Slowing down a sex-structured invasion

Background/Question/Methods

Understanding and predicting range expansion are key objectives in many basic and applied contexts. Dispersal is a key driver of range expansion. Among dioecious organisms, dispersal is often sex-biased, with females and males moving different distances. Sex-biased dispersal could alter the sex ratio at the expansion's leading edge. However, demographic stochasticity could also affect leading-edge sex ratios, even if there are no deterministic differences between female and male dispersal. Whether demographic stochasticity dampens or amplifies the influence of sex-biased dispersal, and how this might inform strategies for managing invasive range expansion, remain open and important questions. I 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. I also examined whether these factors affect the efficacy of alternative management strategies (targeted local extinctions) for decelerating sex-structured range expansion.

Results/Conclusions

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. However, I also found that demographic stochasticity can weaken the influence of sex-biased dispersal, especially when sex differences are small. Sex-biased dispersal and demographic stochasticity both affected where in the expanding wave local extinctions cause the greatest deceleration in spread. These 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.