Leaf beetles damage willow trees by stripping away their leaves, but

Question

Which of the following most logically completes the passage?

Leaf beetles damage willow trees by stripping away their leaves, but a combination of parasites and predators generally keeps populations of these beetles in check. Researchers have found that severe air pollution results in reduced predator populations. The parasites, by contrast, are not adversely affected by pollution; nevertheless, the researchers’ discovery probably does explain why leaf beetles cause particularly severe damage to willows in areas with severe air pollution, since ________.

(A) neither the predators nor the parasites of leaf beetles themselves attack willow trees

(B) the parasites that attack leaf beetles actually tend to be more prevalent in areas with severe air pollution than they are elsewhere

(C) the damage caused by leaf beetles is usually not enough to kill a willow tree outright

(D) where air pollution is not especially severe, predators have much more impact on leaf-beetle populations than parasites do

(E) willows often grow in areas where air pollution is especially severe

ID: 500265

GMATNinja · Accepted Answer

The conclusion is that "the researchers’ discovery probably does explain why leaf beetles cause particularly severe damage to willows in areas with severe air pollution." How do we arrive at that conclusion?

"Leaf beetles damage willow trees by stripping away their leaves." 
 "A combination of parasites and predators generally keeps populations of these beetles in check." Thus, the parasites and predators help limit the amount of beetles and, presumably, the damage done to the willows. 
 "Researchers have found that severe air pollution results in reduced predator populations." A reduction in predator populations would be good for the leaf beetles. But with fewer predators to keep the beetles in check, the willow trees might suffer. So, air pollution sounds bad for the willow trees and good for the leaf beetles, but we do not know the extent of this effect. 
 "The parasites, by contrast, are not adversely affected by pollution." Thus, air pollution would have no effect on the parasites and thus no effect on the leaf beetles. So, according to the information in the passage, air pollution would not affect the willow trees or the leaf beetles.

Okay, so if air pollution only affects one of the two factors that keep the beetle populations in check, why are the beetles still able to cause particularly severe damage to willows in areas with severe air pollution? Shouldn't the parasites keep the beetle populations in check and thus prevent the beetles from causing particularly severe damage to the trees? We need something else to explain this phenomenon:

We need to explain why the beetles are still able to cause significant damage despite the presence of parasites that keep the beetle populations in check. We don't care whether the predators or parasites damage the willow trees directly. Choice (A) is irrelevant and can be eliminated.

If there are more parasites in areas with severe air pollution, then those parasites would further help to LIMIT the beetle populations. This effect should REDUCE the amount of damage done by the beetles to the willows. Thus, statement (B) does not explain why the beetles can cause particularly severe damage in areas with severe air pollution. Eliminate (B).

We don't care whether the beetles can KILL a willow tree. We are simply trying to explain why the beetles are able to cause particularly severe DAMAGE to the willow trees. Thus, choice (C) does not help explain the observed phenomenon and can be eliminated.

We are told that the predator populations are reduced in areas with severe air pollution. If predators affect the leaf-beetle populations MORE than the parasites, it makes sense that the parasites themselves might not have a huge impact on the beetle populations. That would explain why the beetles are still able to cause such damage even though the parasites are still there. Choice (D) looks good.

We don't care about where willows grow and where they do not. All we know is that the beetles cause severe damage to willows in areas with severe air pollution, and choice (E) does not help explain this phenomenon. Eliminate (E).

Choice (D) is our winner.

KarishmaB · Answer

"a combination of parasites and predators keeps a check" means this is what is happening today. A combination is keeping a check on population. You cannot say what will happen in the absence of either one of two. Say predators are absent which makes parasites double in number. Or perhaps the same number of parasites consume more beetles because more are available for consumption. So the parasites may give up other food sources and prefer beetles. Or it is possible that in the absence of predators, beetle population goes out of control. We don't know what exactly WILL HAPPEN. We can only hypothesise.

Beetles damage trees.
Predators and parasites control beetle population. 
Air pollution reduces predator population. 
Parasite population is not affected.

the researchers’ discovery probably does explain why leaf beetles cause particularly severe damage to willows in areas with severe air pollution, since ________.

The air pollution discovery explains why beetles cause severe damage in areas with severe air pollution - why beetle population is out of control in areas with severe air pollution. The discovery says that air pollution reduces predator population. So it must be something to do with that.

