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The Allee effect is a phenomenon in
biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
characterized by a correlation between
population size In population genetics and population ecology, population size (usually denoted ''N'') is the number of individual organisms in a population. Population size is directly associated with amount of genetic drift, and is the underlying cause of effect ...
or density and the mean individual fitness (often measured as ''
per capita ''Per capita'' is a Latin phrase literally meaning "by heads" or "for each head", and idiomatically used to mean "per person". The term is used in a wide variety of social sciences and statistical research contexts, including government statistic ...
''
population growth rate Population growth is the increase in the number of people in a population or dispersed group. Actual global human population growth amounts to around 83 million annually, or 1.1% per year. The global population has grown from 1 billion in 1800 to ...
) of a population or species.


History and background

Although the concept of Allee effect had no title at the time, it was first described in the 1930s by its namesake,
Warder Clyde Allee Warder Clyde Allee (June 5, 1885 – March 18, 1955) was an American ecologist. He is recognized to be one of the great pioneers of American ecology. Schmidt, Karl Patterson. "Warder Allee: A Biographical Memoir", National Academy of Sciences. Wa ...
. Through experimental studies, Allee was able to demonstrate that
goldfish The goldfish (''Carassius auratus'') is a freshwater fish in the family Cyprinidae of order Cypriniformes. It is commonly kept as a pet in indoor aquariums, and is one of the most popular aquarium fish. Goldfish released into the wild have ...
have a greater survival rate when there are more individuals within the tank. This led him to conclude that aggregation can improve the survival rate of individuals, and that cooperation may be crucial in the overall evolution of social structure. The term "Allee principle" was introduced in the 1950s, a time when the field of ecology was heavily focused on the role of competition among and within species. The classical view of population dynamics stated that due to competition for resources, a population will experience a reduced overall growth rate at higher density and increased growth rate at lower density. In other words, individuals in a population would be better off when there are fewer individuals around due to a limited amount of resources (see
logistic growth A logistic function or logistic curve is a common S-shaped curve (sigmoid curve) with equation f(x) = \frac, where For values of x in the domain of real numbers from -\infty to +\infty, the S-curve shown on the right is obtained, with the ...
). However, the concept of the Allee effect introduced the idea that the reverse holds true when the
population density Population density (in agriculture: standing stock or plant density) is a measurement of population per unit land area. It is mostly applied to humans, but sometimes to other living organisms too. It is a key geographical term.Matt RosenberPopul ...
is low. Individuals within a species often require the assistance of another individual for more than simple reproductive reasons in order to persist. The most obvious example of this is observed in animals that hunt for prey or defend against predators as a group.


Definition

The generally accepted definition of Allee effect is positive density dependence, or the positive correlation between population density and individual fitness. It is sometimes referred to as "undercrowding" and it is analogous (or even considered synonymous by some) to " depensation" in the field of
fishery science Fisheries science is the academic discipline of managing and understanding fisheries. It is a multidisciplinary science, which draws on the disciplines of limnology, oceanography, freshwater biology, marine biology, meteorology, conservation, eco ...
s. Listed below are a few significant subcategories of the Allee effect used in the ecology literature.


Component vs. demographic Allee effects

The ''component Allee effect'' is the positive relationship between any measurable component of individual fitness and population density. The ''demographic Allee effect'' is the positive relationship between the overall individual fitness and population density. The distinction between the two terms lies on the scale of the Allee effect: the presence of a demographic Allee effect suggests the presence of at least one component Allee effect, while the presence of a component Allee effect does not necessarily result in a demographic Allee effect. For example, cooperative hunting and the ability to more easily find mates, both influenced by population density, are component Allee effects, as they influence individual fitness of the population. At low population density, these component Allee effects would add up to produce an overall demographic Allee effect (increased fitness with higher population density). When population density reaches a high number, negative density dependence often offsets the component Allee effects through resource competition, thus erasing the demographic Allee effect. Allee effects might occur even at high population density for some species.


Strong vs. weak Allee effects

The ''strong Allee effect'' is a demographic Allee effect with a critical population size or density. The ''weak Allee effect'' is a demographic Allee effect without a critical population size or density. The distinction between the two terms is based on whether or not the population in question exhibits a critical population size or density. A population exhibiting a weak Allee effect will possess a reduced per capita growth rate (directly related to individual fitness of the population) at lower population density or size. However, even at this low population size or density, the population will always exhibit a positive per capita growth rate. Meanwhile, a population exhibiting a strong Allee effect will have a critical population size or density under which the population growth rate becomes negative. Therefore, when the population density or size hits a number below this threshold, the population will be destined for extinction without any further aid. A strong Allee effect is often easier to demonstrate empirically using time series data, as one can pinpoint the population size or density at which per capita growth rate becomes negative.


Mechanisms

Due to its definition as the positive correlation between population density and average fitness, the mechanisms for which an Allee effect arises are therefore inherently tied to survival and reproduction. In general, these Allee effect mechanisms arise from cooperation or facilitation among individuals in the species. Examples of such cooperative behaviors include better mate finding, environmental conditioning, and group defense against predators. As these mechanisms are more-easily observable in the field, they tend to be more commonly associated with the Allee effect concept. Nevertheless, mechanisms of Allee effect that are less conspicuous such as
inbreeding depression Inbreeding depression is the reduced biological fitness which has the potential to result from inbreeding (the breeding of related individuals). Biological fitness refers to an organism's ability to survive and perpetuate its genetic material. In ...
and
sex ratio The sex ratio (or gender ratio) is usually defined as the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. Many species devia ...
bias should be considered as well.


