Allee Effect

Introduction

Allee effect is the per capita population growth rate, which helps to detect and eliminate the extinction of a population. The Allee effect was explained by Warder Clyde Allee (1885-1955) to determine a positive per capita growth rate. In simple words, we can say that the Allee effect helps us to know at what speed the number of individuals will increase in a population (growth rate) concerning each individual (per capita).

It determines whether a population is going to increase its growth rate or is on the verge of extinction. Following this, any population, like plants and animals (especially endangered) can be prevented from being eliminated or getting extinct. Various factors determine the Allee effect.

What is the Allee Effect?

Allee effect is the relationship between an individual's fitness and population growth rate.

It determines the positive relation of population density with an individual and provides a critical value below which a population cannot exist. If a population is having a strong Allee effect it means that the population is having a positive growth rate and is at low risk of extinction and if a population is having a weak Allee effect it indicates the risk of extinction in near future like those of endangered species i.e. they are having a negative correlation.

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Examples of the Allee Eeffect

There are some instances of the Allee effect in nature, some of which are natural and a few are anthropogenic. Various factors influence the Allee effect in nature.

• Passenger Pigeon − The population of Passenger Pigeon were estimated to be in billions in North America, but due to their hunting on a large scale and habitat destruction of hardwood trees, they became extinct, as they couldn't escape predators. These human activities lead to a negative growth rate of these pigeons.

• Marine systems − High mortality in fish population due to interspecific competition or environmental factors or over-exploitation leads to a negative growth rate, as there is a population decline.

• Fruit flies − Fruit flies are considered one of the most dangerous pests in agriculture that cause harm to more than 400 crops all over the world, using males who are sterile are introduced which is one of the factors that promote the Allee effect and decline population of fruit flies to protect the crops.

Mechanism of Allee Effect

The mechanism of the Allee effect is determined by the reproduction and survivability of individual. Certain factors influence the mechanism of the Allee effect, like mate limitation which plays a major role in the mechanism because if a compatible partner or gamete is not found reproduction will not occur, as sexual reproduction requires the fusion of male and female gametes if there is any difficulty in locating the male or female gametes it limits the chances of reproduction and decline in population size, i.e. negative correlation. Thus, mate limitation reduces reproduction in animal and plant species thereby leading to a weaker Allee effect.

Survivability depends on cooperation between organisms, like breeding, feeding and defense, etc. It leads to more reproductive success and survivability. Cooperative behaviour is common among vertebrates. More co-operation lead to a strong Allee effect. Cooperative behaviour like group feeding and group defence in insects and aquatic animals can also give rise to the Allee effect.

Population interactions like that of prey and predator also result in the Allee effect, increased predation leads to a weak Allee effect in the prey population and decreased predation creates a strong effect, also, it is observed that the risk of predation is less in large prey population determining the Allee effect.

Other mechanisms like genetic mechanisms also lead to the Allee effect, one of them being inbreeding depression, which leads to a decline in population growth, causing negative correlation and a weaker Allee effect.

Types of Allee Effect

Allee effect is of two types −

• Component Allee effect − It is the positive correlation between a fitness component and population size. Fitness components can be fecundity, survivorship of new individuals and viability.

• Demographic Allee effect − It is the positive correlation between per capita population growth and population size.

Factors Contributing to Allee Effect

Various factors lead to the Allee effect which are mate limitation, cooperative behaviours like feeding and defence and environmental factors. Mate limitation occurs when organisms especially lower ones are unable to find suitable mates. Difficulty in locating male or female gametes as in animals or plants leads to a decrease in population growth due to a decline in sexual reproduction. Like there was a weak Allee effect on the population of fruit flies when sterile males were added to the population to protect the crops being damaged by the flies.

Cooperative feeding is the practice of finding food in a group to ensure complete nutrition and survivability. Cooperative defence is also required to avoid predation and thereby increasing chances of survival. This ensures more chances of survival in smaller populations.

Benefits of Allee effect

The Allee effect increases the probability of reintroduction of a species or population that is on the verge of extinction back into the environment.

If we determine the Allee effect concerned with a population we can predict whether a species going to be endangered or threatened, and is at the risk of extinction.

If a population is having a weaker Allee effect, we can make an inference that the population is endangered and going to face elimination.

If a population is having stronger Allee effect, we can make the inference that the population is having high growth rate and fit individuals.

Allee effect on Ecology and Evolution

Evolution

The ability of organisms to adapt according to the environment and evolve affects the Allee effect. For example, increased mate limitation and vulnerability to predators make organisms adapt to better survival strategies and develop traits that increase their growth rate.

Developing anti-predatory traits may increase the chances of the prey population increasing the growth rate.

Release of pheromones, hermaphroditism and parthenogenesis are some ways to avoid mate limitation and increase reproductive success to increase population size.

Ecology

Much environmental disturbance leads to a decline in population growth naturally, and anthropogenic reasons also determine the survivability of species. Allee's effect needs to be taken into consideration to develop management practices and measures to prevent the low-density populations from getting extinct.

Allee's effect on invasive species, prey and predator species needs to be taken into consideration to maintain a balance in the population growth rates and fitness of individuals for the maintenance of the ecosystem.

Conclusion

Allee effect is the positive correlation between individual fitness and population density, which can be of two types i.e. component and demographic Allee effects. There are various factors like mate limitation and Cooperation between species that determine the mechanism of the Allee effect whether it's going to be weaker or stronger. The weak Allee effect leads to a negative association and the stronger Allee effect leads to a positive association. The Allee effect helps in the elimination of the risk of extinction of a population and has a significant role in the evolution and the environment.

FAQs

Q1. How are Allee's effects tested?

Ans. The exact probabilities of the observed frequency of data points with expected in the hiatus area are calculated.

Q2. What are the consequences of the Allee effect?

Ans. It provides a threshold value below which a population cannot exist and gets eliminated or extinct.

Q3. What is the significance of the Allee effect?

Ans. It helps in determining the probability of survival of a population and thereby proper management strategies should be applied to prevent the extinction of endangered species.

Q4. Enlist factors that determine the Allee effect.

Ans. Factors are mate limitation, cooperative feeding, cooperative defence, reproduction, survivability and environmental factors.