The debate on how populations are regulated began in the mid-1930s when A.J. Nicholson proposed that controlling factors must act density dependently. While a huge body of evidence has been collected demonstrating the existence of density-dependent mechanisms, the debate on their general role in determining population size and stability is still ongoing. Nicholson's key argument was that regulatory agents must increase their impact on a population as the size of the population increases. It is this change in impact with density that is termed density dependence. Because predators, pathogens, and parasites can change their population size in response to changes in host abundance and thus change their level of impact with the host population size, they were viewed as key regulatory candidates. The core role of biotic factors in density-dependent regulation came to be a central belief of ecologists of the functional school. This view implies that populations have a long-term equilibrium size or density from which they may be displaced by stochastic factors and toward which they are returned by density-dependent forces. However, the proponents of this view do not suggest that populations are necessarily held tightly to this equilibrium. As D. L.
Lack stated in the 1954 ''most wild animals fluctuate irregularly in numbers between limits that are extremely restricted compared with what their rates of increase would allow.''
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