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The Allee effect is a phenomenon in
biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, Development ...

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 effects ...
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 individuals in a population In biology, a population is a number of all the organisms of the same group or species In biology, a species is the basic unit of biological classification, ...
) 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. Washin ...

. Through experimental studies, Allee was able to demonstrate that goldfish 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 Logistic may refer to: Mathematics * Logistic function A logistic function or logistic curve is a common S-shaped curve (sigmoid curve A sigmoid function is a function (mathematics), mathematical function having a characteristic "S"-shaped ...
). However, the concept of the Allee effect introduced the idea that the reverse holds true when the population density 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 sciences. 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 Fitness (often denoted w or ω in population genetics models) is the quantitative representation of natural and sexual selection File:Sexual Selection with Peafowl.gif, 250px, Sexual se ...
and
sex ratio The sex ratio is the ratio In mathematics, a ratio indicates how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, ...
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 organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology ...

, 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 The meerkat (''Suricata suricatta'') or suricate is a small mongoose found in southern Africa. It is characterised by a broad head, large eyes, a pointed snout, long legs, a thin tapering tail, and a brindled coat pattern. The head-and-body l ...

s. Meanwhile, other species move in synchrony to confuse and avoid predators such as schools of
sardines "Sardine" and "pilchard" are common names that refer to various small, Oily fish, oily forage fish in the herring family Clupeidae. The term "sardine" was first used in English during the early 15th century and may come from the Mediterranean ...
and flocks of
starlings Starlings are small to medium-sized passerine birds in the family (biology), family Sturnidae. The name "Sturnidae" comes from the Latin word for starling, ''sturnus''. Many Asian species, particularly the larger ones, are called mynas, and many ...

. 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'') is a canine Canine may refer to: Zoology * dog-like mammals (i.e. members of the canid subfamily Caninae) ** ''Canis'', a genus including dogs, wolves, coyotes, and jackals ** Dog, the domestic dog * Ca ...

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 Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer ...
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 thumb File:Genetic Variation and Inheritance.svg, Parents have similar gene coding in this specific situation where they reproduce and variation in the offspring is seen. Offspring containing the variation also reproduce and passes down traits t ...
'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 is a reduction in fitness of the species through the process of
genetic drift Genetic drift (allelic drift or the Sewall Wright effect) is the change in the frequency of an existing gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical stru ...

, as well as
inbreeding depression Inbreeding depression is the reduced biological fitness Fitness (often denoted w or ω in population genetics models) is the quantitative representation of natural and sexual selection File:Sexual Selection with Peafowl.gif, 250px, Sexual se ...
. This overall fitness decrease of a species is caused by an accumulation of deleterious mutations throughout the population. Genetic variation within a species could range from beneficial to detrimental. Nevertheless, in a smaller sized gene pool, there is a higher chance of a stochastic event in which deleterious alleles become fixed (
genetic drift Genetic drift (allelic drift or the Sewall Wright effect) is the change in the frequency of an existing gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical stru ...

). While evolutionary theory states that expressed deleterious alleles should be purged through natural selection, purging would be most efficient only at eliminating alleles that are highly detrimental or harmful. Mildly deleterious alleles such as those that act later in life would be less likely to be removed by natural selection, and conversely, newly acquired beneficial mutations are more likely to be lost by random chance in smaller genetic pools than larger ones. Although the long-term population persistence of several species with low genetic variation has recently prompted debate on the generality of inbreeding depression, there are various empirical evidences for genetic Allee effects. One such case was observed in the endangered
Florida panther The Florida panther is a North American cougar (''P. c. couguar'') population found in South Florida. It lives in pinelands, tropical hardwood hammocks, and mixed freshwater swamp forests. Males can weigh up to and live within a range that incl ...

(''Puma concolor coryi''). The Florida panther experienced a genetic bottleneck in the early 1990s where the population was reduced to ≈25 adult individuals. This reduction in genetic diversity was correlated with defects that include lower sperm quality, abnormal testosterone levels, cowlicks, and kinked tails. In response, a genetic rescue plan was put in motion and several female pumas from Texas were introduced into the Florida population. This action quickly led to the reduction in the prevalence of the defects previously associated with inbreeding depression. Although the timescale for this inbreeding depression is larger than of those more immediate Allee effects, it has significant implications on the long-term persistence of a species.

## Demographic stochasticity

Demographic stochasticity refers to variability in population growth arising from sampling random births and deaths in a population of finite size. In small populations, demographic stochasticity will decrease the population growth rate, causing an effect similar to the Allee effect, which will increase the risk of population extinction. Whether or not demographic stochasticity can be considered a part of Allee effect is somewhat contentious however. The most current definition of Allee effect considers the correlation between population density and mean individual fitness. Therefore, random variation resulting from birth and death events would not be considered part of Allee effect as the increased risk of extinction is not a consequence of the changing fates of individuals within the population. Meanwhile, when demographic stochasticity results in fluctuations of sex ratios, it arguably reduces the mean individual fitness as population declines. For example, a fluctuation in small population that causes a scarcity in one sex would in turn limit the access of mates for the opposite sex, decreasing the fitness of the individuals within the population. This type of Allee effect will likely be more prevalent in
monogamous Monogamy ( ) is a form of dyadic relationship Relationship most often refers to: * Interpersonal relationship The concept of interpersonal relationship involves social associations, connections, or affiliations between two or more peo ...
species than
polygynous Polygyny (; from Neoclassical Greek πολυγυνία from πολύ- ''poly-'' "many", and γυνή ''gyne'' "woman" or "wife") is the most common and accepted form of polygamy, entailing the marriage of a man with several women. Most count ...
species.

