As Gilbert White, Darwin, and others observed long ago, all species

Question

Project RC Butler 2020  -  Participate and win GMAT Club Tests.   
Passage # 157 Date: 18-Jun-2020
 This post is a part of Project RC Butler 2020 .  Click here for Details

1991 02 SECTION A 17-23

As Gilbert White, Darwin, and others observed long ago, all species appear to have the innate capacity to increase their numbers from generation to generation. The task for ecologists is to untangle the environmental and biological factors that hold this intrinsic capacity for population growth in check over the long run. The great variety of dynamic behaviors exhibited by different populations makes this task more difficult: some populations remain roughly constant from year to year; others exhibit regular cycles of abundance and scarcity; still others vary wildly, with outbreaks and crashes that are in some cases plainly correlated with the weather, and in other cases not.

To impose some order on this kaleidoscope of patterns, one school of thought proposes dividing populations into two groups. These ecologists posit that the relatively steady populations have “density-dependent” growth parameters; that is, rates of birth, death, and migration which depend strongly on population density. The highly varying populations have “density-independent” growth parameters, with vital rates buffeted by environmental events; these rates fluctuate in a way that is wholly independent of population density.

This dichotomy has its uses, but it can cause problems if taken too literally. For one thing, no population can be driven entirely by density-independent factors all the time. No matter how severely or unpredictably birth, death and migration rates may be fluctuating around their long-term averages, if there were no density-dependent effects, the population would, in the long run, either increase or decrease without bound (barring a miracle by which gains and losses canceled exactly). Put another way, it may be that on average 99 percent of all deaths in a population arise from density-independent causes, and only one percent from factors varying with density. The factors making up the one percent may seem unimportant, and their cause may be correspondingly hard to determine. Yet, whether recognized or not, they will usually determine the long-term average population density.

In order to understand the nature of the ecologist’s investigation, we may think of the density-dependent effects on growth parameters as the “signal” ecologists are trying to isolate and interpret, one that tends to make the population increase from relatively low values or decrease from relatively high ones, while the density-independent effects act to produce “noise” in the population dynamics. For populations that remain relatively constant, or that oscillate around repeated cycles, the signal can be fairly easily characterized and its effects described, even though the causative biological mechanism may remain unknown. For irregularly fluctuating populations, we are likely to have too few observations to have any hope of extracting the signal from the overwhelming noise. But it now seems clear that all populations are regulated by a mixture of density-dependent and density-independent effects in varying proportions.
   1 
 1. The author of the passage is primarily concerned with

(A) discussing two categories of factors that control population growth and assessing their relative importance
(B) describing how growth rates in natural populations fluctuate over time and explaining why these changes occur
(C) proposing a hypothesis concerning population sizes and suggesting ways to test it
(D) posing a fundamental question about environmental factors in population growth and presenting some currently accepted answers
(E) refuting a commonly accepted theory about population density and offering a new alternative

2 
 2. It can be inferred from the passage that the author considers the dichotomy discussed in the second paragraph to be

(A) applicable only to erratically fluctuating populations
(B) useful, but only if its limitations are recognized
(C) dangerously misleading in most circumstances
(D) a complete and sufficient way to account for observed phenomena
(E) conceptually valid, but too confusing to apply on a practical basis

3 
 3. Which of the following statements can be inferred from the last paragraph?

(A) For irregularly fluctuating populations, doubling the number of observations made will probably result in the isolation of density-dependent effects.
(B) Density-dependent effects on population dynamics do not occur as frequently as do density-independent effects.
(C) At present, ecologists do not understand any of the underlying causes of the density-dependent effects they observe in population dynamics.
(D) Density-dependent effects on growth parameters are thought to be caused by some sort of biochemical “signaling” that ecologists hope eventually to understand.
(E) It is sometimes possible to infer the existence of a density-dependent factor controlling population growth without understanding its causative mechanism.

4 
 4. According to the passage, which of the following is a true statement about density-dependent factors in population growth?

(A) They ultimately account for long-term population levels.
(B) They have little to do with long-term population dynamics.
(C) They are always more easily isolated and described than those that are density-independent.
(D) They include random environmental events.
(E) They contradict current ecological assumptions about population dynamics.

