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QOTD: Leaf beetles damage willow trees by

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QOTD: Leaf beetles damage willow trees by [#permalink]

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New post 02 Aug 2017, 12:11
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  55% (hard)

Question Stats:

64% (01:39) correct 36% (02:23) wrong based on 755 sessions

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Verbal Question of The Day: Day 73: Critical Reasoning


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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

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Re: QOTD: Leaf beetles damage willow trees by [#permalink]

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New post 02 Aug 2017, 12:13
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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:

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

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.

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

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).

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

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.

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

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.

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

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.
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Re: QOTD: Leaf beetles damage willow trees by [#permalink]

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New post 02 Aug 2017, 12:45
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souvik101990 wrote:
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 ________.


Leaf beetles damage willow trees, but parasites + predators keep the population of these beetles in check.
Severe air pollution => reduced predator population. Parasites not adversely affected.
Researchers find that leaf beetles cause severe damage to willows in areas with severe air pollution.
Why?


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

This does not explain why the damage is high in areas with severe air pollution. OUT!


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

And these parasites attack leaf beetles => the damage should be less in areas with severe pollution if the parasites are more prevalent here. OUT!


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

This would be same in areas with pollution, or without pollution and again, this doesn't explain the researcher's findings. OUT!


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

Predators have much more impact on leaf-beetle population, but in areas with severe pollution, predators numbers decrease. This would explain why the beetles are able to cause more damage to the willow tree leaves.
KEEP!


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

This doesn't help why the damage would be more in areas where pollution is severe. OUT!


D is the answer for me!
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Re: QOTD: Leaf beetles damage willow trees by [#permalink]

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New post 03 Aug 2017, 08:07
souvik101990 wrote:

Verbal Question of The Day: Day 73: Critical Reasoning


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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

Every question of the day will be followed by an expert reply by GMATNinja in 12-15 hours. Stay tuned! Post your answers and explanations to earn kudos.


The answer is D

The argument lists beetles as the cause of the damage of the willows.
Severe pollution can reduce the damage to willow trees by beetles because predator and parasite have detrimental effects on the beetles.


[b]Conclusion : 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


Only D gives us the reasoning why conclusion is correct .



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QOTD: Leaf beetles damage willow trees by [#permalink]

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New post 02 Jun 2018, 17:30
Question Stem:-

1) Leaf beetles:-
1.1) damage willow trees by stripping away their leaves, but -----------> Leaf beetles are BAD for Trees
1.2) a combination of parasites and predators generally keeps populations of these beetles in check--------------> a combination of parasites and predators
does not allow the population of Leaf beetles to explode. -------------->which HELPS trees as lesser no. of Leaf beetles will cause Less damage to trees.

2) Researchers have found that severe air pollution results in reduced predator populations. --------------------> Severe air pollution is reduces predator populations.-----------> Reduced predator population means GOOD for Leaf beetles.

3) The parasites, by contrast, are not adversely affected by pollution; --------------------> Severe air pollution does not affect parasites----------> Does not do any good to Leaf beetles.

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

Analysis, "a combination of parasites and predators generally keeps populations of these beetles in check". If say, a combination of, 10 parasites and 10 predators, is required but due to severe air pollution (which affects predator population but does not affect parasite population) the predator population reduces to 5, leading to unavailability of the right combination. This is good for Leaf Beetles as the right combination is not available then Leaf Beetle population will not be kept in check.

Elimination and Selection of right answer choice:-
Option D says--------->where air pollution is NOT especially severe, predators have much more impact on leaf-beetle populations than parasites do----------->Air polution not severe then no impact on predator population----------->which means, as per our hypothetical case above, the required combination, of 10 predators & 10 parasites, is available then these 10 predators have much more impact on leaf-beetle populations than these 10 parasites do which keeps the Leaf Beetle population in check. Hence the right combination is reached and the objective is reached.

Option (D) comes very close to the analysis. No other options speak of the right number required to keep the population of leaf beetles in check.
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Re: QOTD: Leaf beetles damage willow trees by [#permalink]

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New post 19 Jun 2018, 22:27
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.
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Re: QOTD: Leaf beetles damage willow trees by   [#permalink] 19 Jun 2018, 22:27
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