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现象解释型

Although numbers of animals in a given region may fluctuate from year to year, the fluctuations are often temporary and, over long periods, trivial. 主题句Scientists have advanced three theories of population control to account for this relative constancy.

The first theory attributes a relatively constant population to periodic climatic catastrophes that decimate populations with such frequency as to prevent them from exceeding some particular limit. （第一个观点：气候影响）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. （反驳：正常的季节变化有影响）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.第一个理论：气候影响  反驳：正常的季节变化有影响

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 number of animals increases. The mechanisms that manage regulation may vary. （第二个理论：密度理论
                    密度增加－）数量减少）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 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 glands that in turn may regulate population by lowering sexual activity and inhibiting sexual maturation. （接着说明：数量增加－）食物减少－》死亡增加  数量增加－》H变化－》限制性活劝）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.第二个理论：密度理论，反驳：没法画精确的图

A third theory, proposed by Wynne-Edwards and termed “epideictic,” argues that organisms have evolved a “code” in the form of social or epideictic behavior displays, such as winter-roosting aggregations or group vocalizing; such codes provide organisms with information on population size in a region so that they can, if necessary, exercise reproductive restraint. However, Wynne-Edwards’ theory, linking animal social behavior and population control, has been challenged, with some justification, by several studies.第三个理论：基因理论，被正确地反驳