The Allee effect is a phenomenon in biology characterized by a positive correlation between population density and the per capita population growth rate.
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The Allee effect was first written on extensively by its namesake Warder Clyde Allee. The general idea is that for smaller populations, the reproduction and survival rates of individuals increase with population density, although this effect usually disappears as increased intraspecific competition occurs.
The effect may be due to any number of causes. In some species, reproduction—finding a mate in particular—may be increasingly difficult as the population density decreases in large areas. The Allee effect either limits that small population to a small area where they grow, or the population dies off. Other species may use strategies (such as schooling in fish) that are more effective for larger populations.
A distinction is made between a "strong Allee effect", where a population exhibits a "critical size or density", below which the population declines on average, and above which it increases on average, and a "weak Allee effect", where a population lacks a "critical density", but where, at lower densities, the population growth rate rises with increasing density.
From Warder Clyde Allee, American ecologist.
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