hadimadi wrote:
ArtVandaley wrote:
The argument talks about the time when pollution is severe, though option D is providing information for the "not severe" pollution.
D says that when pollution is not severe, predators have more impact than parasites. But, that doesn't mean when pollution is severe, parasites have more impact.
They can have equal impact.
Logically I am not able to derive anything from D that explains the damage to willow trees by leaf beetles.
egmat mikemcgarry please help.
Posted from my mobile device GMATNinja,
AndrewNI have to agree with this comment. From question stem we have the premises:
1. Combination of parasites and predators keeps beetles population in check
2. Severe air pollution -> Fewer predators and equal or more parasites
Now, adding premises (D) to the above:
3. Not severe air pollution -> predators impact on beetles greater than that of parasites.
This statement is logically equivalent to:
Parasites have greater impact than predators on beetles -> severe air pollutionNow pulling 1,2,3 together, I can't see how we can derive that: Severe air pollution -> particularly severe damage by beetles (this is the conclusion of the argument)
It could be, for example, that 1,2,3 are all true, and also, that during severe air pollution, comparably little damage is done by the beetles. All we need is find an example: Imagine we have significantly MORE parasites now. In fact, so many more that we now even kill more beetles than the predators would have. That could lead to comparably little damage, while 1,2,3 are still true ...
Hello,
hadimadi. The
highlighted part above from your post is an
assumption, not a logically equivalent statement to what is presented in the passage. We cannot comment on whether, in the
presence of severe air pollution, parasites have a greater impact than do predators on the leaf beetle population. Perhaps the two are equally effective in keeping that population in check, but their combined effectiveness is not strong enough to produce the same outcome as in less polluted areas—i.e. some sort of delicate population-controlling balance has been thrown off, and leaf beetles are getting away with damaging willow trees. You may have studied the three basic ways of altering a conditional statement in formal logic. Note that these alterations may or may not produce truthful, logically sound statements themselves.
Statement: If p, then q.
Converse: If q, then p.
Inverse: If not p, then not q.
Contrapositive: If not q, then not p.
Now, if we replace the statement with pertinent information from answer choice (D), we get the following:
Statement: [If] air pollution is not especially severe, predators have much more impact on leaf-beetle populations than parasites do.
Converse: [If] predators have much more impact on leaf-beetle populations than parasites do, air pollution is not especially severe.
Inverse: [If] air pollution is especially severe, predators do
not have much more impact on leaf-beetle populations than parasites do.
Contrapositive: [If] predators
do not have much more impact on leaf-beetle populations than parasites do, air pollution is especially severe.
Perhaps you can see why your statement from earlier is not the same as any of the variations above. The fact of the matter is that the passage tells us that
leaf beetles cause particularly severe damage to willows in areas with severe air pollution. It could still be true that within an interpretation of
much more impact, even in severely polluted areas, predators could still be the primary driver in keeping leaf beetle populations at bay. But again, if these predators suffer from a
reduced population in such areas, the delicate balance could be thrown off, resulting in the damage described in the passage. Answer choice (D) supplies a reasonable explanation to the end of the sentence in question. No other answer choice does, although I would be open to hearing a counterargument if you missed the question.
Thank you for thinking to ask. (I think
GMATNinja and
KarishmaB have done a fine job handling the material in earlier posts.)
- Andrew
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