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.

　　[考點(diǎn)解析]

　　1.「答案」A

　　「考點(diǎn)解析」這是一道中心主旨題。本文的中心主旨句在第三段的首句。如果考生能夠抓住第三段的首句就等于抓住了整篇文章的論述結(jié)構(gòu)。該句中的“dichotomy”（兩分法） 就是正確選項(xiàng)A中的“two categories”。考生在解題時(shí)應(yīng)首先抓住每篇文章的中心主旨句，因?yàn)檫@決定了對(duì)全文結(jié)構(gòu)的認(rèn)識(shí)以及對(duì)原文整體的把握。

　　2.「答案」B

　　「考點(diǎn)解析」這是一道細(xì)節(jié)推導(dǎo)題。根據(jù)題干中的“dichotomy”可將本題的答案信息來(lái)源迅速確定在第二段的首句。如果考生能夠正確理解該句中“but”一詞前后的內(nèi)容，就可以找出本題的正確選項(xiàng)B.考生在解題時(shí)一定要正確理解原文所傳達(dá)的含義，即進(jìn)行正確的細(xì)節(jié)推導(dǎo)。

　　3.「答案」D

　　「考點(diǎn)解析」這是一道審題定位與中心主旨題。根據(jù)本題題干中的“different populations”可將本題的答案信息來(lái)源確定在首段的第一句，因?yàn)樵摼渲械摹癮ll species”實(shí)際上指的就是題干中的“different populations”。從第一段的首句入手并且仔細(xì)閱讀首段的第二句，就可以找出本題的正確選項(xiàng)D，因?yàn)樵撨x項(xiàng)所傳達(dá)的信息與原文第一段第一、二句所傳達(dá)的信息不一致。考生在解題時(shí)一定要學(xué)會(huì)迅速審題定位的能力。

　　4.「答案」D

　　「考點(diǎn)解析」本題是一道歸納推導(dǎo)題。題干已明確指出該題的答案信息來(lái)源在第三段。第三段主要陳述“density-dependent factors”的作用，通過(guò)仔細(xì)閱讀第三段，尤其是第三段的比較后三句話(huà)，考生可以得知本文作者在第三段是在強(qiáng)調(diào)“density-dependent factors”的作用�？梢�(jiàn)D是本題的正確選項(xiàng)。考生在理解原文時(shí)一定要注意掌握歸納推導(dǎo)的能力。

　　5.「答案」D

　　「考點(diǎn)解析」本題是一道寫(xiě)作手法題。這是一道比較難的題目，旨在考察考生的語(yǔ)言功底。本文作者在論述自己的觀點(diǎn)時(shí)運(yùn)用了各種寫(xiě)作手段。選項(xiàng)A、B、C中所涉及的內(nèi)容分別在第一段、第三段和第二段中出現(xiàn)�？忌�一定要提高��(duì)原文各種寫(xiě)作手段的認(rèn)識(shí)。

　　[參考譯文]

　　正如吉爾伯特？懷特、達(dá)爾文以及其它科學(xué)家很久以前觀察的那樣，所有的物種都具有一種天生的能力來(lái)一代一代地增加它們種群的數(shù)量。生態(tài)學(xué)家的使命就是調(diào)查清楚環(huán)境和生態(tài)因素長(zhǎng)期以來(lái)是如何控制這種致使種群增長(zhǎng)的內(nèi)在能力的。由于不同種群所表現(xiàn)出來(lái)的動(dòng)態(tài)行為是如此多樣化，致使這項(xiàng)任務(wù)顯得非常困難：有些種群長(zhǎng)期以來(lái)大致保持不變；有些表現(xiàn)出周期性的繁榮和稀少；而還有些變化非常大，時(shí)而暴增，時(shí)而銳減，這種增減有時(shí)顯然與氣候有關(guān)，而有時(shí)又與氣候無(wú)關(guān)。

　　為了賦予這些千變?nèi)f化的模式一種規(guī)律，科學(xué)家中的一種流派提出將所有種群劃分為兩類(lèi)。這部分生態(tài)學(xué)家假設(shè)：那些相對(duì)穩(wěn)定的種群具有和“種群密度相關(guān)”的增長(zhǎng)參數(shù)；也就是說(shuō)，這些種群的出生率、死亡率以及遷徙規(guī)律都大大地依賴(lài)于種群的密度。那些變化很大的種群具有“種群密度無(wú)關(guān)”的增長(zhǎng)參數(shù)，其生存率隨環(huán)境因素的變化而變化，并且以一種完全獨(dú)立于種群密度的方式進(jìn)行波動(dòng)。

　　這種二分法有它的用途，但如果死板地應(yīng)用也會(huì)引發(fā)一些問(wèn)題。一方面，沒(méi)有任何種群在所有的時(shí)間里完全只受“種群密度無(wú)關(guān)”因素的影響。無(wú)論出生、死亡率以及遷徙規(guī)律相對(duì)于長(zhǎng)期的平均值的變化是如何的劇烈和不可預(yù)測(cè)，如果沒(méi)有種群密度無(wú)關(guān)因素，從長(zhǎng)遠(yuǎn)來(lái)看，這個(gè)種群的數(shù)量將無(wú)限地增加或減少（從而阻礙了得失正好相消的奇跡的發(fā)生）。換一句話(huà)講，也許種群內(nèi)平均99%的死亡都由“種群密度無(wú)關(guān)”因素導(dǎo)致的，只有1%由隨種群密度變化而變化的因素所導(dǎo)致。導(dǎo)致這1%的因素也許看起來(lái)并不重要，而且它們的影響也相應(yīng)地很難確定。但是，無(wú)論認(rèn)識(shí)到或沒(méi)有認(rèn)識(shí)到，它們都將對(duì)該種群密度長(zhǎng)期的平均值起決定作用。

　　為了能夠理解生態(tài)學(xué)家們研究的性質(zhì)，我們可以把種群密度相關(guān)對(duì)增長(zhǎng)參數(shù)的影響視為一種“信號(hào)”，生態(tài)學(xué)家試圖分離并解釋這些“信號(hào)”，這種“信號(hào)”將使種群中個(gè)體數(shù)量從相對(duì)低的數(shù)值增長(zhǎng)或從相對(duì)高的數(shù)值減少，同時(shí)，密度無(wú)關(guān)因素將作為一種“噪音”來(lái)影響種群的動(dòng)態(tài)變化。對(duì)于那些數(shù)量保持相對(duì)不變，或者以一種循環(huán)周期進(jìn)行擺動(dòng)的種群而言，這種“信號(hào)”將會(huì)相當(dāng)容易提取特征并描述它的影響，即使我們?nèi)圆恢�道作為原因的生物機(jī)制。對(duì)那些變化不規(guī)則的種群，我們可能因?yàn)橛^察太小而無(wú)法從繁多的“噪音”中提取那一信號(hào)。但是，現(xiàn)在顯得很清楚的是，所有種群都受“種群密度相關(guān)”和“種群密度無(wú)關(guān)”兩類(lèi)因素在不同程度上的共同制約。