Leaf beetles damage willow trees by stripping away their leaves, but a combination of parasites and predators generally keeps populations...

GMAT Critical Reasoning : (CR) Questions

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Critical Reasoning

Logically Completes

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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 ______.

Which of the following most logically completes the passage?

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

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

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

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

willows often grow in areas where air pollution is especially severe

Solution

Passage Analysis:

Text from Passage	Analysis
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.	What it says: Leaf beetles hurt willow trees by eating their leaves, but parasites and predators usually control beetle numbers What it does: Sets up the basic ecosystem - introduces the problem (beetle damage) and the natural solution (biological controls) What it is: Author's background information Visualization: Normal situation: 100 beetles → Parasites + Predators control → Only 20 beetles cause damage
Researchers have found that severe air pollution results in reduced predator populations.	What it says: Bad air pollution makes predator numbers go down What it does: Introduces a disruptive factor that affects one part of the control system we just learned about What it is: Research finding Visualization: With pollution: 100 beetles → Fewer predators (say 30% less) + Parasites → More beetles survive to cause damage
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 _______.	What it says: Parasites aren't hurt by pollution, but the predator research finding still explains why polluted areas have worse beetle damage What it does: Creates a puzzle - if parasites are fine, why does losing some predators lead to "particularly severe" damage? What it is: Author's reasoning setup for completion Visualization: Polluted areas: 100 beetles → 30% fewer predators + same parasites → Should only have moderate increase, but actually seeing "particularly severe" damage

Argument Flow:

We start with a balanced ecosystem where beetles damage trees but parasites and predators keep them controlled. Then we learn pollution reduces predators but not parasites. The puzzle is: if only one control mechanism is weakened, why do we see particularly severe damage? The blank needs to explain this gap.

Main Conclusion:

The research finding about pollution reducing predators explains why beetle damage is particularly severe in polluted areas (pending completion of the blank).

Logical Structure:

The argument sets up a cause-effect relationship but leaves a gap. We have: pollution → fewer predators → worse beetle damage. But the "particularly severe" damage seems disproportionate to just losing predators while keeping parasites. The completion needs to explain why losing predators has such a big impact even when parasites remain.

Prethinking:

Question type:

Logically Completes - We need to find what logically fills in the blank to complete the author's reasoning about why beetle damage is particularly severe in polluted areas

Precision of Claims

The key claim is about the severity of damage being 'particularly severe' - this suggests something more than just a moderate increase that losing some predators would cause

Strategy

Look for scenarios that explain why losing predators (while keeping parasites) would lead to disproportionately severe damage. We need to bridge the gap between 'fewer predators but same parasites' and 'particularly severe damage'. The completion should explain why predators are more crucial than parasites or why their roles aren't equally substitutable

Answer Choices Explained

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

'neither the predators nor the parasites of leaf beetles themselves attack willow trees' - This tells us what predators and parasites don't do, but it doesn't help explain why losing predators (while keeping parasites) would lead to particularly severe beetle damage. Whether or not these control agents attack willows directly is irrelevant to understanding why reduced predators cause such severe problems. This doesn't complete the logical reasoning.

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

'the parasites that attack leaf beetles actually tend to be more prevalent in areas with severe air pollution than they are elsewhere' - This actually works against the argument's logic. If parasites are more common in polluted areas, they should be able to compensate even better for the reduced predators, making beetle damage less severe, not more severe. This contradicts what we're trying to explain.

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

'the damage caused by leaf beetles is usually not enough to kill a willow tree outright' - This tells us about the severity of beetle damage in general, but doesn't explain the comparison between polluted and non-polluted areas. Whether beetles kill trees completely or not doesn't help us understand why pollution leads to particularly severe damage.

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

'where air pollution is not especially severe, predators have much more impact on leaf-beetle populations than parasites do' - This perfectly explains the puzzle! If predators are normally the primary controllers of beetle populations (doing most of the work while parasites play a smaller role), then when pollution reduces predators but leaves parasites unchanged, we lose the main control mechanism. The parasites alone can't compensate for this major loss, explaining why damage becomes particularly severe rather than just moderately worse.

willows often grow in areas where air pollution is especially severe

'willows often grow in areas where air pollution is especially severe' - This tells us about where willows tend to grow, but doesn't explain the causal relationship between pollution, reduced predators, and severe beetle damage. Even if willows commonly grow in polluted areas, this doesn't help us understand why the reduction in predators leads to particularly severe damage.

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