Allee Effect

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What is the Allee Effect?

The Allee effect may be a process in Biology identified by a correspondence between density or population size and the mean individual fitness. Allee effect is categorized by the essence of dependence on density which is relatively lower. 

A region is claimed to possess a robust Allee effect when the population shrinks to lower densities and a weak Allee effect when the proliferation rate is high and positive. The null hypothesis is when the proliferation rates are decreasing at low density but are still positive. The difference between the two types is distinctly based on the property of showing critical population density or size by a specific region.

A population showing a sturdy Allee effect will have a critical density or population size where an increase becomes negative. On the other hand, the Allee-effect exhibiting population will have a decreased per capita rate of growth every week at low population size which always exhibits a positive per capita rate of growth. Hence a population will become extinct when the population size hits a level below the edge.

Allee witnessed that aggregation was at positive effects on land isopods and their survival which when subjected to isolation desiccated rapidly. His fact-based instances of the benefits of aggregation conflicted with the Logistic model and the Malthusian paradigm. Taking into consideration both kinds of Allee effects, the following view can be generalized: An Allee effect is indeed a positive link between the absolute average individual fitness and population size over a period of time. This association may however produce a critical population density to a point where the population cannot persist.

The Allee Effect is of the following Two Types:

  • Component Allee Effect: 

This is often exhibited by a population where there's a positive association between some fitness components.

  • Demographic Allee Effect: 

On the other hand this happens when the component Allee effect produces a positive association between per capita increase and population size.

The difference between the 2 effects is often understood by the measure of the allee effect. For a demographic Allee effect to exist, there should be a minimum of one component allee effect. The demographic Allee effect won't necessarily exist if component Allee effects exist. For eg., collaborative hunting and therefore the ability to seek out mates are component Allee effects, since they influence the fitness of people during a population.

When the population is low, the sum of component Allee effects produces demographic allee effects. But in the case of increased population density, the negative density dependence is compensated by the component Allee effects through competition for resources, this results in depleting the demographic Allee effect.

Mechanism of Allee Effect

The mechanism of the allee effect is tied to survival and reproduction. The various mechanisms of the Allee effect are as follows:

Ecological Mechanism

The factors in the ecological mechanism which contribute to the Allee effect includes- limitation of mates, defense cooperation, cooperative feeding, and environmental conditioning

Mate limitation refers to the problem of finding an appropriate mate for amphimixis at a lower population size. This is usually found in non-mobile organisms like plants, plankton, and sessile vertebrates.

Co-operative defense or cooperation defense refers to protection against predators by group anti-predator behaviour. Predator vigilance behaviors are prevalent and are exhibited by many species, the rate at which it is exhibited is also high. This leads to less time and energy spent on foraging. This reduces the fitness of an individual living in smaller groups.

Cooperative feeding involves groups checking out food so as to survive. The species that search in packs won't be ready to locate their prey as efficiently in comparison to smaller groups.

Environmental conditioning is the mechanism where the individuals work together so as to enhance their environment for the advantage of the species changing the biotic and abiotic factors.


The exploitation of the population by humans has ultimately led to the extinction of many species. When rare species are more desirable than the common species the costs for rare species exceed. This has resulted in anthropogenic Allee effects where rare species become extinct but the common and non-threatened species survive.

Genetic Mechanisms

When the population size is decreased the genetic diversity is lost. Allee effect is observed under such a situation. As the population size decreases, the fitness of the species is further reduced.

Examples of Allee Effect in Various Taxa

Following are the examples of the Allee effect:

  • Fruit flies are one of the most dangerous pests in agriculture as they have the potential to attack over 400 varieties of crops globally. Usage of sterile males is one of the techniques used to control them which creates the Allee effect. To make sure the Allee effect is avoided, natural enemies in large numbers are released.

  • When density is low, the Allee effect can cause the extinction of a species due to the low mating encounters or of fertilization as illustrated by pollination from fig trees.

  • It is important in fish schooling as it is a group of living animals. If the harvesting pressure is high, it may lead to a potential population disintegration.

  • Some species have a really less probability of locating responsive mates or have a prejudiced sex-ratio thanks to the stochasticity of the democracy at low population sizes.

The allee effect can reduce the speed of range expansion of a population and may prevent biological invasions. The limitation of mates is the most commonly observed evidence that causes the allee effect in both plants and animals.

FAQ (Frequently Asked Questions)

Q1. Define Allee Effect .

Ans - The Allee effect is a process in biology identified by a correspondence between density or population size and therefore the mean individual fitness that is categorized by the essence of dependence on density which is relatively lower. 

Q2. What are the Factors that Contribute to the Allee Effect?

Ans - The factors that contribute to the allee effect include- mate limitation, cooperative defense, environmental conditioning, and cooperative feeding.