Editing Allee effect (section)

==Definition==

The generally accepted definition of Allee effect is positive density dependence, or the positive correlation between population density and individual fitness. It is sometimes referred to as "undercrowding" and it is analogous (or even considered synonymous by some) to "[[depensation]]" in the field of [[fishery science]]s.<ref name=book1 /><ref>{{Cite journal |vauthors=Stephens PA, Sutherland WJ, Freckleton RP |s2cid=54663313 |year=1999 |title=What is the Allee effect? |journal=Oikos |volume=87 |issue=1 |pages=185–190 |doi=10.2307/3547011|jstor=3547011 |bibcode=1999Oikos..87..185S }}</ref> Listed below are a few significant subcategories of the Allee effect used in the ecology literature.

===Component vs. demographic Allee effects===
The ''component Allee effect'' is the positive relationship between any measurable component of individual fitness and population density. The ''demographic Allee effect'' is the positive relationship between the overall individual fitness and population density.

The distinction between the two terms lies on the scale of the Allee effect: the presence of a demographic Allee effect suggests the presence of at least one component Allee effect, while the presence of a component Allee effect does not necessarily result in a demographic Allee effect. For example, cooperative hunting and the ability to more easily find mates, both influenced by population density, are component Allee effects, as they influence individual fitness of the population. At low population density, these component Allee effects would add up to produce an overall demographic Allee effect (increased fitness with higher population density). When population density reaches a high number, negative density dependence often offsets the component Allee effects through resource competition, thus erasing the demographic Allee effect.<ref name=kramer>{{Cite journal |vauthors=Kramer AM, Dennis B, Liebhold AM, Drake JM |year=2009 |title=The evidence for Allee effects |journal=Population Ecology |volume=51 |pages=341–354 |doi=10.1007/s10144-009-0152-6 |issue=3|bibcode=2009PopEc..51..341K |s2cid=36920194 }}</ref> Allee effects might occur even at high population density for some species.<ref name=book1 />

===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 />