SAMPLE 15

　　[生態學]

　　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.

　　[答案與考點解析]

　　1. 【答案】A

　　【考點解析】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構。該句中的“dichotomy”(兩分法) 就是正確選項A中的“two categories”。考生在解題時應首先抓住每篇文章的中心主旨句，因為這決定了對全文結構的認識以及對原文整體的把握。

　　2. 【答案】B

　　【考點解析】這是一道細節推導題。根據題干中的“dichotomy”可將本題的答案信息來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”一詞前后的內容，就可以找出本題的正確選項B�？忌�在解題時一定要正確理解原文所傳達的含義，即進行正確的細節推導。

　　3. 【答案】D

　　【考點解析】這是一道審題定位與中心主旨題。根據本題題干中的“different populations”可將本題的答案信息來源確定在首段的第一句，因為該句中的“all species”實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閱讀首段的第二句，就可以找出本題的正確選項D，因為該選項所傳達的信息與原文第一段第一、二句所傳達的信息不一致�？忌�在解題時一定要學會迅速審題定位的能力。

　　4. 【答案】D

　　【考點解析】本題是一道歸納推導題。題干已明確指出該題的答案信息來源在第三段。第三段主要陳述“density-dependent factors”的作用，通過仔細閱讀第三段，尤其是第三段的最后三句話，考生可以得知本文作者在第三段是在強調“density-dependent factors”的作用�？梢奃是本題的正確選項�？忌�在理解原文時一定要注意掌握歸納推導的能力。

　　5. 【答案】D

　　【考點解析】本題是一道寫作手法題。這是一道比較難的題目，旨在考察考生的語言功底。本文作者在論述自己的觀點時運用了各種寫作手段。選項A、B、C中所涉及的內容分別在第一段、第三段和第二段中出現�？忌�一定要提高對原文各種寫作手段的認識。

　　[參考譯文]

　　正如吉爾伯特·懷特、達爾文以及其它科學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一代一代地增加它們種群的數量。生態學家的使命就是調查清楚環境和生態因素長期以來是如何控制這種致使種群增長的內在能力的。由于不同種群所表現出來的動態行為是如此多樣化，致使這項任務顯得非常困難：有些種群長期以來大致保持不變;有些表現出周期性的繁榮和稀少;而還有些變化非常大，時而暴增，時而銳減，這種增減有時顯然與氣候有關，而有時又與氣候無關。

　　為了賦予這些千變萬化的模式一種規律，科學家中的一種流派提出將所有種群劃分為兩類。這部分生態學家假設：那些相對穩定的種群具有和“種群密度相關”的增長參數;也就是說，這些種群的出生率、死亡率以及遷徙規律都大大地依賴于種群的密度。那些變化很大的種群具有“種群密度無關”的增長參數，其生存率隨環境因素的變化而變化，并且以一種完全獨立于種群密度的方式進行波動。

　　這種二分法有它的用途，但如果死板地應用也會引發一些問題。一方面，沒有任何種群在所有的時間里完全只受“種群密度無關”因素的影響。無論出生、死亡率以及遷徙規律相對于長期的平均值的變化是如何的劇烈和不可預測，如果沒有種群密度無關因素，從長遠來看，這個種群的數量將無限地增加或減少(從而阻礙了得失正好相消的奇跡的發生)。換一句話講，也許種群內平均99%的死亡都由“種群密度無關”因素導致的，只有1%由隨種群密度變化而變化的因素所導致。導致這1%的因素也許看起來并不重要，而且它們的影響也相應地很難確定。但是，無論認識到或沒有認識到，它們都將對該種群密度長期的平均值起決定作用。

　　為了能夠理解生態學家們研究的性質，我們可以把種群密度相關對增長參數的影響視為一種“信號”，生態學家試圖分離并解釋這些“信號”，這種“信號”將使種群中個體數量從相對低的數值增長或從相對高的數值減少，同時，密度無關因素將作為一種“噪音”來影響種群的動態變化。對于那些數量保持相對不變，或者以一種循環周期進行擺動的種群而言，這種“信號”將會相當容易提取特征并描述它的影響，即使我們仍不知道作為原因的生物機制。對那些變化不規則的種群，我們可能因為觀察太小而無法從繁多的“噪音”中提取那一信號。但是，現在顯得很清楚的是，所有種群都受“種群密度相關”和“種群密度無關”兩類因素在不同程度上的共同制約。