Allee Effect

Allee effect is a biological phenomenon help in expressing the correlation between the survival capacity and the density of population. It determines the connection between the ability of the organisms to thrive by taking into account various aspects of the population. The phenomenon was first identified in the goldfish population by  Warder Clyde Allee. He demonstrated that fish population lived long when they were few in numbers but as the density per unit area has raised, the overall fitness and the survival rate of the fish population was declined. The same formula can be generalized even on the human population. Allee effect is directly connected to the growth and health of population which depends upon the number of individuals living per square unit in a given territory. To put it in another way, the greater the density, the higher the competition and lesser the longevity and the quality of life of organisms. Therefore, as the population became highly dense,  the Allee effect has also proved strong. A stronger allee effect is a negative sign in a given population as it can result in a greater struggle to survive due to high competition between the organisms.  

Types of Allee effect

Component Allee Effect

The component Allee effect is the positive correlation between a measurable component of individual fitness with the population density while the demographic Allee effect is the positive correlation between the overall individual fitness and population density. The component Allee effect occurs when the Allee effect is associated with one or a few particular components of a population. For example, if we take the 2 components operative hunting and the ability to find mates, they both are different but are influenced by population density. To put it in another way, the 2 skills and their success rates depend upon how many of the other organisms competing with the help of the same skills. At low population density, many component Allee effects would add up to produce an overall demographic Allee effect (increased fitness with higher population density).

Demographic Allee Effect

When population density goes to peak,  it often offsets the component Allee effects through resource competition, thus erasing the demographic Allee effect. Demographic allee effect occurs when there is a correlation between the population density and per capita population growth. Unlike component Allee effect, demographic Allee effect is not just about one or 2 aspects of the population, rather it is inclusive of many aspects such as; individual's age, body weight, but it influences a multitude of population aspects. The demographic Allee effect gives rise to a positive relationship between the overall individual fitness and the population density in a given area. 

Strong and weak Allee effects

A strong Allee effect is a demographic Allee effect influence against substantially a large population size( critical population size0  or density while the weak Allee effect is a demographic Allee effect that does not take into account of critical population size or density. The difference between the 2 phenomena is based on whether the population in question involves a critical population size. With this regard, a population showing a weak Allee effect tend to have a reduced per capita growth rate. This is because weak all effect is directly related to individual fitness of a given population when the individuals live in a  population that is low, however, on the demographic Allee effect viewpoint, weak Alle effect improves population growth. Conversely, a population manifesting a strong Allee effect tend to be a critical population size ( high density), which results in the high competition. Consequently, when the population density goes well below the threshold, it is clear that they go into extinction without any further aid. 

Mechanisms of Allee Effects

Allee effect is a consequence of both natural and man-made (induced) acts. The 4 main mechanisms of Allee effect are mate limitation, cooperative defence, cooperative feeding and environmental conditioning or habitat alteration. Let us look at each one. 

Mate limitation

Mate limitation is a condition when species find it difficult to search for a mate to reproduce. This happens if the population is thin and sparsely distributed, for example, wind-pollinated plants exhibit a lower fitness in thin populations as this can cause poor exchange of the pollen grains.  \

Cooperative defence

Some species live with a strict predator vigilance behaviour to protect themselves. During the cooperative defence, the individual, which is not the target of the predator, helps the members of the group attacked by the predator and during a defensive action. This can save the other organisms while the helper individual may die. The ultimate effect here is the loss of an individual which consequently reduces survival. It also results in the lesser time available for hunting and foraging, consequently, the individual does not meet the fitness required for their survival, and is known as cooperative defence. It is common in smaller groups.

Cooperative feeding

It occurs when the species participate in group foraging for their survival. Some animals cannot locate and capture the prey more efficiently in smaller groups, this can force them to develop a massive group for cooperative feeding, Cooperative feeding is commonly seen in African wild dogs.

Environmental conditioning

It is a term used to describe the ability of individual species to modify the biotic and abiotic component which can enhance their welfare and safety. Some abiotic changes that could be altered to help the survival of species are temperature, turbulence, toxins, and hormones in the living habitats.

Anthropogenic Allee effect

The uncontrolled exploitation of the population by humans against the weaker and smaller sized communities of species results in extinction, this is known as anthropogenic allee effect.  The net effect of the allee mechanism is the genetic drift and the hampering of the overall survival ability of species due to the decrease in the fitness of the minority community. To combat this, species must undergo random variation so that they can enhance their reproductive rates. 

Genetic mechanisms

Most negative genetic mechanisms cause a decline in population consequently, there is a loss of genetic diversity therefore, it is a form of an observable Allee effect. Smaller species size is correlated with negative mechanisms such as genetic drift, species immigration, inbreeding depression which altogether cause a massive reduction of the overall fitness.

Benefits of allee effect

Our Ecosystem works by balancing threats that affect living organisms. Each time when the minority species are suppressed, the system induces a defence mechanism through which individuals can unlearn the destructive behaviours and learn the new techniques that can protect against the exploitation by predators. So the conclusion is when a few species lose new and better organisms make an entry, which is the hallmark of evolution.

Examples of the Allee effect 

As the species face the shortage of mating encounters, species undergo drastic extinction due to decreased fertilization and this is even worse when the size of the affected species is very small. The Allee effect has demonstrated a highly significant role to help those organisms live in small groups. Allee effect can sometimes collapse the whole population if the harvesting pressure is too strong. Take for example of fruit flies, they can attack more than 400 crops worldwide, they are considered to be one of the worst insect pests of agriculture. By releasing the sterile males , which is based on one of the principles of Allee effect, fruit flies can be killed.

Read More 

  1. Genetics and Gene therapy

  2. Introduction to genetics and heredity

  3. Gene therapy

  4. Chromosomal theory of inheritanceC

  5. hromosomes-structure and functions

  6. Mechanism of Evolution

Check your understanding

  1. What is an allee effect?

  2. Distinguish between strong and weak Allee effect.

  3. How can we benefit from the Allee effect?​​​​​​

  4. What is Mate limitation?

  5. Explain the correlation between Anthropogenic Allee effect and the survival capacity of individuals.

  6. Explain any 4 Mechanisms of Allee Effect.

  7. Differentiate between the component and demographic Allee Effect.

  8. Cooperative hunting reduces the population density, Explain this with evidence.



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