Most animal populations are characterized by balanced sex ratios, but there
exist several exceptions in which the sex ratio at birth is skewed. An
interesting hypothesis proposed by Clark (1978) to explain male-biased sex
ratios is the local resource competition theory: the bias may be expected in
those species in which males disperse more than females, which are thus more
prone to local competition for resources. Here we discuss some of the ideas
underlying Clark's theory using a spatially explicit approach. In particular, we
focus on the role of spatiotemporal heterogeneity as a possible determinant of
biased sex ratios. We model spatially structured semelparous populations where
either Ricker density dependence or environmental stochasticity can generate
irregular spatiotemporal patterns. The proposed discrete-time model describes both
genetic and complex population dynamics assuming that (1) sex ratio is
genetically determined, (2) only young males can disperse, and (3) individuals
locally compete for resources. The analysis of the model shows that no skewed
sex ratios can arise in homogeneous habitats. Temporal asynchronized
fluctuations between two distinct patches coupled with dispersal of young males
is the minimum requirement for obtaining skewed sex ratios of demographic nature
in local adult populations. However, the establishment of a male-biased sex
ratio at birth in the long run is possible if dispersal is genetically
determined and there is genetic linkage between sex ratio determination and
dispersal.
