Allee effect

The Allee effect is a phenomenon in biology characterized by a positive correlation between population density and the per capita growth rate.

Description
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 of individuals decrease. This effect usually saturates or disappears as populations get larger.

Causes
The effect may be due to any number of causes. In some species, reproduction&mdash;finding a mate in particular&mdash;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.

Strong vs Weak Allee effect
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, although there is acceleration in population size at small densities.