(D) where air pollution is not especially severe, predators have much more impact on leaf-beetle populations than parasites do

It says that predators control beetle population more i.e. they consume more beetles than do parasites. This explains that in case of severe pollution, predators population will reduce and hence beetle population will see a significant increase. Hence the trees will see more damage. 
With this information, it does seem that the discovery explains the severe damage to willows in areas with severe pollution.

Answer (D)

Nightfury14 · Answer

Question Type: Completes the passage

Leaf beetles damage willow trees by stripping away their leaves, but a combination of parasites and predators generally keeps populations of these beetles in check. Researchers have found that severe air pollution results in reduced predator populations. The parasites, by contrast, are not adversely affected by pollution; nevertheless, the researchers’ discovery probably does explain why leaf beetles cause particularly severe damage to willows in areas with severe air pollution, since __________.

A. neither the predators nor the parasites of leaf beetles themselves attack willow trees
 B. the parasites that attack leaf beetles actually tend to be more prevalent in areas with severe air pollution than they are elsewhere
 C. the damage caused by leaf beetles is usually not enough to kill a willow tree outright
  D. where air pollution is not especially severe, predators have much more impact on leaf-beetle populations than parasites do 
 E. willows often grow in areas where air pollution is especially severe

Statement D) Contrasts the difference in impact caused by the parasites to the predators and hence gives a proper justification for severe damage in areas with air pollution.

aragonn · Answer

Fact 1:  Of the two elements that keep leaf beetle populations in check -- predators and parasites -- only predators are adversely affected by severe air pollution. 
 Fact 2:  In areas with severe air pollution, leaf beetles cause particularly severe damage to willows.

Pre-thinking : 
The correct answer choice must explain why leaf beetles cause particularly severe damage to willows in areas with severe air pollution, even though air pollution does not adversely affect parasites. Few speculations:
- May be less parasites found in this area.
- Something else found in area that live on these parasite.
- parasites don't go near willow trees.
 
(A) neither the predators nor the parasites of leaf beetles themselves attack willow trees  --- the duos attack on willow tree is not a matter to talk about. either way it is not affecting.
 
(B) the parasites that attack leaf beetles actually tend to be more prevalent in areas with severe air pollution than they are elsewhere  --- well yes it should be but it is not. actually a contrast to a fact mentioned above, wrong.

(C) the damage caused by leaf beetles is usually not enough to kill a willow tree outright --- irrelevant.

(D) where air pollution is not especially severe, predators have much more impact on leaf-beetle populations than parasites do   --- This choice want to say that when no air pollution, predators have much more impact on leaf-beetle populations than parasites do, which means if we remove predators from the picture, parasite will make less impact on beetles' population. this is why beetles are impacting willow trees.

(E) willows often grow in areas where air pollution is especially severe --- this on is not pointing any light on the situation.

ArtVandaley · Answer

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

kornn · Answer

Dear  AjiteshArun   VeritasKarishma ,

I have one question on the passage:

a combination of parasites and predators generally keeps populations of these beetles in check

From the portion above, can we infer that an absence of either parasites or predators will not keep the beetles in check?

"A combination of"    should mean that  both parasites and predators are necessary  to keep beetles in check. Hence, according to me, it follows that the absence of either of the 2 elements should result in not keeping the beetles in check.

I am confused here because I think the argument stands sound on its own without any additional premise to support.

Thank you as always Sir !

AjiteshArun · Answer

Hi varotkorn , That's a great question.

I think that one of the ways we can use combination is to show that the things within the group contribute individually, so maybe that's what this question is going for. I'm not sure about that though, and it'd be great to get more opinions on this.

shameekv1989 · Answer

Predators impact greater than parasites impact in less severe air pollution does not necessarily mean that parasites will have no impact in absence of predators.

May be now that the predators are not present, parasites will not have any competition and will thrive on the leaf beetles.

For eg. When both predators and parasites are present and say there are 100 leaf beetles, 
Predators take care of 90 beetles and only 10 by parasites
Now that predators are not present 
All these 100 can be taken care by parasites. There is no limit as such from the argument on the check that parasites can keep.

MentorTutoring  - Could you please help!

