decline.
- B
Pollution could be a threat to cod, but that doesn't show that the increase in seals has significantly harmed them. The author hasn't claimed cod face no other dangers. We need evidence against the attempt to discount the seals, not merely another possible problem for the fish. Pollution and an effect from seals could also coexist.
- C
This describes water in which seals thrive and cod don't form dense populations. It doesn't tell us that the seals cause the low cod density or that their growth explains a decline. A population can be sparse because of the temperature itself. We'd still need a connection between more seals and fewer cod.
- DCorrect
Capelin are the shared food source we were looking for. Cod eat almost nothing else, while capelin are a staple food for harp seals. More seals could consume more of that food and leave less available for cod.
The seals need not eat the cod to hurt them. Competition for something cod depend on supplies an indirect route from a larger seal population to a smaller cod population. We don't have to prove that all capelin disappear or that seals are the only cause. This makes their significant contribution more plausible despite the small place cod occupy in their diet.
- E
If the decline began before the seal increase, that increase cannot explain why the decline started. This gives the author's skepticism some support rather than weakening it. It doesn't prove that the later seal increase had no effect on an already declining population, but it supplies no new reason to blame that increase.
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