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】这是一道中心主旨题。本文的中心主旨句在第三段的首句。如果考生能够抓住第三段的首句就等于抓住了整篇文章的论述结构。该句中的“dichotomy”(两分法) 就是正确选项A中的“two categories”。考生在解题时应首先抓住每篇文章的中心主旨句，因为这决定了对全文结构的认识以及对原文整体的把握。

2. 【答案】B

【考点解析】这是一道细节推导题。根据题干中的“dichotomy”可将本题的答案信息来源迅速确定在第二段的首句。如果考生能够正确理解该句中“but”一词前后的内容，就可以找出本题的正确选项B。考生在解题时一定要正确理解原文所传达的含义，即进行正确的细节推导。

3. 【答案】D

【考点解析】这是一道审题定位与中心主旨题。根据本题题干中的“different populations”可将本题的答案信息来源确定在首段的第一句，因为该句中的“all species”实际上指的就是题干中的“different populations”。从第一段的首句入手并且仔细阅读首段的第二句，就可以找出本题的正确选项D，因为该选项所传达的信息与原文第一段第一、二句所传达的信息不一致。考生在解题时一定要学会迅速审题定位的能力。

4. 【答案】D

【考点解析】本题是一道归纳推导题。题干已明确指出该题的答案信息来源在第三段。第三段主要陈述“density-dependent factors”的作用，通过仔细阅读第三段，尤其是第三段的最后三句话，考生可以得知本文作者在第三段是在强调“density-dependent factors”的作用。可见D是本题的正确选项。考生在理解原文时一定要注意掌握归纳推导的能力。

5. 【答案】D

【考点解析】本题是一道写作手法题。这是一道比较难的题目，旨在考察考生的语言功底。本文作者在论述自己的观点时运用了各种写作手段。选项A、B、C中所涉及的内容分别在第一段、第三段和第二段中出现。考生一定要提高对原文各种写作手段的认识。

[参考译文]

正如吉尔伯特·怀特、达尔文以及其它科学家很久以前观察的那样，所有的物种都具有一种天生的能力来一代一代地增加它们种群的数量。生态学家的使命就是调查清楚环境和生态因素长期以来是如何控制这种致使种群增长的内在能力的。由于不同种群所表现出来的动态行为是如此多样化，致使这项任务显得非常困难：有些种群长期以来大致保持不变;有些表现出周期性的繁荣和稀少;而还有些变化非常大，时而暴增，时而锐减，这种增减有时显然与气候有关，而有时又与气候无关。

为了赋予这些千变万化的模式一种规律，科学家中的一种流派提出将所有种群划分为两类。这部分生态学家假设：那些相对稳定的种群具有和“种群密度相关”的增长参数;也就是说，这些种群的出生率、死亡率以及迁徙规律都大大地依赖于种群的密度。那些变化很大的种群具有“种群密度无关”的增长参数，其生存率随环境因素的变化而变化，并且以一种完全独立于种群密度的方式进行波动。

这种二分法有它的用途，但如果死板地应用也会引发一些问题。一方面，没有任何种群在所有的时间里完全只受“种群密度无关”因素的影响。无论出生、死亡率以及迁徙规律相对于长期的平均值的变化是如何的剧烈和不可预测，如果没有种群密度无关因素，从长远来看，这个种群的数量将无限地增加或减少(从而阻碍了得失正好相消的奇迹的发生)。换一句话讲，也许种群内平均99%的死亡都由“种群密度无关”因素导致的，只有1%由随种群密度变化而变化的因素所导致。导致这1%的因素也许看起来并不重要，而且它们的影响也相应地很难确定。但是，无论认识到或没有认识到，它们都将对该种群密度长期的平均值起决定作用。

为了能够理解生态学家们研究的性质，我们可以把种群密度相关对增长参数的影响视为一种“信号”，生态学家试图分离并解释这些“信号”，这种“信号”将使种群中个体数量从相对低的数值增长或从相对高的数值减少，同时，密度无关因素将作为一种“噪音”来影响种群的动态变化。对于那些数量保持相对不变，或者以一种循环周期进行摆动的种群而言，这种“信号”将会相当容易提取特征并描述它的影响，即使我们仍不知道作为原因的生物机制。对那些变化不规则的种群，我们可能因为观察太小而无法从繁多的“噪音”中提取那一信号。但是，现在显得很清楚的是，所有种群都受“种群密度相关”和“种群密度无关”两类因素在不同程度上的共同制约。