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[[File:Predator prey curve.png|500px|thumb]]
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A '''population cycle''' in zoology is a phenomenon where [[population]]s rise and fall over a predictable period of time. There are some species where population numbers have reasonably predictable patterns of change although the full reasons for population cycles is one of the major unsolved ecological problems. There are a number of factors which influence population change such as availability of food, predators, diseases and climate.
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Each population cycle tends to last as long as a species' life expectancy (i.e. [[lemmings]], [[rabbits]] and [[locusts]])
==Among humans==
{{See also|Societal collapse|Malthusianism}}
There is strong evidence that humans also display population cycles. Societies as diverse as those of England and France during the Roman, medieval, and early modern eras, of Egypt during Greco-Roman and Ottoman rule, and of various dynasties in China all showed similar patterns of political instability and violence becoming considerably more common after times of relative peace, prosperity, and sustained population growth. Quantitatively, periods of unrest included many times more events of instability per decade and occurred when the population was declining, rather than increasing.<ref>{{cite web | url=http://home.vicnet.net.au/~ozideas/poprus.htm | archive-url=https://web.archive.org/web/20110405081151/http://home.vicnet.net.au/~ozideas/poprus.htm | archive-date=2011-04-05 | title=Population crises and cycles in history - OzIdeas }}</ref>
[[File:Wars-Long-Run-military-civilian-fatalities.png|thumb|600x600px|Military and civilian fatalities|center]]
==See also==
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