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  3. Although numbers of animals in a given region may fluctuate from year to yea

Although numbers of animals in a given region may fluctuate from year to yea

游客2025-02-25  38
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. [br] The primary purpose of the passage is to
选项 A、argue against those scientists who maintain that animal populations tend to fluctuate.
B、compare and contrast the density-dependent and epideictic theories of population control.
C、provide example of some of the ways in which animals exercise reproductive restraint to control their own numbers.
D、suggests that theories of population control that concentrate on the social behavior of animals are more open to debate than are theories that do not.
E、summarize a number of scientific theories that attempt to explain why animal populations do not exceed certain limits.
答案 E
解析 主题题型：A．反对动物种群有变化。显然违反作者一开始的叙述。B．比较密度决定和传播理论。说偏了，文中共三个理论，而且作者没有将不同理论正式比较。C．“reproductive restraint”只是第三个理论一家之言。D．关注社会行为的理论比不关注的引起更多争论。文中三个理论全都有反对意见。E．正确。总结一系列科学理论试图解释为什么动物种群不超过一定限度。原文一开始点出动物界这一现象，后面举出三种理论来说明它。
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