Allee effect

Learn more about Allee effect

The Allee effect is a phenomenon in biology named after W. C. Allee, who first wrote extensively on it. It describes a positive relation between population density and the per capita growth rate. In other words, for smaller populations, the reproduction and survival of individuals decrease. This effect usually saturates or disappears as populations get larger.

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. 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, in which there is a population size or density called the critical size/density below which the population declines on average and above which it increases on average, and a weak Allee effect in which there is no critical density, though there is acceleration in population size at small densities.