Overcoming Allee effects through evolutionary, genetic, and demographic rescue

Background/Question/Methods

Studies of biological invasions frequently acknowledge a phenomenological paradox wherein despite the amplified threats of extinction facing small founder populations, successful colonization occurs nonetheless, bringing devastating ecological and economic consequences.  We addressed this paradox by exploring the importance of evolutionary processes given the time constraints generated by ecological factors driving the population to extinction.  When a population is introduced at low density, individuals often experience a reduction in one or more components of fitness due to novel selection pressures that arise from diminished intraspecific interactions and positive density dependence (i.e., component Allee effects).  Although the time to extinction may be limited, there is a chance that the population can adapt and recover on its own (i.e., evolutionary rescue) or through additional immigration contributing to the population size (i.e., demographic rescue) and/or enhancing the genetic variation (i.e., genetic rescue).  Within a spatially-explicit modeling framework, we consider the relative impact of each type of rescue on probability of success by following the evolution of a multi-locus quantitative trait that influences the strength of component Allee effects.  

Results/Conclusions

We demonstrate that because the ecological system is significantly density driven, the effect of demographic rescue provides the greatest opportunity for success.  While highlighting the role of evolution in the invasion process we underscore the importance of the ecological context influencing the persistence of small founder populations.