The Ecological and Evolutionary Drivers of Female-Biased Sex Ratios: Two-Sex Models of Perennial Seagrasses

Among sexually reproducing species, differences between the sexes within species are ubiquitous. Despite the clear effect of sex differences on sex ratios and population growth rates, demographic models rarely consider both sexes explicitly. Here I explore the causes of extreme female-biased sex ratios in two marine angio-sperms (Phyllospadix spp.). Using demographic data, I develop two-sex matrix projection models to assess the magnitude of demographic differences necessary to generate observed sex ratios and the consequences of sex differences for population growth rates. I demonstrate that small sex differences in survival can generate biased sex ratios, but the importance of sexual reproduction differs markedly between species. Even in the absence of a direct trade-off between sexual and asexual reproduction, the presence of two reproductive modes affects both the importance of sex and the sex-ratio bias. Using sensitivity analyses, I quantify the contribution of shared and sex-specific vital rates and show that until males become rare, the sensitivity of sex-specific vital rates is small relative to that of shared vital rates. I demonstrate that placing sex differences in the context of a demographic model that includes biologically motivated life-history trade-offs can explain the maintenance of sex-specific life histories and the persistence of skewed sex ratios.