2. 外语
  3. Although numbers of animals in a given region may fluctuate from year to year, the fluctuations are often temporary and, over lo

Although numbers of animals in a given region may fluctuate from year to year, the fluctuations are often temporary and, over lo

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问题     Although numbers of animals in a given region may fluctuate from year to year, the fluctuations are often temporary and, over long periods, trivial. Scientists have advanced three theories of population control to account for this relative constancy.
    The first theory attributes a relatively constant population to periodic cli- matic catastrophes that decimate populations with such frequency as to prevent them from exceeding some particular limit. In the case of small organisms with short life cycles, climatic changes need not be catastrophic: normal seasonal changes in photoperiod(daily amount of sunlight), for example, can govern population growth. This theory— the density-independent view—asserts that climatic factors exert the same regulatory effect on population regardless of the number of individuals in a region.
    A second theory argues that population growth is primarily density-de- pendent—that is, the rate of growth of a population in a region decreases as the number of animals increases. The mechanisms that manage regulation may vary. For example, as numbers in- crease, the food supply would probably diminish, which would increase mortality. In addition, as Lotka and Volterra have shown, predators can find prey more easily in high-density populations. Other regulators include physiological control mechanisms: for example, Christian and Davis have demonstrated how the crowding that results from a rise in numbers may bring about hor- monal changes in the pituitary and adrenal glands that in turn may regulate population by lowering sexual activity and inhibiting sexual maturation. There is evidence that these effects may persist for three generations in the absence of the original provocation. One challenge for density-dependent theorists is to develop models that would allow the precise prediction of the effects of crowding.
    A third theory, proposed by Wynne- Edwards and termed "epideic-tic," argues that organisms have evolved a "code"in the form of social or epideic- tic behavior displays, such as winterroosting aggregations or group vocalizing; such codes provide organisms with information on population size in a region so that they can, if necessary, ex- ercise reproductive restraint. However, Wynne-Edwards’ theory, linking animal social behavior and population control, has been challenged, with some justification, by several studies.
It can be inferred from the passage that proponents of the density-dependent theory of population control have not yet been able to
选项 A、use their theory to explain the population growth of organisms with short life cycles.
B、reproduce the results of the study of Christian and Davis.
C、explain adequately why the numbers of a population can increase as the population’s rate of growth decreases.
D、make sufficiently accurate predictions about the effects of crowding.
E、demonstrate how predator populations are themselves regulated.
答案D
解析 文中暗示，密度决定论的理论家如今还不能做什么?文中L46—50提出对这些人的一个挑战，他们应努力完成的实际上就是他们现在没有做到的。∴D正确，对数目增加的影响给出完整预测。A、B、C、E不符合上述推理。
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