The Allee effect was described by its namesake Warder Clyde Allee. The theory states that, for very small populations, the reproduction and survival rates of individuals increases with population density. This contrasts with small populations in which increasing population density reduces the growth rate of the population.
In the logistic growth equation the left hand side of the equation represents the per capita population growth rate, which is dependent on the population size N, and decreases with increasing N throughout the entire range of population sizes. In contrast, when there is an Allee effect the per-capita growth rate increases with increasing N over some range of population sizes .
When a population is made up of small sub-populations additional factors to the Allee effect arise.