The allee effect is defined as a “positive relationship between any component of individual fitness and either numbers or density of conspecifics” (Stephens et al. 1999). This concept was first described by Warder Clyde Allee in the 1930’s. This means that populations that have higher population sizes or densities will have greater fitness over smaller populations (Stephens et al. 2009).
This concept is very important in conservation biology and invasive biology. When the population is small (newly established population) many factors can have a large effect on the overall fitness. The allee effect will drive the population into extinction faster than expected. Depending on the magnitude of the allee effect, the rate of population establishment will vary (Stephens et al. 1999). The allee effect has been broken down into many sub-effects. Some of the most influential include component, demographic, weak and strong allee effect. The component effect includes one factor that contributes to an individual’s fitness while the demographic effect averages all factors that contribute to an individual’s fitness. A strong effect includes an allee threshold which is a critical population size below which the growth rate is negative. A weak allee effect does not have an allee threshold but still has a demographic allee effect (Berec et al. 2007).
Berec, L., Angulo, E., & Courchamp, F. (2007). Multiple Allee effects and population management. Trends in Ecology & Evolution, 22(4), 185–191. doi:10.1016/j.tree.2006.12.002
Stephens PA, Sutherland WJ, Freckleton RP (1999). "What is the Allee effect?". Oikos 87: 185–190.