Mechanisms Allee effect

1 mechanisms

1.1 ecological mechanism
1.2 human induced
1.3 genetic mechanisms
1.4 demographic stochasticity

mechanisms

due definition positive correlation between population density , average fitness, mechanisms allee effect arises therefore inherently tied survival , reproduction. in general, these allee effect mechanisms arise cooperation or facilitation among individuals in species. examples of such cooperative behaviors include better mate finding, environmental conditioning, , group defense against predators. these mechanisms more observable in field, tend more commonly associated allee effect concept. nevertheless, mechanisms of allee effect less conspicuous such inbreeding depression , sex ratio bias should considered well.

ecological mechanism

although numerous ecological mechanisms allee effects exist, list of commonly cited facilitative behaviors contribute allee effects in literature include: mate limitation, cooperative defense, cooperative feeding, , environmental conditioning. while these behaviors classified in separate categories, note can overlap , tend context dependent (will operate under conditions – example, cooperative defense useful when there predators or competitors present).

mate limitation
mate limitation refers difficulty of finding compatible , receptive mate sexual reproduction @ lower population size or density. problem encountered species utilize passive reproduction , possess low mobility, such plankton, plants , sessile invertebrates. example, wind-pollinated plants have lower fitness in sparse populations due lower likelihood of pollen landing on conspecific.


cooperative defense
another possible benefit of aggregation protect against predation group anti-predator behavior. many species exhibit higher rates of predator vigilance behavior per individual @ lower density. increased vigilance might result in less time , energy spent on foraging, reducing fitness of individual living in smaller groups. 1 striking example of such shared vigilance exhibited meerkats. meanwhile, other species move in synchrony confuse , avoid predators such schools of sardines , flocks of starlings. confusion effect herding behavior have on predators more effective when more individuals present.


cooperative feeding
certain species require group foraging in order survive. example, species hunt in packs, such african wild dogs, not able locate , capture prey efficiently in smaller groups.


environmental conditioning / habitat alteration
environmental conditioning refers mechanism in individuals work in order improve immediate or future environment benefit of species. alteration involve changes in both abiotic (temperature, turbulence, etc.) or biotic (toxins, hormones, etc.) environmental factors. pacific salmon presents unique case of such component allee effects, density of spawning individuals can affect survivability of following generations. spawning salmon carry marine nutrients acquired ocean migrate freshwater streams reproduce, in turn fertilize surrounding habitat when die, creating more suitable habitat juveniles hatch in following months.

human induced

classic economic theory predicts human exploitation of population unlikely result in species extinction because escalating costs find last few individuals exceed fixed price 1 achieves selling individuals on market. however, when rare species more desirable common species, prices rare species can exceed high harvest costs. phenomena can create anthropogenic allee effect rare species go extinct common species sustainably harvested. anthropogenic allee effect has become standard approach conceptualizing threat of economic markets on endangered species. however, original theory posited using 1 dimensional analysis of 2 dimensional model. turns out 2 dimensional analysis yields allee curve in human exploiter , biological population space , curve separating species destined extinction vs persistence can complicated. high population sizes can potentially pass through proposed allee thresholds on predestined paths extinction.

genetic mechanisms

declines in population size can result in loss of genetic diversity, , owing genetic variation s role in evolutionary potential of species, in turn result in observable allee effect. species population becomes smaller, gene pool reduced in size well. 1 possible outcome genetic bottleneck reduction in fitness of species through process of genetic drift, inbreeding depression. overall fitness decrease of species caused accumulation of deleterious mutations throughout population. genetic variation within species range beneficial detrimental. nevertheless, in smaller sized gene pool, there higher chance of stochastic event in deleterious alleles become fixed (genetic drift). while evolutionary theory states expressed deleterious alleles should purged through natural selection, purging efficient @ eliminating alleles highly detrimental or harmful. mildly deleterious alleles such act later in life less removed natural selection, , conversely, newly acquired beneficial mutations more lost random chance in smaller genetic pools larger ones.

although long-term population persistence of several species low genetic variation has prompted debate on generality of inbreeding depression, there various empirical evidences genetic allee effects. 1 such case observed in endangered florida panther (puma concolor coryi). florida panther experienced genetic bottleneck in 1990s population reduced ~ 25 adult individuals. reduction in genetic diversity correlated defects include lower sperm quality, abnormal testosterone levels, cowlicks, , kinked tails. in response, genetic rescue plan put in motion , several female pumas texas introduced florida population. action led reduction in prevalence of defects associated inbreeding depression. although timescale inbreeding depression larger of more immediate allee effects, has significant implications on long-term persistence of species.

demographic stochasticity

demographic stochasticity refers variability in population growth arising sampling random births , deaths in population of finite size. in small populations, demographic stochasticity decrease population growth rate, causing effect similar allee effect, increase risk of population extinction. whether or not demographic stochasticity can considered part of allee effect contentious however. current definition of allee effect considers correlation between population density , mean individual fitness. therefore, random variation resulting birth , death events not considered part of allee effect increased risk of extinction not consequence of changing fates of individuals within population.

meanwhile, when demographic stochasticity results in fluctuations of sex ratios, arguably reduces mean individual fitness population declines. example, fluctuation in small population causes scarcity in 1 sex in turn limit access of mates opposite sex, decreasing fitness of individuals within population. type of allee effect more prevalent in monogamous species polygynous species.