Our present understanding of the stability of natural populations shows a gradual, if uneven, increase in understanding that is still far from complete. That there is a balance of nature, so that all species can continue to coexist, is a common and ancient worldview and has been long held in western thought. Until the twentieth century it was widely assumed that the sizes of populations were held in balance by divine power, occasional outbreaks of pests and a loss of balance, such as a plague of locusts, were seen as divine punishment. Belief in this benevolent balance gradually eroded, until C. S. Elton in 1930 could assert ''The balance of nature' does not exist, and perhaps never has existed. The numbers of wild animals are constantly varying to a greater or less extent, and the variations are usually irregular in period and always irregular in amplitude.'' However, no twentieth-century ecologist, including Elton, viewed populations as simply random variables changing without constraint for it was realized this would rapidly lead to extinction and the collapse of ecosystems. The natural world shows order that needs to be explained. The tightness of the constraints exerted by the environment, both biotic and abiotic, and the nature of the factors impacting population density are presumed to influence the degree of population stability. But, in addition, it came to be realized that the life history strategy followed by the organism also had an influence and was itself undergoing natural selection. Some species had evolved to produce large numbers of offspring and were capable of explosive growth when conditions allowed. In contrast, others produced a small number of offspring and their populations were considerably more stable. These two strategies were termed r (for reproductive rate) and k (for carrying capacity) selected. The populations of r-selected species were viewed as more variable than the k-selected species, which remain closer to their environmental carrying capacity. The distinction between r- and k-selected species is now less frequently applied in ecological arguments, in part because the distinction is too rigid. How population stability is determined and regulated remains a major area of research and is still capable of generating considerable controversy.
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