Editing Allee effect (section)

==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]] and [[sex ratio]] 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.<ref name=kramer /> 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]], plants and sessile invertebrates.<ref name="Berec L 2006">{{Cite journal |vauthors=Berec L, Angulo E, Courchamp F |year=2007 |title=Multiple Allee effects and population management |journal=Trends in Ecology & Evolution |volume=22 |pages=185–191 |doi=10.1016/j.tree.2006.12.002 |pmid=17175060 |issue=4|bibcode=2007TEcoE..22..185B |hdl=10261/45491 |hdl-access=free }}</ref> 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.<ref>{{Cite journal |vauthors=Davis HG, Taylor CM, Lambrinos JG, Strong DR |year=2004 |title=Pollen limitation causes an Allee effect in a wind-pollinated invasive grass (''Spartina alterniflora'') |journal=Proceedings of the National Academy of Sciences |volume=101 |pages=13804–13807 |doi=10.1073/pnas.0405230101 |issue=38|bibcode = 2004PNAS..10113804D |pmid=15317944 |pmc=518837|doi-access=free }}</ref>

;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.<ref>{{Cite journal |vauthors=Mooring MS, Fitzpatrick TA, Nishihira TT, Reisig DD |year=2004 |title=Vigilance, predation risk, and the Allee effect in desert bighorn sheep |journal=Journal of Wildlife Management |volume=68 |pages=519–532 |doi=10.2193/0022-541X(2004)068[0519:VPRATA]2.0.CO;2 |editor1-last=Hall |issue=3|s2cid=53981797 }}</ref> One striking example of such shared vigilance is exhibited by [[meerkat]]s.<ref>{{Cite journal |vauthors=Clutton-Brock TH, Gaynor D, McIlrath GM, MacColl AD, Kansky R, Chadwick P, Manser M, Skinner JD, Brotherton PN |s2cid=51855211 |year=1999 |title=Predation, group size and mortality in a cooperative mongoose, ''Suricata suricatta'' |journal=Journal of Animal Ecology |volume=68 |pages=672–683 |doi=10.1046/j.1365-2656.1999.00317.x |issue=4|doi-access=free |bibcode=1999JAnEc..68..672C }}</ref> Meanwhile, other species move in synchrony to confuse and avoid predators such as schools of [[sardines]] and flocks of [[starlings]]. The confusion effect that this herding behavior would have on predators will be more effective when more individuals are present.<ref name=book1 />

;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]]s, would not be able to locate and capture prey as efficiently in smaller groups.<ref name="Berec L 2006"/>

;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]] 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.<ref>{{Cite journal |vauthors=Rinella DJ, Wipfli MS, Stricker CA, Heintz RA, Rinella MJ |year=2012 |title=Pacific salmon (''Oncorhynchus'' spp.) runs and consumer fitness: growth and energy storage in stream-dwelling salmonids increase with salmon spawner density |journal=Canadian Journal of Fisheries and Aquatic Sciences |volume=69 |issue=1 |pages=73–84 |doi=10.1139/f2011-133|bibcode=2012CJFAS..69Q..73R }}</ref> 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.<ref name=":0">{{Cite journal|last1=Courchamp|first1=Franck|last2=Angulo|first2=Elena|last3=Rivalan|first3=Philippe|last4=Hall|first4=Richard J.|last5=Signoret|first5=Laetitia|last6=Bull|first6=Leigh|last7=Meinard|first7=Yves|date=2006-11-28|title=Rarity Value and Species Extinction: The Anthropogenic Allee Effect|journal=PLOS Biology|volume=4|issue=12|pages=e415|doi=10.1371/journal.pbio.0040415|pmid=17132047|issn=1545-7885|pmc=1661683 |doi-access=free }}</ref> The anthropogenic Allee effect has become a standard approach for conceptualizing the threat of economic markets on endangered species.<ref name=":1">{{Cite journal|last1=Holden|first1=Matthew H.|last2=McDonald-Madden|first2=Eve|date=2017-09-21|title=High prices for rare species can drive large populations extinct: the anthropogenic Allee effect revisited|journal=Journal of Theoretical Biology|volume=429|pages=170–180|doi=10.1016/j.jtbi.2017.06.019|pmid=28669883|arxiv=1703.06736|bibcode=2017JThBi.429..170H|s2cid=4877874}}</ref> However, the original theory was posited using a one dimensional analysis of a two dimensional model.<ref name=":0" /><ref name=":1" />  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.<ref name=":1" />

