Primary purpose items most often deal with the passage as a whole (as does the first question in this set), but the same guidelines apply here: The "primary" purpose/function should accurately cover MOST of what's happening (including anything that's clearly presented as a conclusion), but not necessarily every word. Anything unsupported immediately eliminates an answer choice.





¶3 doesn't mention anything about the practical situations faced by ecological researchers. A hypothetical case is laid out for the purposes of the 99%/1% discussion, but that case remains entirely theoretical. (It's only in ¶4 that there's some mention of the fact that ecologists' ability to substantiate this stuff is limited in certain cases.)

Not supported. Eliminate.




This passage does not engage in advocacy (= persuasive writing that argues for one side of an issue and/or against the other).




The easiest way to eliminate this choice is to notice that it isolates "death rates"—a statistic that only appears in ¶3 as one out of a set of three types of statistics (birth rates, death rates, and migration rates).
These three factors are mentioned in identical context, on absolutely equal footing, so it's not possible that just one of the three is somehow "PRIMARY" = more important than the other two!




Interestingly, this choice is incorrect for precisely the same reason as the previous one. When three factors are presented in exactly the same context with exactly the same degree of emphasis, all three of them must be accorded the same degree of importance in any discussion, meaning that no one of them can be "primary".

What's more, ¶3 makes no attempt at all to give any actual example of this phenomenon. In fact, the only specific point it makes on that front is to note that, in many cases, those details WON'T be known and will be quite hard to describe.




Most of ¶3 is dedicated to the principle that the author illustrates using the hypothetical case of a population whose numbers are 99% driven by density-independent factors and 1% driven by density-dependent factors. The point of that illustration is that, for long-term purposes, that 1% will eventually win out and determine the population average—thus underscoring the importance of exactly what is named in this choice.


Correct choice is E.