The Allee effect limits the loss of genetic variability during population bottlenecks

Allee effects create a minimum (critical) density for population persistence.  In the context of a specific habitat, this critical density defines a minimum population size during population bottlenecks or founder events.  By setting a lower limit on population size, Allee effects constrain the loss of genetic variability due to genetic drift. In fact, the minimum population size required by Allee effects can be much larger than the extremely small population sizes generally assumed possible in the literature on bottlenecks. As a result, much of the original genetic variation may be retained in recovered or newly-founded populations. In the alpine copepod Hesperodiaptomus shoshone for example, models, surveys and experimental data suggest populations with densities less than 0.5 - 5 individuals/m³ are unable to persist due to mate limitation. Combining this range of critical density with reproductive characteristics and habitat sizes we estimated minimum effective population sizes for H. shoshone.  The distribution of habitat sizes for this species in the Sierra Nevada suggests that >90% of populations are resistant to significant changes in heterozygosity or genetic distance and 70-75% of populations will lose <10% of allelic richness during bottlenecks or founder events. To our knowledge, this is the first consideration of the role of Allee effects in the loss of genetic variability during population bottlenecks. Our results reinforce the fact that even moderate limits on minimum population size lead to significantly different genetic outcomes than those predicted when ignoring limits on bottleneck size, and suggest Allee effects could potentially decrease the influence of genetic drift in other taxa.