Editing Allee effect (section)

===Strong vs. weak Allee effects===
[[File:A graph showing the difference between strong and weak Allee effects.svg|thumb|right|Allee effects are classified by the nature of density dependence at low densities. If the population shrinks for low densities, there is a strong Allee effect. If the proliferation rate is positive and increasing then there is a weak Allee effect. The null hypothesis is that proliferation rates are positive but decreasing at low densities.]]

The ''strong Allee effect'' is a demographic Allee effect with a critical population size or density. The ''weak Allee effect'' is a demographic Allee effect without a critical population size or density.

The distinction between the two terms is based on whether or not the population in question exhibits a [[extinction threshold|critical population size or density]]. A population exhibiting a weak Allee effect will possess a reduced per capita growth rate (directly related to individual fitness of the population) at lower population density or size. However, even at this low population size or density, the population will always exhibit a positive per capita growth rate. Meanwhile, a population exhibiting a strong Allee effect will have a critical population size or density under which the population growth rate becomes negative. Therefore, when the population density or size hits a number below this threshold, the population will be destined for extinction without any further aid. A strong Allee effect is often easier to demonstrate empirically using time series data, as one can pinpoint the population size or density at which per capita growth rate becomes negative.<ref name=book1 />