Allee Effect

What is the Allee Effect?

The Allee effect is a process in biology identified by a correspondence between density or population size and the mean individual fitness. It is believed to be widely common and occurs in sparsely populated regions.

Allee effect is categorized by the essence of dependence on density which is comparatively lower. A region is said to have a strong Allee effect when population shrinks to lower densities and weak Allee effect when the proliferation rate is increasing and positive. When the proliferation rates are decreasing at low density but are positive, then it is said to be a null hypothesis.

Difference between the two types is majorly based on the property of exhibiting critical population size or density by a specific region.

A population exhibiting a strong Allee effect will have a critical density or population size where population growth becomes negative. Whereas, a week Allee-effect exhibiting population will have a decreased per capita growth rate at low population size which always exhibits a positive per capita growth rate. Hence a population will become extinct when the population size hits a level below the threshold.

Allee observed that aggregation had positive effects on land isopods and their survival which when subjected to isolation desiccated rapidly. His factual instances of benefits of aggregation contradicted the Logistic model and the Malthusian paradigm. Considering both kinds of Allee effects, the following can be generalized: An Allee effect is a positive correlation between absolute average individual fitness and size of the population over a period of time. This association may produce critical population density to a point where the population cannot persevere.

The allee effect exhibits the following two manifestations:

  • Component Allee Effect: This is exhibited by a population where there is a positive association between some fitness component.
  • Demographic Allee Effect: This occurs when the component allee effect produces a positive association between per capita population growth and population size.

The difference between the two effects can be understood by the measure of the allee effect. For a demographic allee effect to exist, there should be at least one component allee effect. But for a component allee effect to exist, the demographic allee effect might not necessarily exist. For eg., collaborative hunting and the ability to find mates are component allee effects, since they influence the fitness of individuals in a population.

When the population is low, the sum total of component allee effect produces demographic allee effects. When the population density increases, the negative density dependence compensates the component allee effects through competition for resources, thereby depleting the demographic allee effect.

See also: Population Growth

Mechanism of Allee Effect

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

Ecological Mechanism

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

Mate limitation refers to the difficulty in finding a suitable mate for sexual reproduction at lower population size. This is usually found in plants, planktons and sessile vertebrates.

Cooperative defence refers to protection against predators by group anti-predator behaviour. Higher rates of predator vigilance behaviour are exhibited by many species. This results in less time and energy spent on foraging. This reduces the fitness of an individual living in smaller groups.

Cooperative feeding involves group searching for food in order to survive. The species that search in packs will not be able to locate their prey as efficiently when compared to smaller groups.

Environmental conditioning is the mechanism where the individuals work together in order to improve their environment for the benefit of the species. This changes the biotic and abiotic factors.


The exploitation of the population by humans has resulted in the extinction of species. When rare species are more desirable that the common species the prices for rare species exceed. This leads to anthropogenic allee effect where the rare species become extinct but the common species survive.

Genetic Mechanisms

When the population size is decreased the genetic diversity is lost. Under such a situation, allee effect is observed. As the population size decreases, the fitness of the species is also reduced.

Examples Of Allee Effect In Various Taxa

Following are the examples of allee effect:

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

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

  • It is important in schooling fishes which are group living animals. If harvesting pressure is high, it may cause a potential population disintegration.

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

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

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Related Links: Organisms And Population Attributes

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