5 
 5. According to the passage, all of the following behaviors have been exhibited by different populations EXCEPT:

(A) roughly constant population levels from year to year
(B) regular cycles of increases and decreases in numbers
(C) erratic increases in numbers correlated with the weather
(D) unchecked increases in numbers over many generations
(E) sudden declines in numbers from time to time

6 
 6. The discussion concerning population in lines 24-40 serves primarily to

(A) demonstrate the difficulties ecologists face in studying density-dependent factors limiting population growth
(B) advocate more rigorous study of density-dependent factors in population growth
(C) prove that the death rates of any population are never entirely density-independent
(D) give an example of how death rates function to limit population densities in typical populations
(E) underline the importance of even small density-dependent factors in regulating long-term population densities

7 
 7. In the passage, the author does all of the following EXCEPT:

(A) cite the views of other biologists
(B) define a basic problem that the passage addresses
(C) present conceptual categories used by other biologists
(D) describe the results of a particular study
(E) draw a conclusion

auradediligodo · Accepted Answer

Hi everyone,
Got 6/7 correct in 13:30 minutes, including a little less than 6 minutes to read.

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P1

The author starts with presenting a fact: populations grow over the long term. Then the author proceeds by explaining the difficulties related to analyzing the contributing factors to such growth.

Purpose:  To portrait the difficulties that scientists can encounter while investigating the variable related to populations' growth.

P2

In this paragraph the author divides populations in 2 groups: the first one depends primarily on density factors while the second one depends on factors unrelated to density.

Purpose:  To present 2 kind of populations: density and non density dependent.

P3

Here the author clarifies than no population depends only on non density factors. The density factors could be as small as 1% but they are there (maybe difficult to determine) and from them the long term growth of the population is determined.

Purpose:  To point out the presence of density population factors in all the populations and its importance.

P4

In this paragraph the author compares density dependent and non dependent factors respectively to a signal and noise. The signal is easy to determine in populations whose growth is constant or whose oscillations are constant while it is more difficult to isolate the signal from the noise in those populations whose density non dependent factors are significant.

Purpose:  to explain density dependent and non dependent factors through the concept of signal and noise and to claim that the two are always present.

Main point

The purpose of this passage is to claim that all populations' growths are characterized by both density dependent and independent factors.

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1. The author of the passage is primarily concerned with

Pre-thinking

Main point question

The purpose of this passage is to claim that all populations' growths are characterized by both density dependent and independent factors.

(A)   discussing two categories of factors that control population growth and assessing their relative importance
 In line with pre.thinking

(B)  describing how growth rates in natural populations fluctuate  over time and explaining why these changes occur
 Not in line with prethinking

(C) proposing a hypothesis concerning population sizes and suggesting ways to test it
 no hypothesis

(D) posing a fundamental question about environmental factors in population growth and presenting some currently accepted answers
 No question mentioned

(E) refuting a commonly accepted theory about population density and offering a new alternative
 no commonly accepted theory is refuted

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2. It can be inferred from the passage that the author considers the dichotomy discussed in the second paragraph to be

Pre-thinking

Inference question

This dichotomy has its uses, but it can cause problems if taken too literally.

Useful but we need to be careful and not to think that they are exclusive

(A) applicable only to erratically fluctuating populations
 Cannot be inferred

(B)   useful, but only if its limitations are recognized
 In line with pre-thinking

(C)  dangerously misleading  in most circumstances
 Cannot be inferred

(D) a  complete  and  sufficient  way to account for observed phenomena
 Cannot be inferred

(E) conceptually valid, but too  confusing  to apply on a practical basis
 Cannot be inferred

------------------------------------------------------------------------------------------

3. Which of the following statements can be inferred from the last paragraph?

Pre-thinking

Inference question

Let's evaluate the options

(A) For irregularly fluctuating populations, doubling the number of observations made will probably result in the isolation of density-dependent effects.
 Cannot be inferred. Too specific

(B) Density-dependent effects on population dynamics do not occur as frequently as do density-independent effects.
 Cannot be inferred

(C) At present, ecologists do not understand any of the underlying causes of the density-dependent effects they observe in population dynamics.
 Cannot be inferred

(D) Density-dependent effects on growth parameters are thought to be caused by some sort of  biochemical  “signaling” that ecologists hope eventually to understand.
 Cannot be inferred

(E)   It is sometimes possible to infer the existence of a density-dependent factor controlling population growth without understanding its causative mechanism.
   For populations that remain relatively constant, or that oscillate around repeated cycles, the signal can be fairly easily characterized and its effects described, even though the causative biological mechanism may remain unknown.