=== Genetic mechanisms ===
Declines in population size can result in a loss of genetic diversity, and owing to [[genetic diversity|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 [[population bottleneck|genetic bottleneck]] is a reduction in fitness of the species through the process of [[genetic drift]], as well as [[inbreeding depression]].<ref>{{Cite journal |author =Frankham R |s2cid=6088451 |year=1996 |title=Relationship of genetic variation to population size in wildlife- a review |journal=Conservation Biology |volume=10 |pages=1500–1508 |doi=10.1046/j.1523-1739.1996.10061500.x |issue=6|bibcode=1996ConBi..10.1500F }}</ref> This overall fitness decrease of a species is caused by an accumulation of [[Mutation#Harmful mutations|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]]). 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.<ref name=book1 /><ref>{{Cite book |last=Gillespie |first=J. H. |year=2004 |title=Population genetics: a concise guide |publisher=Johns Hopkins University Press |location=Baltimore, Maryland, USA |isbn=0-8018-8008-4 }}</ref>

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.<ref>{{Cite journal |author =Reed DH |year=2010 |title=Albatrosses, eagles and newts, Oh My!: exceptions to the prevailing paradigm concerning genetic diversity and population viability? |journal=Animal Conservation |volume=13 |pages=448–457 |doi=10.1111/j.1469-1795.2010.00353.x |issue=5|doi-access=free |bibcode=2010AnCon..13..448R }}</ref> One such case was observed in the endangered [[Florida panther]] (''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.<ref>{{Cite journal |year=2010 |title=Genetic restoration of the Florida panther |journal=Science |volume=329 |pages=1641–1645 |bibcode=2010Sci...329.1641J |last1=Johnson|first1=WE|last2=Onorato |first2=DP|last3=Roelke |first3=MW|last4=Land |first4=ED|last5=Cunningham |first5=M|last6=Belden |first6=RC|last7=McBride |first7=R|last8=Jansen |first8=D|last9=Lotz |first9=M|s2cid=206527881 |display-authors=8|doi=10.1126/science.1192891 |pmid=20929847 |issue=5999|pmc=6993177 }}</ref> 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.<ref>{{Cite book|chapter-url=http://oxfordindex.oup.com/view/10.1093/acprof:oso/9780198525257.003.0001|chapter=Demographic and environmental stochasticity|last1=Lande|first1=Russell|last2=Engen|first2=Steinar|last3=SÆther|first3=Bernt-Erik|pages=1–24|language=en|doi=10.1093/acprof:oso/9780198525257.003.0001|title=Stochastic Population Dynamics in Ecology and Conservation|year=2003|isbn=9780198525257|access-date=2017-10-11|archive-date=2018-11-27|archive-url=https://web.archive.org/web/20181127064611/http://oxfordindex.oup.com/view/10.1093/acprof:oso/9780198525257.003.0001|url-status=dead}}</ref> In small populations, demographic stochasticity will decrease the population growth rate, causing an effect similar to the Allee effect,<ref>{{cite journal | vauthors = Lande R | year = 1998 | title = Demographic stochasticity and Allee effect on a scale with isotropic noise | journal = Oikos | volume = 83 | issue = 2| pages = 353–358 | doi=10.2307/3546849| jstor = 3546849 | bibcode = 1998Oikos..83..353L }}</ref><ref>{{Cite Q| Q56486427}}</ref> 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.<ref>{{Cite journal |vauthors=Bessa-Gomes C, Legendre S, Clobert J |year=2004 |title=Allee effects, mating systems and the extinction risk in populations with two sexes |journal=Ecology Letters |volume=7 |pages=802–812 |doi=10.1111/j.1461-0248.2004.00632.x |issue=9|bibcode=2004EcolL...7..802B }}</ref>
 
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 [[monogamy|monogamous]] species than [[polygyny|polygynous]] species.<ref>{{cite journal |vauthors=Engen S, Lande R, Saether BE |year=2003 |title=Demographic stochasticity and Allee effects in populations with two sexes |journal=Ecology |volume=84 |pages=2378–2386 |doi=10.1890/02-0123 |issue=9|bibcode=2003Ecol...84.2378E }}</ref>