What is the Allee Effect?

The Allee effect is a biological process that describes a positive relationship between absolute mean individual fitness and population size within a given range. Such a positive association can lead to (but not necessarily) a critical population size that the population cannot fall short of. It is believed to be prevalent and seen in sparsely populated areas.

History

Allee effects occur in small or sparse populations and are believed to be common in nature, although they are rarely detected. Population growth exposed to Allee effects declines at low intensity. The first emergence of this phenomenon is by Warder Clyde Allee (1885–1955), a University of Chicago zoologist and animal ecologist, whose special interest is group behavior in animals.

Charles Darwin observed that the large population size is an important measure against extinction in the presence of predators or other natural enemies. (Darwin 1872, as cited in Stephens et al. 1999). However, biologists in the early twentieth century accepted both the Malthusian principle that specific intra-competition does not decrease with population size – and its mathematical expression (Gause 1934) in the logistic model for population growth.

Allee, an intelligent observer of animal behavior, realized that in many species it was inadequacy that limits population growth, not competition. For example, he observed that clustering had positive effects on the survival of land isopods that, when isolated, undergo rapid drying. (Allee 1927). Experimental examples of the benefits of fusion seem to contradict both the Malthusian paradigm and the logistical model. Most of Allee’s early work focused on documenting this phenomenon in animal populations. (Allee 1931).

Definition

The allee effect is classified according to the essence of the dependence on the relatively lower intensity. An area is said to have a strong Allee effect when the population density decreases, and a weak Allee effect when the growth rate increases and is positive. This hypothesis is said to be a null hypothesis when propagation rates decrease at low density and are positive.

The difference between the two species is largely based on the ability to exhibit critical population size or density relative to a particular region. A population population growth would be negative where it would have a critical density or population size exhibiting strong Allee effect. Whereas, a population that exhibits a weekly Allee effect will always have a per capita growth rate at a low population size that exhibits positive per capita growth rate. Therefore, the population will become extinct when the population size reaches a level below the threshold.

Allee observes that clustering has positive effects on land isopods and their survival, which, when subjected to isolation, dries quickly. Examples of the true benefits of consolidation contradict the Logistics model and the Malthusian paradigm. Given both types of Allee effects, the following can be generalized: An Allee effect is a positive correlation between the absolute mean individual fitness and the size of the population over a given period of time. This relationship can produce critical population density to a point where the population cannot sustain.

The Allee effect occurs in the following two ways;

• Component Allee Effect: This is exhibited by a population in which there is a positive relationship between some fitness components.

• Demographic Allee Effect: This occurs when the component allee effect creates a positive relationship between population growth per capita 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 must be at least one component allee effect. However, for a component allee effect to exist, the demographic allee effect may not necessarily exist. For example, cooperative hunting and mate-finding ability are component-independent effects as they affect the fitness of individuals in a population.

When the population is low, the sum of the component total effect produces all of the demographic effects. When population density increases, the component compensates for all its effects through negative density dependence, competition for resources, and thus depletes the demographic allee effect.

Mechanism of the Allee Effect

The mechanism of the Allee effect is linked to survival and reproduction. The mechanism of the Allee effect is as follows:

Ecological Mechanism

Factors contributing to the Allee effect include restraint, cooperative defense, environmental conditioning, and cooperative feeding. Partner constraint refers to the difficulty of finding a suitable mate for sexual reproduction at lower population size. This is commonly found in plants, plankton, and sessile vertebrates.

Collaborative defense refers to protecting against predators by group anti-predatory behavior. Many species exhibit higher rates of alertness than predators. This results in less time and energy spent looking for food. This reduces the fitness of an individual who lives in smaller groups.

Cooperative diets involve a group looking for food to survive. Species that search in packs will not be able to find their prey efficiently compared to smaller groups. Environmental conditioning is the mechanism by which individuals work together to improve their environment for the benefit of the species. This alters biotic and abiotic factors.

Human Sourced Mechanisms

The exploitation of the population by humans results in the extinction of the species. Rare species are more valuable than rare species when more desirable than common species. This leads to the anthropogenic allee effect, with rare species extinct but common species surviving.

Genetic Mechanisms

Genetic diversity is lost when the population size gets smaller. In such a case, the allee effect is observed. As the population size decreases, the vitality of the species decreases.

Examples of Allee Effect in Various Taxa

Below are examples of the allee effect:

• Fruit flies are considered to be one of the most dangerous pests in agriculture, as they have the potential to attack about 400 crops worldwide. The use of sterile male metve flies is one of the techniques used to control them and creates the Allee effect. A large number of natural enemies are released to make sure the Allee effect is prevented.

• When density is low, the Allee effect can result in species extinction due to the rarity of mating encounters or fertilization indicated by pollination of fig trees.

• It is important in breeding fish, which are animals living in groups. If harvest pressure is high, it could lead to a potential population fragmentation.

• Some endangered species are much less likely to find sensitive mates or have a biased sex ratio due to the stochasticity of democracy at low population sizes.

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