AndrewN · Answer

Hello,  shameekv1989 . The problem is that the passage does not outline what  will  happen in areas in which the threat of one of parasites or predators to leaf beetles has been mitigated. We can only speculate what parasites may do to the beetles in the absence of predators (of leaf beetles). Perhaps some unknown predator  of leaf beetle parasites  thrives in areas with severe air pollution. We simply do not know. The fact of the matter is that  leaf beetles cause particularly severe damage to willows in areas with severe air pollution , and it is our task to figure out why  the researchers' discovery probably does explain  such a phenomenon. Of the answer choices provided, only (D) fits the linear logic of the passage, for reasons explained above by  VeritasKarishma  and  GMATNinja , among others.

I hope that helps. If you have further questions, feel free to ask. Thank you for tagging me, and good luck with your studies.

- Andrew

AnirudhaS · Answer

OE:
  Situation   Leaf beetles damage willow trees, but predators and
parasites keep leaf beetle populations in check. Air pollution reduces
populations of predators but not of parasites. Leaf beetles damage
willows especially severely in areas with severe air pollution.
  Reasoning   What would support the conclusion that air pollution’s
effects on the predator populations (but not on the parasite
populations) explains why leaf beetles damage willows the most in
areas with severe air pollution? The word since preceding the blank
space at the end of the passage indicates that the space should be
filled with a premise supporting the conclusion stated immediately
before the since. To support this conclusion, it would help to have
evidence that predators play a predominant role in keeping leaf beetle
populations in check, and thus that the reduction of predator
populations by air pollution could be sufficient to enable leaf beetle
populations to grow and cause especially severe damage.
A. The fact that neither the predators nor the parasites directly
contribute to harming the trees offers no reason to conclude that a
difference in how they are affected by pollution would contribute to
the harm that the beetles cause to the trees.
B. If the parasites are more prevalent in areas with severe air pollution,
then they are more likely to keep leaf beetle populations in check in
those areas, despite the reduced predator populations. Thus, the
decline in predator populations would more likely be insufficient to
explain why the leaf beetles cause more damage in those areas.
C. This observation is irrelevant to whether the decline in predator
populations explains why leaf beetles damage willow trees more
severely in areas with severe air pollution.
D.  Correct.  This indicates that predators play a predominant role in
keeping leaf beetle populations in check, so, as explained above, it
supports the argument’s conclusion.
E. This is not clearly relevant to whether the decline in predator
populations explains why leaf beetles damage willow trees more
severely in areas with severe air pollution. The argument’s
conclusion could just as easily be true regardless of whether willows
grow in such polluted areas frequently or infrequently.
 The correct answer is D.

kskumar · Answer

can anyone explain it has mentioned in the passage that but a combination of parasites and predators generally keep populations of these beetles in check then how can only one can help more than other.

GMATNinja

GMATNinja

GMATNinja · Answer

Check out our earlier analysis of this question  here , which might help to clear things up.

Regarding your exact question, you're right that the populations of leaf beetles are kept in check by a " combination  of parasites and predators." But the question is essentially asking us to provide a  reason  which can explain the researchers' finding -- namely, that "leaf beetles cause particularly severe damage to willows in areas with severe air pollution."

We know that "severe air pollution results in reduced predator populations," but that the "parasites, by contrast, are not adversely affected" by severe pollution. So if parasites were  just as effective  as predators at reducing leaf beetle populations, this would  fail  to explain the researchers' finding. In other words, if the parasites were just as effective as the predators, we'd expect that in areas with severe pollution, the parasites would continue to keep the beetles in check.

Answer choice (D) says that "where air pollution is not especially severe, predators have much more impact on leaf-beetle populations than parasites do." It's true that we can't  conclude  this from the passage, as you suggest. But it does provide the  reason  we've been looking for. Because if predators have a much larger impact on the beetles than parasites, we'd expect leaf beetle damage to be severe in areas with severe pollution.

I hope that helps!

hadimadi · Answer

GMATNinja ,  AndrewN

I 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 pollution

Now 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 ...

AndrewN · Answer

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 :   air pollution is not especially severe, predators have much more impact on leaf-beetle populations than parasites do.
 Converse :   predators have much more impact on leaf-beetle populations than parasites do, air pollution is not especially severe.
 Inverse :   air pollution is especially severe, predators do  not  have much more impact on leaf-beetle populations than parasites do.
 Contrapositive :   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

hadimadi · Answer

Hi Andrew,

I agree that I didn't find the correct contraposition to the statement.

