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 population 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. The author of the text 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 size and suggesting ways to test it.

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

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

4. The discussion concerning population in the third paragraph 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] underline the importance of even small density-dependent factors in regulating 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.

[答案与考点解析]

1. 【答案】A

【考点解析】本题是一道标点符号题。通过本题题干中的“solitary”一词可将本题的答案信息中心确定在第一段第十行即第一段第四句话，通过仔细阅读和理解本句话以及本句前后的两句话，可以得出本题的正确选项A。其实美国的女权主义运动者并不“孤单”(solitary)，因为她们的思想和行动是和欧洲大陆的女权主义者有着千丝万缕的联系，是国际女权运动的一部分。考生在解题时要善于理解标点符号，更要善于对原文的细节进行推导。

2. 【答案】B

【考点解析】本题是一道细节推导题。根据本题题干中的“European historians”可将本题的答案信息来源迅速确定在第二段第四句，通过阅读本句分号前后的内容，可以推导出作者的态度是否定的，其否定原因是第二段第四句分号前半部分所表达的内容。本题的正确答案应该是B。考生在解题时一定要注意原文细节的推导，尤其是分句之间存在因果关系的时候。

3. 【答案】D

【考点解析】这是一道归纳推导题。但是本题的题干确没有明确给出本题在原文中的准确信息来源。这时考生就会迷失答题思路。请同学们一定要记住：每当自己迷失答题思路时，一定要多想一想全文的中心主旨句和每段的主题句，这会帮助考生寻找到解题的思路。本题的正确答案应该是D，因为选项D所表达的内容和本文尾段第一、二句所表达的内容相反。选项A、B、C的内容分别在第三段第一句、第三段第二句以及第二段首句涉及。考生在解题时一定要牢记段落主题句。

4. 【答案】A

【考点解析】这是一道审题定位题。从本题题干中的“envisioned”(设想，预想)一词可将本题的答案信息来源迅速确定在倒数第二段第一句和第二句，因为倒数第二段第一句含有“project”(计划，规划)一词，倒数第二段第二句含有表示未来的“would”一词。通过仔细阅读倒数第二段的第一、二句话，发现这两句话都包含“globe”或“world”，可见本题的正确选项应该是A。考生在解题时一定要善于利用题干中的词语迅速而准确地进行审题定位。

5. 【答案】B

【考点解析】这是一道反推题。通过本题题干中的“most Saint-Simonians”可将本题的答案信息来源迅速确定在尾段的第一句。根据尾段第一句进行反推即逆向思维，可得出本题的正确答案是B。考生在解题时一定要时时牢记反推题型，并且经常利用自己的逆向思维能力。