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发布时间：2006-06-23 17:08     点击：

　　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 (5)  account for this relative constancy.

　　The first theory attributes a relatively constant popu- lation to periodic climatic catastrophes that decimate populations with such frequency as to prevent them from exceeding some particular limit. In the case of (10) 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 (15) exert the same regulatory effect on population regard- less of the number of individuals in a region.

　　A second theory argues that population growth is primarily density-dependent——that is, the rate of growth of a population in a region decreases as the (20) number of animals increases. The mechanisms that manage regulation may vary. For example, as numbers increase, the food supply would probably diminish, which would increase mortality. In addition, as Lotka and Volterra have shown, predators can find prey more (25) 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 hormonal changes in the pituitary and adrenal (30) 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 (35) develop models that would allow the precise prediction of the effects of crowding.

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