------------------------------------------------------------------------------------------

4. According to the passage, which of the following is a true statement about density-dependent factors in population growth?

Pre-thinking

Detail question

Let's evaluate the options

(A)   They ultimately account for long-term population levels.
 From P3:    they will usually determine the long-term average population density.

(B) They have little to do with long-term population dynamics.
 Opposite

(C) They are always more easily isolated and described than those that are density-independent.
 Always sounds too extreme

(D) They include random environmental events.
 not mentioned

(E) They contradict current ecological assumptions about population dynamics.
 not mentioned

------------------------------------------------------------------------------------------

5. According to the passage, all of the following behaviors have been exhibited by different populations EXCEPT:

Pre-thinking

Detail question

Let's evaluate the options

(A) roughly constant population levels from year to year
 Mentioned in P1

(B) regular cycles of increases and decreases in numbers
 Same as for option A

(C) erratic increases in numbers correlated with the weather
 Same as for option A

(D)   unchecked increases in numbers over many generations
 Not mentioned

(E) sudden declines in numbers from time to time
 Same as for option A

------------------------------------------------------------------------------------------

6. The discussion concerning population in lines 24-40 serves primarily to

Pre-thinking

Function question

The purpose is to claim the importance of density dependent factors in assessing the long term growth averages

(A) demonstrate the difficulties ecologists face in studying density-dependent factors limiting population growth
 Not in line with pre.thinking

(B) advocate more rigorous study of density-dependent factors in population growth
 Not in line with pre.thinking

(C) prove that the death rates of any population are never entirely density-independent
 the death rates have little to do here

(D) give an example of how death rates function to limit population densities in typical populations
 Not in line with pre.thinking

(E)   underline the importance of even small density-dependent factors in regulating long-term population densities
 in line with pre.thinking

------------------------------------------------------------------------------------------

7. In the passage, the author does all of the following EXCEPT:

Pre-thinking

Detail question

Let's evaluate the options

(A) cite the views of other biologists
 p2

(B) define a basic problem that the passage addresses
 P1

(C) present conceptual categories used by other biologists
 P2

(D)   describe the results of a particular study
 A particular study is never mentioned

(E) draw a conclusion
 P3/P4

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It's a great day to be alive!

robu1 · Answer

In qc7, where it is mentioned conceptual categories used by other biologists
 P2

bM22 · Answer

Hi  robu1 ,

Option C for 7th question is implied by lines: "To impose some order on this kaleidoscope of patterns, one school of thought proposes dividing populations into two groups. These ecologists posit that the relatively steady populations have “density-dependent” growth parameters; that is, rates of birth, death, and migration which depend strongly on population density.".
The author discussed about the different categories used by other biologists and ecologists for dividing population.

Hope this helps.
Thanks.

Skythelimit · Answer

P1-- Introduction of the crux of the passage: even though animals are naturally bound to increase in population, there are factors that makes the affects pop growth. These factors are difficult to distinguish because of the pattern of discrepancy in population is not same.

P2-- In response to the difficulty, there is two division of the factors: density-dependent DD and density independent DI

P3-- but one cannot use this division completely because DD has some significance even when it is least present.

P4-- Information about way of investigation. DD-signal and DI- noise. Pop variation as effect of both DD and DI

Question 20. A) paragraph 3 details about how significant DD is even if there is only 1% . 
I had selected option C because I actually did not understand Paragraph 3 in my 1st go. Option C is true only when pop remain constant or have repeated cycles. But for irregular there is no hope to extract signal (DD). So, option C is not true for all conditions.

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As Gilbert White, Darwin, and others observed long ago, all species

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As Gilbert White, Darwin, and others observed long ago, all species

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