Q14 - Guam has 40 times more spiders than nearby

[ohthatpatrick]

Question Type:
Strengthen

Stimulus Breakdown:
Conclusion: Spider population has increased because of the loss of bird species.
Evidence: a tree snake killed off most of the bird species on the island, and many birds preyed on spiders / messed with their webs.

Answer Anticipation:
Since this is a Causal Explanation, we can use the 2-pronged causal prephrase:
CURIOUS FACT: Why are there so many spiders on Guam?
AUTHOR'S EXPLANATION: the declining population of birds from 1940 - 1980 (birds prey on spiders and steal their webs).

We can strengthen by ruling out an alternate explanation for why there are so many spiders (NOT better habitat ... NOT better food supply ... NOT better mating conditions), or we can strengthen by increasing the plausibility of the author's story.

On Strengthen, correct answers more often do the latter. The most common answer is some form of covariation, so we might anticipate "the parts of the islands that DO still have lots of birds DO NOT have as many spiders", or "prior to 1940, Guam had a similar number of spiders to nearby islands", or maybe "the ten bird species eliminated included many birds who eat spiders / ruin webs".

Correct Answer:
A

Answer Choice Analysis:
(A) YES, this boosts the plausibility of the author's story. The loss of birds not only means fewer spiders being eaten and fewer webs being stolen but also means more insect food supply for spiders.

(B) Their tactic of counting spiders doesn't help us solve the causal mystery.

(C) We'd expect this, since there are 40 times more spiders, but this doesn't help us solve the causal mystery of WHY there are 40 times more spiders / webs.

(D) Irrelevant whether they've proliferated. If anything this would weaken because it would suggest that even though we lost 10 of 12 species, there might still be close to as many birds if the remaining 2 species have greatly increased in numbers.

(E) This is miles away from being relevant. We don't need to be convinced that brown snakes are STILL getting rid of birds. The author has already established that 10 of 12 species are gone. It's an island, so they're not coming back.

Takeaway/Pattern: The correct answer functioned by increasing the plausibility of the author's hypothesis. The pressure points for talking about population increases/decline with non-human species are predators, prey (food supply), habitat, and mating.

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