2017英语专八阅读100篇高级篇附答案解析（81）

2017英语专八阅读100篇高级篇附答案解析（81）

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

　　To impose some order on this kaleidoscope of patterns, one school ofthought proposes dividing populations into two groups. These ecologists positthat the relatively steady populations have “density-dependent” growth parameters; that is, ratesof birth, death, and migration which depend strongly on population density. Thehighly 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 populationdensity.

　　This dichotomy has its uses, but it can cause problems if taken tooliterally. For one thing, no population can be driven entirely bydensity-independent factors all the time. No matter how severely orunpredictably birth, death, and migration rates may be fluctuating around theirlong-term averages, if there were no density-dependent effects, the populationwould, in the long run, either increase or decrease without bound . Put another way, it may bethat on average 99 percent of all deaths in a population arise fromdensity-independent causes, and only one percent from factors varying withdensity. The factors making up the one percent may seem unimportant, and theircause 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 ofthe density-dependent effects on growth parameters as the “signal” ecologists are trying to isolateand interpret, one that tends to make the population increase from relativelylow values or decrease from relatively high ones, while the density-independenteffects act to produce “noise” in the population dynamics. For populations that remain relativelyconstant, or that oscillate around repeated cycles, the signal can be fairlyeasily characterized and its effects described, even though the causativebiological mechanism may remain unknown. For irregularly fluctuatingpopulations, we are likely to have too few observations to have any hope ofextracting the signal from the overwhelming noise. But it now seems clear thatall populations are regulated by a mixture of density-dependent anddensity-independent effects in varying proportions.

　　1. The author of the text is primarily concerned with

　　[A] discussing two categories of factors that control population growthand assessing their relative importance.

　　[B] describing how growth rates in natural populations fluctuate overtime and explaining why these changes occur.

　　[C] proposing a hypothesis concerning population size and suggesting waysto test it.

　　[D] posing a fundamental question about environmental factors inpopulation growth and presenting some currently accepted answer.

　　2. It can be inferred from the text that the author considers thedichotomy discussed to 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. According to the text, all of the following behaviors have beenexhibited 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.

　　4. The discussion concerning population in the third paragraph servesprimarily to

　　[A] demonstrate the difficulties ecologists face in studyingdensity-dependent factors limiting population growth.

　　[B] advocate more rigorous study of density-dependent factors inpopulation growth.

　　[C] prove that the death rates of any population are never entirelydensity-independent.

　　[D] underline the importance of even small density-dependent factors inregulating long-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.

专四专八精选好课 暖心助学

新东方好老师 手把手带学