Differential reproductive expenditure between the sexes is one of the primary drivers of biased flowering sex ratios in plant populations. Greater reproductive investment by females is often associated with male-biased flowering sex ratios because of the earlier onset and more frequent flowering of males, and because of greater female mortality. These processes can drive sex-ratio variation among species and lead to associations between life-history traits related to sex-specific differences in reproductive costs, e.g. fleshy fruits and pollen and seed dispersal mechanisms. Differential costs of reproduction between the sexes may also drive among-population sex-ratio variation for species occupying heterogeneous environments. For example, if females incur greater reproductive costs they may delay flowering or flower less frequently, which could lead to a positive association between male bias and the proportion of non-reproductive individuals. Similarly, higher female reproductive expenditure leading to greater susceptibility to stress should also result in more male-biased sex ratios when resources are limited, or with increasing environmental stress along altitudinal, latitudinal or moisture gradients). Consequently, biased sex ratios among populations of flowering plants are likely to reflect complex interactions between sex-based differences in the costs of reproduction, life history and ecological context.
A new paper in Annals of Botany considers within-species variation in sex ratios in relation to ecological correlates, demography and life history.
Ecological context and metapopulation dynamics affect sex-ratio variation among dioecious plant populations. (2013) Annals of Botany 111 (5): 917-923. doi: 10.1093/aob/mct040
Populations of dioecious flowering plants commonly exhibit heterogeneity in sex ratios and deviations from the equilibrium expectation of equal numbers of females and males. Yet the role of ecological and demographic factors in contributing towards biased sex ratios is currently not well understood. Species-level studies from the literature were analysed to investigate ecological correlates of among-population sex-ratio variation and metapopulation models and empirical data were used to explore the influence of demography and non-equilibrium conditions on flowering sex ratios. The survey revealed significant among-population heterogeneity in sex ratios and this was related to the degree of sampling effort. For some species, sex-ratio bias was associated with the proportion of non-reproductive individuals, with greater male bias in populations with a lower proportion of individuals that were flowering. Male-biased ratios were also found at higher altitudes and latitudes, and in more xeric sites. Simulations and empirical data indicated that clonal species exhibited greater heterogeneity in sex ratios than non-clonal species as a result of their slower approach to equilibrium. The simulations also indicated the importance of interactions between reproductive mode and founder effects, with greater departures from equilibrium in clonal populations with fewer founding individuals. The results indicate that sex-based differences in costs of reproduction and non-equilibrium conditions can each play important roles in affecting flowering sex ratios in populations of dioecious plants.