In the 1950s a radically different view on the nature of regulation from that presented by Nicholson was proposed. In contrast to the views of the functional school, H. C. Andrewartha and L. C. Birch argued that density-independent agents such as climate and resource availability limited the growth of populations and there is only limited time when they had the opportunity to increase. Termed the environmental school, this viewpoint emphasizes constraint by predominantly physical conditions and by implication argues against the existence of long-term equilibrium population sizes. A strong argument for this viewpoint is given by T. C. R. White, ''Surviving on this earth is, and always has been, especially for the very young, a struggle, a chancy business Nor is there an 'optimum' or 'equilibrium' density of a population in nature — only the maximum number that can survive each generation in a population that is pressing hard against the variable but limited supply of resources in its environment.'' From this viewpoint populations are constantly expanding and being pushed back by adverse events. Through time, they may occasionally expand exponentially, then, as conditions change, suddenly decline.
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