# Effects on range-expanding populations

Demographic and mathematical studies demonstrate that the existence of an Allee effect can reduce the speed of range expansion of a population and can even prevent biological invasions. Recent results based on spatio-temporal models show that the Allee effect can also promote genetic diversity in expanding populations. These results counteract commonly held notions that the Allee effect possesses net adverse consequences. Reducing the growth rate of the individuals ahead of the colonization front simultaneously reduces the speed of colonization and enables a diversity of genes coming from the core of the population to remain on the front. The Allee effect also affects the spatial distribution of diversity. Whereas spatio-temporal models which do not include an Allee effect lead to a vertical pattern of genetic diversity (i.e., a strongly structured spatial distribution of genetic fractions), those including an Allee effect lead to a "horizontal pattern" of genetic diversity (i.e., an absence of genetic differentiation in space).

# Mathematical models

A simple mathematical example of an Allee effect is given by the cubic growth model :$\frac = r N \left\left( \frac - 1 \right\right) \left\left( 1 - \frac \right\right),$ where the population has a negative growth rate for $0< N < A$, and a positive growth rate for $A < N < K$ (assuming $0 < A < K$). This is a departure from the logistic growth equation :$\frac = r N \left\left( 1- \frac \right\right)$ where :N = population size; :r = intrinsic rate of increase; :K =
carrying capacity The carrying capacity of an Natural environment, environment is the maximum population size of a biological species that can be sustained by that specific environment, given the food, habitat, Drinking water, water, and other resources available. T ...
; :A = critical point; and :dN/dt = rate of increase of the population. After dividing both sides of the equation by the population size N, in the logistic growth the left hand side of the equation represents the per capita population growth rate, which is dependent on the population size N, and decreases with increasing N throughout the entire range of population sizes. In contrast, when there is an Allee effect the per-capita growth rate increases with increasing N over some range of population sizes , N Spatio-temporal models can take Allee effect into account as well. A simple example is given by the reaction-diffusion model :$\frac =D \frac+ r N \left\left( \frac - 1 \right\right) \left\left( 1 - \frac \right\right),$ where : D=
diffusion coefficient Diffusivity, mass diffusivity or diffusion coefficient is a proportionality constant between the molar flux due to molecular diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally fro ...
; : $\frac$ = one-dimensional
Laplace operator In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities an ...
. When a population is made up of small sub-populations additional factors to the Allee effect arise. If the sub-populations are subject to different environmental variations (i.e. separated enough that a disaster could occur at one sub-population site without affecting the other sub-populations) but still allow individuals to travel between sub-populations, then the individual sub-populations are more likely to go extinct than the total population. In the case of a catastrophic event decreasing numbers at a sub-population, individuals from another sub-population site may be able to repopulate the area. If all sub-populations are subject to the same environmental variations (i.e. if a disaster affected one, it would affect them all) then fragmentation of the population is detrimental to the population and increases extinction risk for the total population. In this case, the species receives none of the benefits of a small sub-population (loss of the sub-population is not catastrophic to the species as a whole) and all of the disadvantages (
inbreeding Inbreeding is the production of offspring In biology, offspring are the young creation of living organisms, produced either by a Asexual reproduction, single organism or, in the case of sexual reproduction, two organisms. Collective offspri ...
depression, loss of
genetic diversity Genetic diversity is the total number of genetic characteristics in the genetic makeup of a species, it ranges widely from the number of species to differences within species In biology, a species is the basic unit of biological classificati ...
and increased vulnerability to environmental instability) and the population would survive better unfragmented.

# Allee principles of aggregation

Clumping results due to individuals aggregating in response to: local habitat or landscape differences, daily and seasonal weather changes, reproductive processes, or as the result of social attractions.

# References

* *

* Berryman, AA (1997)
''Underpopulation (Allee) effects''
Entomology Entomology () is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. Th ...
Department,
Washington State University Washington State University (Washington State, WSU, or Wazzu) is a public In public relations Public relations (PR) is the practice of managing and disseminating information from an individual or an organization An organizat ...

. Retrieved 19 May 2008.
''Allee effect''
Warner College of Natural Resources,
Colorado State University Colorado State University (Colorado State or CSU) is a Public university, public Land-grant university, land-grant research university in Fort Collins, Colorado. It is the flagship university of the Colorado State University System. Colorado St ...
. Retrieved 19 May 2008. * Stephens, PA, Sutherland, WJ and Freckleton, RP (1999)
"What is the Allee effect?" (summary)
''Oikos'', 87, 185-90, at Evolutionary Biology Group, Department of
Zoology Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the Animal, animal kingdom, including the anatomy, structure, embryology, evolution, Biological class ...
,
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. Updated 22 November 2005. Retrieved 19 May 2008
''Classics: the Allee effect''
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