There are many phenomena related to the field of biology and many pieces of research have been performed on living organisms since time immortal. One such phenomenon is the Allee effect. The Allee effect in biology is a phenomenon that is primarily studied with reference to the correlation between population size (density) and the mean individual fitness of a population or species.


Warder Clyde Allee for the first time described this phenomenon; thus, the name Allee effect. He did many experimental studies and researches, and then Allee was able to demonstrate using the example of goldfish that a goldfish grow and reproduce more rapidly when there are more individuals within the tank in which they are kept. This led him to the conclusion that any aggregation can improve the survival rate of individuals, and that if they live within the group cooperating their chances of survival increase. This term “Allee principle” came into action on a time when the field of ecology was greatly focused on the role of competition, its effects, causes, and advantages among and within the species.

A very famous view of population dynamics stated that due to the competition for the resources, a population will have a reduced overall growth rate even at a very high density and will have an increased growth rate at a lower density. In other words, individuals in a population would live and perform at a better rate when there are fewer individuals around due to a limited amount of resources because there the competition reduces. However, the concept of the Allee effect introduced the idea that the opposite is true when the population density is low. individuals in their surroundings need one another to complete their survival. They need prey to hunt for and feed upon; they need a mate for reproduction; they even live in groups in their surroundings. All this shows the level of relationship and the amount of competition an organism faces.


Allee effects occur in a small group of populations and, although they are very rarely detected, they are still believed to be commonly found in nature. Charles Darwin, a very renowned person observed that large population size is an important boundary against the extinction of any species in the presence of predators or it’s other natural enemies.

Allee being a close observer of animal behavior noticed that in many species it was under-crowding (places that had less individual population density)  and not a competition, that was the major cause of limited population growth. He also observed that aggregation had positive effects on the survival of any individual, which faced rapid loss when subjected to isolation. This was because these species have always lived in aggregation and do not know the techniques and methods of surviving all alone. His theories and examples of the benefits of aggregation seemed to speak up against both the Malthusian paradigm (gave population curve) and the logistic model.

A positive relationship between fitness and population size can take place usually by different types of mechanisms that affect the reproduction rate and survival of a particular organism. Any imbalance in the species can cause an imbalance in nature because each species has its place and has a unique role to play in the environment. For example a well-situated mate limitation in an area may automatically result in under crowding in sexually reproducing species because for such type of reproduction a contact is required between the male and a female organism.


Mate limitation tends to reduce reproduction rate when any plant or animals release the male and female gametes into the environment or when males and females have difficulty in finding each other. When the behavioral aspect such as breeding, feeding, and defense are cooperative, they become more efficient or successful in larger social groups, which will results in increased reproductive success or survivor-ship in an organism may it be a plant or an animal. Although this effect is mostly seen in large animals and plants too they can also be observed in pests, mites, predatory insects, bugs, etc.

If we talk about the other mechanisms of ecology, then we can say that they do not require cooperation in respect of behavioral but only the presence of that specific individual/individual. Demographic and genetic mechanisms give rise to Allee effects. In animals the active dispersal far away from the regions of low-density populations can result in declining rates of population growth. For many organisms, in which the population size is small in such species phenomena such as inbreeding depression can cause an Allee effect by reducing average fitness as population size decreases by this.


When the size of populations that faces strong Allee effects is small, these populations tend towards extinction. This fact states that a thorough understanding of Allee effects and their mechanisms is needed because there is an urgent need to develop good management practices to control environmental issues. A significant step here can be the conservation of rare species. In general, subjecting of species to strong Allee effects requires that minimum population size should be distributed over a population-specific area to avoid extinction of these species.

Another environmental issue that involves the Allee effect is the management of those species that come from an outside environment and occupy a canopy leading to the decline of the main species. Such species are called invasive species. Population biologists have questioned why only a small population of introduced species ultimately can thrive in some locations and establish so well that it is hard for us to eventually get rid of them. Allee effect is a very interesting subject matter in itself if taken into positive and good consideration.

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