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Population decline
Population decline can refer to the decline in population of any organism, but this article refers to population decline in humans. It is a term usually used to describe any great reduction in a human population.〔(Depopulate ) - Webster's Online Dictionary〕 It can be used to refer to long-term demographic trends, as in sub-replacement fertility, urban decay, white flight or rural flight, but it is also commonly employed to describe large reductions in population due to violence, disease, or other catastrophes.
== Definition ==
Sometimes known as depopulation, population decline is the reduction over time in a region's population. The decline can be caused by several factors including sub-replacement fertility (along with limited immigration), heavy emigration, disease, famine, and war. History is replete with examples of large-scale depopulations. Many wars, for example, have been accompanied by significant depopulations. Before the 20th century, population decline was mostly observed due to disease, starvation and/or emigration. The Black Death in Europe, the arrival of Old World diseases to the Americas, the tsetse fly invasion of the Waterberg Massif in South Africa, and the Great Irish Famine all caused sizable population declines. In modern times, the AIDS epidemic caused declines in the population of some African countries. Less frequently, population declines are caused by genocide or mass execution; for example, in the 1970s, the population of Cambodia declined because of wide-scale executions by the Khmer Rouge.
Sometimes the term underpopulation is applied to a specific economic system. It does not refer to carrying capacity, and is not a term in opposition to overpopulation, which deals with the total possible population that can be sustained by available food, water, sanitation and other infrastructure. "Underpopulation" is usually defined as a state in which a country's population has declined too much to support its current economic system. Thus the term has nothing to do with the biological aspects of carrying capacity, but is an economic term employed to imply that the transfer payment schemes of some developed countries might fail once the population declines to a certain point. An example would be if retirees were supported through a social security system which does not invest savings, and then a large emigration movement occurred. In this case, the younger generation may not be able to support the older generation.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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