GRE阅读题目解析:物种繁殖后代能力

　　P5

　　1

　　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.

　　正如 GW，D，以及其他人早就观察到的，所有的物种都表现出一种与生俱来的能力，每一代都繁殖出更多后代。

　　2

　　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.

　　生态学家的任务，就是理清长期抑制这种内的繁殖能力的环境和生物因素。

　　3

　　The great variety of dynamic behaviors exhibited by different populations 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.

　　不同种群所表现出的各种动态变化，增加了任务难度：有些种群的规模多年基本保持不变;另外一些则表现出时多时寡的规律周期;还有一些种群的数量会突然激增或骤减，起伏剧烈，这种波动有时与气候因素直接相关，有时又无关。

　　4

　　To impose some order on this kaleidoscope of patterns, one school of thought proposes dividing populations into two groups.

　　非要在如此纷繁的模型中找到规律，某学派建议把种群分成两类。

　　5

　　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.

　　采用这种方法的生态学家假设，相对稳定的种群有若干 “ 密度依赖 ” 增长参数;也就是说，出生率，死亡率，迁徙率很大程度上取决于种群密度。

　　6

　　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.

　　数量变化巨大的种群则有 “ 非密度依赖 ” 增长参数，这些参数受环境事件的极大影响;这类种群的各种速率(出生，死亡，迁徙)波动与种群密度毫无关系。

　　7

　　This dichotomy has its uses, but it can cause problems if taken too literally.

　　这种分类方法有它的道理，但太过强调这种二分对立会出问题。

　　8

　　For one thing, no population can be driven entirely by density-independent factors all the time.

　　一来，没有哪个种群 始终 只 受到密度依赖因素的驱动。

　　9

　　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).

　　无论出生率，死亡率和迁徙率围绕它们的长期平均值，做多么剧烈或不可预测的波动，如果没有密度依赖参数的影响，长期来看，种群数量都会不受限制地增加或减少(不考虑那种增加量与减少量恰好抵消的情况)。

　　(barring 此处做介词，除了…)

　　10

　　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.

　　换言之，也许平均来看种群内 99% 的死亡都是由非密度依赖因素引起的，只有 1% 与密度依赖因素有关。

　　11

　　The factors making up the one percent may seem unimportant, and their cause may be correspondingly hard to determine.

　　影响这 1% 的因素看似无关紧要，但它们的原因也能相应地难以确定。

　　12

　　Yet, whether recognized or not, they will usually determine the long-term average population density.

　　然而，无论能否确定，长期来看它们通常会影响平均种群密度。

　　13

　　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.

　　为了理解生态学家研究的本质，我们不妨把增长参数中的密度依赖效果，理解成一种生态学家们想要分离和解析的信号，当信号值较低时，倾向于使种群数量上升，反之则降低，而非密度依赖效果则在种群动态变化中充当“噪音”的角色。

　　14

　　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.

　　对于那些数量一直较稳定，或按特定周期循环演化的种群，信号以及效果可以被相当容易地描述，尽管背后发挥作用的生物机制可能仍然未知。

　　15

　　For irregularly fluctuating populations, we are likely to have too few observations to have any hope of extracting the signal from the overwhelming noise.

　　对于那些不规律波动的种群，我们可能没有积累足够的观察，无法从巨大的噪音中解释信号。

　　16

　　But it now seems clear that all populations are regulated by a mixture of density-dependent and density-independent effects in varying proportions.

　　但有一点似无争议，所有的种群都受密度依赖和非密度依赖作用混合调节，比例不同而已。

　　7.The author of the passage 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 sizes and suggesting ways to test it

　　(D) posing a fundamental question about environmental factors in population growth and presenting some currently accepted answers

　　(E) refuting a commonly accepted theory about population density and offering a new alternative

　　选 A

　　讨论了两种因素，无争议，密度依赖和非密度依赖。

　　怎么看出谁相对重要呢，主要根据尾段开头(句 13)，把一种比喻为信号，另一种比喻为噪音，就算外行也明白信号比噪音重要。

　　8. It can be inferred from the passage that the author considers the dichotomy discussed in the second paragraph to be

　　(A) applicable only to erratically fluctuating populations

　　(B) useful, but only if its limitations are recognized

　　(C) dangerously misleading in most circumstances

　　(D) a complete and sufficient way to account for observed phenomena

　　(E) conceptually valid, but too confusing to apply on a practical basis

　　选 B

　　句 7：

　　This dichotomy has its uses, but it can cause problems if taken too literally.

　　9.Which of the following statements can be inferred from the last paragraph(sentences 13-16)?

　　(A) For irregularly fluctuating populations, doubling the number of observations made will probably result in the isolation of density-dependent effects.

　　(B) Density-dependent effects on population dynamics do not occur as frequently as do density-independent effects.

　　(C) At present, ecologists do not understand any of the underlying causes of the density-dependent effects they observe in population dynamics..

　　(D) Density-dependent effects on growth parameters are thought to be caused by some sort of biochemical "signaling" that ecologists hope eventually to understand.

　　(E) It is sometimes possible to infer the existence of a density-dependent factor controlling population growth without understanding its causative mechanism.

　　选 E

　　A 很天真可爱的一个选项。但就算 double 了也不一定能。

　　B 未知项。文中没说。

　　C 错在 any，看到 any 字样的，在单选中这个选项基本就属于自废武功了，多选不好这样讲。

　　D 是不是 biochemical 不能确定，文中一点这方面的暗示都无。

　　For the following question, consider each of the choices separately and select all that apply

　　10. According to the passage, all of the following behaviors have been exhibited by different populations

　　(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

　　选 ABC

　　都在句 3：

　　The great variety of dynamic behaviors exhibited by different populations 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.

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