Dispersal evolution in the presence of Allee effects can speed up or slow down invasions

Successful invasions by sexually reproducing species depend on the ability of individuals to mate. Finding mates can be particularly challenging at low densities (a mate-finding Allee effect), a factor that is only implicitly accounted for by most invasion models, which typically assume asexual populations. Existing theory on singlesex populations suggests that dispersal evolution in the presence of amate-finding Allee effect slows invasions. Here we develop a two-sex model to determine how mating system, strength of an Allee effect, and dispersal evolution influence invasion speed. We show that mating system differences can dramatically alter the spread rate. We also find a broader spectrum of outcomes than earlier work suggests. Allowing dispersal to evolve in a spreading context can sometimes alleviate the mate-finding Allee effect and slow the rate of spread. However, we demonstrate the opposite when resource competition among females remains high: evolution then acts to speed up the spread rate, despite simultaneously exacerbating the Allee effect. Our results highlight the importance of the timing of mating relative to dispersal and the strength of resource competition for consideration in future empirical studies.