Mate finding, Allee effects, and selection for sex-biased dispersal

Background/Question/Methods

Dispersal is a complicated behavior that involves decision-making in three distinct stages: when to leave, where and how far to travel, and when and where to settle. Yet within the theoretical literature, dispersal is often modeled as a simple trait, without explicit consideration of the decisions individuals make in any of these stages. For sexually reproducing organisms, an important factor in the settling decision is the location of suitable mates. Difficulty finding mates can lead to an Allee effect where lower population growth occurs at low densities. Surprisingly there is little overlap between research on movement to find mates and movement to disperse, and theory from each of these fields even generate opposing predictions. Here we develop a two sex individual based model to examine the evolution of settling behavior by bridging the mate-finding and dispersal literature. We determine under what conditions sex-biased dispersal evolves, and see if dispersal can help a population to overcome an Allee effect.

Results/Conclusions

Our model includes a parameter that controls how 'sensitive' individuals are to the local sex ratio when deciding when to settle, allowing us to replicate both seemingly contradictory predictions from the mate-finding and dispersal literatures. When sensitivity is high, we observe sex-biased dispersal, where either males or females move to find mates (matching the mate-finding literature finding). When sensitivity is low, we observe no sex-bias in dispersal and a correlation in distance between the sexes (matching the dispersal literature finding). Finally we show that under some conditions movement can help a population overcome a mate-finding Allee effect, but in other conditions movement can actually exacerbate the Allee effect.