Ecological mechanism

Although numerous ecological mechanisms for Allee effects exist, the list of most commonly cited facilitative behaviors that contribute to Allee effects in the literature include: mate limitation, cooperative defense, cooperative feeding, and environmental conditioning. While these behaviors are classified in separate categories, they can overlap and tend to be context dependent (will operate only under certain conditions – for example, cooperative defense will only be useful when there are predators or competitors present). ;Mate limitation: Mate limitation refers to the difficulty of finding a compatible and receptive mate for sexual reproduction at lower population size or density. This is generally a problem encountered by species that utilize passive reproduction and possess low mobility, such as
plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...
, plants and sessile invertebrates. For example, wind-pollinated plants would have a lower fitness in sparse populations due to the lower likelihood of pollen successfully landing on a conspecific. ;Cooperative defense: Another possible benefit of aggregation is to protect against predation by group anti-predator behavior. Many species exhibit higher rates of predator vigilance behavior per individual at lower density. This increased vigilance might result in less time and energy spent on foraging, thus reducing the fitness of an individual living in smaller groups. One striking example of such shared vigilance is exhibited by
meerkat MeerKAT, originally the Karoo Array Telescope, is a radio telescope consisting of 64 antennas in the Meerkat National Park, in the Northern Cape of South Africa. In 2003, South Africa submitted an expression of interest to host the Square Kilom ...
s. Meanwhile, other species move in synchrony to confuse and avoid predators such as schools of
sardines "Sardine" and "pilchard" are common names for various species of small, oily forage fish in the herring family Clupeidae. The term "sardine" was first used in English during the early 15th century, a folk etymology says it comes from the I ...
and flocks of
starlings Starlings are small to medium-sized passerine birds in the family Sturnidae. The Sturnidae are named for the genus ''Sturnus'', which in turn comes from the Latin word for starling, ''sturnus''. Many Asian species, particularly the larger ones, ...
. The confusion effect that this herding behavior would have on predators will be more effective when more individuals are present. ;Cooperative feeding: Certain species also require group foraging in order to survive. As an example, species that hunt in packs, such as the
African wild dog The African wild dog (''Lycaon pictus''), also called the painted dog or Cape hunting dog, is a wild canine which is a native species to sub-Saharan Africa. It is the largest wild canine in Africa, and the only extant member of the genus '' Ly ...
s, would not be able to locate and capture prey as efficiently in smaller groups. ;Environmental conditioning / habitat alteration: Environmental conditioning generally refers to the mechanism in which individuals work together in order to improve their immediate or future environment for the benefit of the species. This alteration could involve changes in both abiotic (temperature, turbulence, etc.) or biotic (toxins, hormones, etc.) environmental factors.
Pacific salmon ''Oncorhynchus'' is a genus of fish in the family Salmonidae; it contains the Pacific salmon and Pacific trout. The name of the genus is derived from the Greek ὄγκος (ónkos, “lump, bend”) + ῥύγχος (rhúnkhos, “snout”), in r ...
present a potential case of such component Allee effects, where the density of spawning individuals can affect the survivability of the following generations. Spawning salmon carry marine nutrients they acquired from the ocean as they migrate to freshwater streams to reproduce, which in turn fertilize the surrounding habitat when they die, thus creating a more suitable habitat for the juveniles that would hatch in the following months. While compelling, this case of environmental conditioning by salmon has not been rigorously supported by empirical evidence.


Human induced

Classic economic theory predicts that human exploitation of a population is unlikely to result in species extinction because the escalating costs to find the last few individuals will exceed the fixed price one achieves by selling the individuals on the market. However, when rare species are more desirable than common species, prices for rare species can exceed high harvest costs. This phenomenon can create an "anthropogenic" Allee effect where rare species go extinct but common species are sustainably harvested. The anthropogenic Allee effect has become a standard approach for conceptualizing the threat of economic markets on endangered species. However, the original theory was posited using a one dimensional analysis of a two dimensional model. It turns out that a two dimensional analysis yields an Allee curve in human exploiter and biological population space and that this curve separating species destined to extinction vs persistence can be complicated. Even very high population sizes can potentially pass through the originally proposed Allee thresholds on predestined paths to extinction.


Genetic mechanisms

Declines in population size can result in a loss of genetic diversity, and owing to
genetic variation Genetic variation is the difference in DNA among individuals or the differences between populations. The multiple sources of genetic variation include mutation and genetic recombination. Mutations are the ultimate sources of genetic variation, ...
's role in the evolutionary potential of a species, this could in turn result in an observable Allee effect. As a species' population becomes smaller, its gene pool will be reduced in size as well. One possible outcome from this
genetic bottleneck A population bottleneck or genetic bottleneck is a sharp reduction in the size of a population due to environmental events such as famines, earthquakes, floods, fires, disease, and droughts; or human activities such as specicide, widespread violen ...
is a reduction in fitness of the species through the process of
genetic drift Genetic drift, also known as allelic drift or the Wright effect, is the change in the frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene variants to disappear completely and there ...
, as well as
inbreeding depression Inbreeding depression is the reduced biological fitness which has the potential to result from inbreeding (the breeding of related individuals). Biological fitness refers to an organism's ability to survive and perpetuate its genetic material. In ...
. This overall fitness decrease of a species is caused by an accumulation of
deleterious mutations {{Short pages monitor