Logical Reasoning

PT153 · S3 · Q14 Guam has 40 times more spiders

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Guam has 40 times more spiders than nearby islands have.

Conclusion

The loss of bird species has caused the increase in spider population.

Evidence

Many birds prey on spiders and some use spiderwebs in constructing nests.

Evaluate

Since we're trying to explain the cause of a Curious Comparison (Guam has way more spiders than nearby islands), we can use our Curious Fact / Explanation mindset.

Curious Fact ... why does Guam have so many more spiders than nearby islands? Author's Explanation ... because Guam has lost 10 of its 12 bird species (in 1940 they introduced a tree snake that decimated 10 of 12 native species by the 1980s, and these birds used to prey on spiders and use their webs in constructing nests).

Since we're Strengthening, we would either - rule out an Alternate Explanation for why Guam has so many spiders or - add plausibility to the Author's Explanation, that a lack of birds is what's driving the increase.

The most common way to Strengthen causal arguments like this is to provide an idea that shows , so we might see an answer like - in areas of the island where birds still live, there aren't that many spiders - on the nearby islands where there aren't many spiders, there are still lots of birds

Goal

Look for an answer increasing the plausibility that birds are the Causal Difference-Maker when it comes to the spider population or ruling out an alternate explanation for why there are lots of spiders.

14.

Which one of the following, if true, most strengthens the biologists' conclusion?

  1. Correct

    Birds compete with spiders for

    Why this is right

    This adds yet another reason why the presence of birds could suppress spider populations and why the absence of birds could allow the spider population to grow. Since they're competing for the same food supply, when birds are out of the picture, there's more food for spiders, so the islands insect population (the prey) can support a larger predator population (the spiders). Given that birds and spiders are limited by the same resource, it helps the plausibility of the "lack of birds enable lots of spiders" explanation.

    Skill tested: Strengthen · how this choice captures the argument's function is the move to repeat next time.

    58% picked this

  2. The biologists counted spiderwebs as

    No Impact

    We don't have any idea whether counting spiderwebs is or isn't a reliable method of ascertaining the spider population, so we can't judge this answer's impact. Even if we conceded that counting spiderwebs is a good way to judge the spider population's size, this would only help us better trust the premise that there are 40 times more spiders. It wouldn't have anything to do with the conclusion, which is about why we have more spiders.

    5% picked this

  3. Spiderwebs are more prevalent on

    No Impact

    This doesn't get at what's causing there to be lots of spiders; it's just presenting a fact that is consistent with the already established fact that Guam has 40 times as many spiders as do any nearby islands. Of course spiderwebs are more prevalent. There are 40 times as many spiders. I bet there's more spider poop too. This is telling us something we would have already known from common sense. It isn't helping us ascertain anything about the cause of this giant population of spiders, webs, poops.

    21% picked this

  4. The two bird species remaining

    Weakens, if anything

    If it turned out that the remaining two bird populations had greatly increased, now that all the other birds were dead, that would undermine the plausibility of the author's story. She is thinking that there are fewer birds than before, thus more spiders than before. All we know is that there are fewer bird species than before, but it's possible the 2 remaining ones have proliferated enough that the overall number of birds is still comparable. So the more birds we think there are, the more we'd be weakening the argument.

    7% picked this

  5. Brown tree snakes have proven

    No Impact

    We don't care whether the brown tree snakes still exist, are extinct, are easy to kill, are hard to kill. All that logically matters is that 10 of 12 bird species are now gone, and we're trying to assess the author's claim that the disappearance of these 10 bird species is the reason there are now so many spiders.

    9% picked this

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