How do mate-finding Allee effects affect evolutionary rescue?

Abstract

In sexually reproducing populations, the challenge of finding mates at low densities can impose a strong demographic Allee effect. Environmental change can cause a population to fall below its Allee threshold by reducing the population size or by increasing the threshold if the latter depends on affected life-history traits. Evolutionary rescue then relies on overcoming the Allee effect, which gets increasingly difficult as the population declines. Despite mate-finding Allee effects being common, most models of evolutionary rescue assume that mating is assured even at low densities. Here, we set up a population genetic model to study the potential for evolutionary rescue of a population below its Allee threshold. For the analysis, we combine stochastic computer simulations with mathematical arguments. As expected, mate limitation can severely impede rescue, but the extent differs across sexual systems. We further show that it shifts the optimal sex ratio for dioecious but not for androdioecious populations, alters optimal evolutionary routes when there are trade-offs between increasing mate-finding efficiency and fecundity, and enhances the importance of standing genetic variation relative to de novo mutants. Overall, our results highlight the importance of accounting for positive density dependence in the assessment of a population's scope for evolutionary rescue.