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  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.
Which of the following, if true, would best support the density-dependent theory of population control as it is described in the passage?
选项 A、As the number of foxes in Minnesota decrease, the growth rate of this population of foxes begins to increase.
B、As the number of woodpeckers in Vermont decreases, the growth rate of this population of woodpeckers also begins to decrease.
C、As the number of prairie dogs in Oklahoma increases, the growth rate of this population of prairie dogs also begins to increase.
D、After the number of beavers in Tennessee decreases, the number of predators of these beavers begins to increase.
E、After the number of eagles in Montana decreases, the food supply of this population of eagles also begins to decrease.
答案A
解析 哪一个能支持密度决定论?密度决定论的内容，见原文L25～29，种群数增加，增长就会放慢，反之亦然。∴A正确，当一种狐狸数下降时，其种群增长速度会加快。B、C、D、E明显和原文论述相反或无关。
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