#23 - Biologists found that off the northeast coast of

[wulflov]

Not to beat a dead horse, but after reviewing the above, I'm still not sure how I'm supposed to arrive at (B) as the indisputably most correct answer.

It seems to me like there are two "puzzles" prompted by the question: why is the death rate among the fish "extraordinarily high" while the P-plankton death rate is only 10%; and why are these species dying out when "no other species in the ecosystem appear to be affected."

I think answer (D) does more to resolve the first question--why is there a differential death rate--while answer (B) answers the second question--why these species and not others (though even then, not really--why is it attacking only the respiratory systems of those fish species and that plankton?)--while leaving unanswered the question of the differential death rates.

In addition, it would seem to make sense that the introduction of "a new strain of bacteria" would, by definition, be impacting another species in the ecosystem--the new bacteria itself.

The only reason I can think of to justify (B) is that it explains the full causal chain of events--the plankton deaths and subsequent fish deaths--while (D) explains the differential death rates, but not the cause, which maybe is what the test makers are going for when they ask for an "explanation."

What's the right way to think about this?

[Luke Haqq]

Hi wulflov,

Happy to help with this one.

You write,

It seems to me like there are two "puzzles" prompted by the question: why is the death rate among the fish "extraordinarily high" while the P-plankton death rate is only 10%; and why are these species dying out when "no other species in the ecosystem appear to be affected."

The underlined part is what I think may be a cause for some of the difficulty in locating the best of the answer choices. Namely, the stimulus states, "Biologists found...the P-plankton population has recently dropped 10 percent." That language is close to but can be distinct from referring to a "death rate." For example, if biologists found a 10% drop in population over a year, or a decade, then it might make sense to refer it as "only 10%." But the stimulus refers to the fact that the population "recently dropped" by this chunk, suggesting some significant cause.

So the issue to be resolved in this question is why both P-plankton and fish X, Y, Z are "beginning to show extraordinarily high death rates," with answer (B) suggesting this is because of the introduction of a new strain of bacteria that attacks both of them. In other words, there's a causal relationship, the cause is unknown, and (B) resolves that by positing a cause that explains the effects.

Hope that helps!