As Gilbert White，Darwin ， and others observed long ago， all species appear to have theinnate capacity to increase their numbers from generation to generation. The task forecologistsis to untangle the environmentaand biologicalfactorsthat hold this intrinsiccapacity for population growth in check over the long run. The great variety of dynamicbehaviorsexhibitedby differentpopulationmakes thistaskmore difficult：sompopulations remain roughly constant from year to year; others exhibit regular cycles ofabundance and scarcity; still others vary wildly， with outbreaks and crashes that arein some cases plainly correlated with the weather， and in other cases not.To impose some order on this kaleidoscopeof patterns ， one school of thought proposesdividing populations into two groups. These ecologists posit that the relatively steadypopulations havedensity-dependent growth parameters; that is， rates ofbirth ， death ，and migrationwhich depend strongly on population density. The highly varying populationshave density-independent growth parameters， with vital rates buffeted by environmentalevents ;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. Forone thing ， no population can be driven entirely by density-independent factors all thetime. No matter how severely or unpredictably birth， death ， and migration rates may befluctuatingaround theirlong-termaverages ， ifthere were no density-dependenteffects ，the populationwould ， in the long run ， eitherincrease or decrease without bound (barringa miracle by which gains and losses canceled exactly)。 Put another way， it may be thaton average 99 percent of all deaths in a populationarise from density-independentcauses ，and only one percent from factors varying with density.The factorsmaking up the one percentmay seem unimportant， and their cause may be correspondingly hard to determine. Yet，whether recognized or not， they will usually determine the long-term average populationdensity.In order to understand the nature of theecologist ’s investigation， we may think ofthe density-dependent effectson growth parameters as the signal ecologists are trying toisolateand interpret， one that tends to make the population increase from relativelylowvalues or decrease from relatively high ones， while the density-independent effects actto produce noise in the populationdynamics.For populationsthatremain relativelyconstant ， or that oscillate around repeated cycles， the signal can be fairly easilycharacterized and its effects described， even though the causative biological mechanismmay remain unknown. For irregularly fluctuating populations， we are likely to have toofew observations to have any hope of extracting the signal from the overwhelming noise.But it now seems clear that all populationsare regulatedby a mixture of density-dependentand density-independent effects in varying proportions.

1. The author of the text is primarily concerned with

[A] Discussing two categories of factorsthat controlpopulationgrowth and assessingtheir relative importance.

[B] Describinghow growth ratesin naturalpopulationsfluctuateover time andexplaining why these changes occur.

[C] Proposing a hypothesisconcerning population size and suggesting ways to test it.

[D] Posing a fundamental question about environmentalfactorsin populationgrowth andpresenting some currently accepted answer.

2. It can be inferred from the text that the author considers the dichotomy discussedto be

[A] Applicable only to erratically fluctuating populations.

[B] instrumental， but only if its limitations are recognized.

[C] Dangerously misleading in most circumstances.

[D] A complete and sufficient way to account for observed phenomena.

3.to the text ， allof the followingbehaviors have been exhibitedby differentpopulations 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.

4. The discussion concerning population in the third paragraph serves primarily to

[A] Demonstrate the difficultiesecologistsface in studying density-dependentfactorslimiting population growth.

[B] Advocate more rigorous study of density-dependent factors in population growth.

[C] Prove that the death rates of any populationare never entirelydensity-independent.

[D] underline the importance of even small density-dependent factors in regulatinglong-term population densities.

5. In the text， the author does all of the following EXCEPT

[A] Cite the views of other biologists.

[B] Define a basic problem that the text addresses.

[C] Present conceptual categories used by other biologists.

[D] Describe the results of a particular study.