Sex in space: population dynamic consequences

Sex, so important in the reproduction of bigametic species, is nonetheless often ignored in explorations of the dynamics of populations. Using a growt h model of dispersal-coupled populations we can keep track of fluctuations in numbers of females and males. The sexes may differ from each other in th eir ability to disperse and their sensitivity to population density. As a f urther complication, the breeding system is either monogamous or polygamous . We use the harmonic mean birth function to account for sex-ratio-dependen t population growth in a Moran-Ricker population renewal process. Incorpora ting the spatial dimension stabilizes the dynamics of populations with mono gamy as the breeding system, but does not stabilize the population dynamics of polygamous species. Most notably, in populations coupled with dispersal , where the sexes differ in their dispersal ability there are rarely stable and equal sex ratios. Rather, a two-point cycle, four-point cycle and even tually complex behaviour of sex-ratio dynamics will emerge with increasing birth rates. Monogamy often leads to less noisy sex-ratio dynamics than pol ygamy. In our model, the sex-ratio dynamics of coupled populations differ f rom those of an isolated population system, where a stable 50:50 sex ratio is achievable with equal density-dependence costs for females and males. Wh en sexes match in their dispersal ability, the population dynamics and sex- ratio dynamics of coupled populations collapse to those of isolated populat ions.