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 climatic 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-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. 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. 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 “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.作者: MarilynR 时间: 2010-4-26 06:57
42. Which of the following statements would provide the most of logical continuation of the final paragraph of the passage?
(A) Thus Wynne-Edwards’ theory raises serious questions about the constancy of animal population in a region.
(B) Because Wynne-Edwards’ theory is able to explain more kinds of animal behavior than is the density-dependent theory, epideictic explanations of population regulation are now widely accepted.
(C) The results of one study, for instance, have suggested that group vocalizing is more often used to defend territory than to provide information about population density.
(D) Some of these studies have, in fact, worked out a systematic and complex code of social behavior that can regulate population size.
(E) One study, for example, has demonstrated that birds are more likely to use winter-roosting aggregations than group vocalizing in order to provide information on population size.
紧跟第三个理论段的最后，
先看d选项
However,Wynne-Edwards’ theory, linking animal social behavior and population control, has been challenged, with some justification, by several studies.、
D) Some of these studies have, in fact, worked out a systematic and complex code of social behavior that can regulate population
d选项跟however，前的内容是一致的不管是复杂不复杂，都是在规范群的数量，
在看c
C) The results of one study, for instance, have suggested that group vocalizing is more often used to defend territory than to provide information about population density
说的是例如，一个研究结果表明，group vocalizing更用来保护领地而不是数量的信息提供
c是对应了，however的强转折，所以c作者: MarilynR 时间: 2010-4-27 06:45
有道理，你这么你说我点醒了，应该关注however转折后面的
linking animal social behavior and population control 才是亮点