However, I still don't agree with (D):

The yellow part in your argument says 'could be thrown off' which isn't the same as 'will be thrown off'. If something could be thrown off and as a result, produce the described effect, it doesn't mean that it will happen.

So in total we need to be sure that the balance will be thrown off and that this will cause the described effect by the argument (which we aren't because we have a 'could be thrown off')

Thanks

AndrewN · Answer

Look at the final part of the passage again (my highlight):

We do not need proof positive, but a reasonable explanation that falls in line with the information provided in the passage. If you do not find answer choice (D) compelling, that is fine, but you still want to throw your reasoning behind some other answer choice that fits a  probable  explanation.

- Andrew

anon7164 · Answer

Hi, I have a doubt here:

While I see the merit of option D and agree that it is a good answer, I was torn between D & E and I finally went for E. I'm not entirely convinced about why E is wrong.

My reasoning is as follows:

The passage says that a combination of predators and parasites control the beetle population.
In areas with severe air pollution, predators are much fewer in number, leaving just the parasites.
If willow trees are more common in areas with severe air pollution, for example 80% of willow trees grow in areas with pollution and 20% in other areas, then beetles attacking the majority (80%) of these willow trees do not face any predators.

One out of the two factors that keeps the beetle population in check is absent from 80% of willow trees. We are not given any information about the effectiveness of either of these factors in keeping the beetle population in check - perhaps both predators and parasites contribute equally, 50% each. Maybe the split is 90-10, but we can't assume anything so we should assume the more conservative estimate - that they contribute equally.

If they contribute equally and one factor is missing and the other factor probably cannot grow in number to compensate for it, then is it not reasonable to conclude that this could be the cause for the more severe beetle damage that trees in polluted areas face compared to trees in other areas?

GMATNinja   KarishmaB   AndrewN  or any other experts, some help would be greatly appreciated. Thank you!

AndrewN · Answer

Hello,  anon7164 . The  highlighted  portions above are unfounded assumptions, and can quickly lead you to pursue an entirely incorrect line of reasoning. Look at  exactly  what the passage states:

Reduced predator populations  should not translate to  leaving just the parasites . The predators could very well still be around in polluted areas, just not in the same numbers, and the equilibrium between predators and parasites is disrupted. We certainly cannot claim that the beetles attacking most willow trees in polluted areas  do not face any predators . Again, such an extreme conclusion is not supported by the passage. Finally, we should  not  assume that  predators and parasites contribute equally  to keeping the leaf beetle population in check. We have no idea just how much each contributes. We just know that some combination appears to work in areas that are not affected by severe air pollution, because that is what the first line of the passage tells us. Think of a reaction in chemistry in which a catalyst is added to an otherwise stable solution and suddenly a reaction occurs. Without getting into specifics of volume or mass, we cannot offer a reasonable estimate of just how much the solution or the catalyst contributed to the reaction. We would only be able to say that together, they set off a certain reaction.

The problem with answer choice (E) is that just because  willows often grow in areas where air pollution is especially severe , that fact alone does not likely  explain why leaf beetles cause particularly severe damage to willows  in those areas. Consider the opposite situation. What if willows  rarely  grew in severely polluted areas? Would that change the tendency of leaf beetles to damage the willows in those areas? Not at all. You should feel better about abandoning (E).

If you  see the merit of option (D) , then I do not need to disabuse you of any problematic thinking regarding it. We seem to be in agreement that it is a fine answer choice for the question at hand, particularly in light of the alternatives.

Thank you for following up, and good luck with your studies.

- Andrew

KarishmaB · Answer

Let me add one point here:

We need to explain why the discovery explains why leaf beetles cause particularly severe damage to willows in areas with severe air pollution.
We do NOT need to explain why the discovery explain why leaf beetles cause particularly severe damage to willows (here it would matter how many trees are in severe pollution areas and how many are in other areas)

We are considering only the severe air pollution areas and what happens there. Whether these areas have more trees or less is irrelevant. We are considering what happens to the trees that are over there. Why do they face severe damage? Because predators control beetle population more efficiently and these areas have fewer predators.

Leaf beetles damage willow trees by stripping away their leaves, but

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Leaf beetles damage willow trees by stripping away their leaves, but

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