Allee effect
Through experimental studies, Allee was able to demonstrate that goldfish have a greater survival rate when there are more individuals within the tank.
[2] This led him to conclude that aggregation can improve the survival rate of individuals, and that cooperation may be crucial in the overall evolution of social structure.
The term "Allee principle" was introduced in the 1950s, a time when the field of ecology was heavily focused on the role of competition among and within species.
However, the concept of the Allee effect introduced the idea that the reverse holds true when the population density is low.
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.
[1] Due to its definition as the positive correlation between population density and average fitness, the mechanisms for which an Allee effect arises are therefore inherently tied to survival and reproduction.
Examples of such cooperative behaviors include better mate finding, environmental conditioning, and group defense against predators.
As these mechanisms are more-easily observable in the field, they tend to be more commonly associated with the Allee effect concept.
Nevertheless, mechanisms of Allee effect that are less conspicuous such as inbreeding depression and sex ratio bias should be considered as well.
Although numerous ecological mechanisms for Allee effects exist, the list of most commonly cited facilitative behaviors that contribute to Allee effects in the literature include: mate limitation, cooperative defense, cooperative feeding, and environmental conditioning.
[5] While these behaviors are classified in separate categories, they can overlap and tend to be context dependent (will operate only under certain conditions – for example, cooperative defense will only be useful when there are predators or competitors present).
[11] The anthropogenic Allee effect has become a standard approach for conceptualizing the threat of economic markets on endangered species.
Even very high population sizes can potentially pass through the originally proposed Allee thresholds on predestined paths to extinction.
Nevertheless, in a smaller sized gene pool, there is a higher chance of a stochastic event in which deleterious alleles become fixed (genetic drift).
Mildly deleterious alleles such as those that act later in life would be less likely to be removed by natural selection, and conversely, newly acquired beneficial mutations are more likely to be lost by random chance in smaller genetic pools than larger ones.
The Florida panther experienced a genetic bottleneck in the early 1990s where the population was reduced to ≈25 adult individuals.
This reduction in genetic diversity was correlated with defects that include lower sperm quality, abnormal testosterone levels, cowlicks, and kinked tails.
[16] In response, a genetic rescue plan was put in motion and several female pumas from Texas were introduced into the Florida population.
This action quickly led to the reduction in the prevalence of the defects previously associated with inbreeding depression.
Although the timescale for this inbreeding depression is larger than of those more immediate Allee effects, it has significant implications on the long-term persistence of a species.
The most current definition of Allee effect considers the correlation between population density and mean individual fitness.
Therefore, random variation resulting from birth and death events would not be considered part of Allee effect as the increased risk of extinction is not a consequence of the changing fates of individuals within the population.
[20] Meanwhile, when demographic stochasticity results in fluctuations of sex ratios, it arguably reduces the mean individual fitness as population declines.
[21] Demographic and mathematical studies demonstrate that the existence of an Allee effect can reduce the speed of range expansion of a population[22][23][24][25] and can even prevent biological invasions.
[26] Recent results based on spatio-temporal models show that the Allee effect can also promote genetic diversity in expanding populations.
[27] These results counteract commonly held notions that the Allee effect possesses net adverse consequences.
This formulation is especially useful when demographic data is employed to identify parameters or when extending the model to stochastic differential equations.
A simple example is given by the reaction-diffusion model where When a population is made up of small sub-populations additional factors to the Allee effect arise.
In this case, the species receives none of the benefits of a small sub-population (loss of the sub-population is not catastrophic to the species as a whole) and all of the disadvantages (inbreeding depression, loss of genetic diversity and increased vulnerability to environmental instability) and the population would survive better unfragmented.
