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Overpopulation occurs when a population of a species exceeds the carrying capacity of its ecological niche. Overpopulation is a function of the number of individuals compared to the relevant resources, such as the water and essential nutrients they need to survive. It can result from an increase in births, a decline in mortality rates, an increase in immigration, or an unsustainable biome and depletion of resources.〔(【引用サイトリンク】url=http://www.daimun.org/pdf/ecosoc3_taraparambi.pdf )〕 ==Animal overpopulation== In the wilderness, the problem of overpopulation of species is often solved by growth in the population of predators. Predators tend to look for signs of weakness in their prey, and therefore usually first eat the old or sick animals. This has the side effects of controlling the prey population and ensuring its evolution in favor of genetic characteristics that enhance escape from predation (and the predator may co-evolve, in response). In the absence of predators, species are bound by the resources they can find in their environment, but this does not necessarily control overpopulation, at least in the short term. An abundant supply of resources can produce a ''population boom'' that ends up with more individuals than the environment can support. In this case, starvation, thirst and sometimes violent competition for scarce resources may affect a sharp reduction in population in a very short lapse (a ''population crash''). Lemmings, as well as other species of rodents, are known to have such cycles of rapid population growth and subsequent decrease. Some species seem to have a measure of self-control, by which individuals refrain from mating when they find themselves in a crowded environment. This voluntary abstinence may be induced by stress or by pheromones. In an ideal setting, when animal populations grow, so do the number of predators that feed on that particular animal. Animals that have birth defects or weak genes (such as the runt of the litter) are unable to compete over food with stronger, healthier animals. In reality, an animal that is not native to an environment may have advantages over the native ones, such being unsuitable for the local predators. If left uncontrolled, such an animal can quickly overpopulate and ultimately destroy its environment. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「overpopulation」の詳細全文を読む スポンサード リンク
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