Apopulation cycle in zoology is a phenomenon wherepopulations 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.
Olaus Magnus, the Archbishop ofUppsala in central Sweden, identified that species of northernrodents had periodic peaks in population and published two reports on the subject in the middle of the 16th century.
In North America, the phenomenon was identified in populations of thesnowshoe hare.[1][2] In 1865, trappers with theHudson's Bay Company were catching plenty of animals. By 1870, they were catching very few. It was finally identified that the cycle of high and low catches ran over approximately a ten-year period.
The most well known example of creatures which have a population cycle is thelemming.[3] The biologistCharles Sutherland Elton first identified in 1924 that the lemming had regular cycles of population growth and decline. When their population outgrows the resources of their habitat, lemmings migrate, although contrary to popular myth, they do not jump into the sea.
Mouse plagues in Australia happen at intervals of about four years.
While the phenomenon is often associated with rodents, it does occur in other species such as theruffed grouse. There are other species which have irregular population explosions such asgrasshoppers where overpopulation results inlocust swarms in Africa and Australia.
There is also an interaction between prey with periodic cycles and predators. As the population expands, there is more food available for predators. As it contracts, there is less food available for predators, putting pressure on their population numbers.
Each population cycle tends to last as long as a species' life expectancy (i.e.lemmings,rabbits andlocusts)
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.[4]
