Sex and stochasticity affect range expansion of experimental invasions

Understanding and predicting range expansion are key objectives in many basic and applied contexts. Among dioecious organisms, there is strong evidence for sex differences in dispersal, which could alter the sex ratio at the expansion's leading edge. However, demographic stochasticity could also affect leading-edge sex ratios, perhaps overwhelming sex-biased dispersal. We used insects in laboratory mesocosms to test the effects of sex-biased dispersal on range expansion, and a simulation model to explore interactive effects of sex-biased dispersal and demographic stochasticity. Sex-biased dispersal created spatial clines in the sex ratio, which influenced offspring production at the front and altered invasion velocity. Increasing female dispersal relative to males accelerated spread, despite the prediction that demographic stochasticity would weaken a signal of sex-biased dispersal. Our results provide the first experimental evidence for an influence of sex-biased dispersal on invasion velocity, highlighting the value of accounting for sex structure in studies of range expansion.