Population growth is determined by the net recruitment rate of individuals to the population. Population growth in a given generation is a linear combination of its initial size, birth, death, immigration, and emigration rates. All four parameters are influenced by the ratio between the sexes in the population. Birth rate depends mainly on the number of females of reproduction age in the population. Here, the ratio between adult males and females affects the probability of a female to mate successfully. The number of females that are actually fertilized is the effective population size that determines the per capita birth rate. Survivorship rates may differ between males and females of all age classes but especially among the young. Theoretical models predict that offspring sex ratio should generally be close to equality after the period of parental care, but can be biased if the cost of rearing offspring differs for sons versus daughters or if mortality is gender-biased.
Population growth rates are also affected by immigration and emigration of individuals to and from other populations. If the probabilities of immigration or emigration are sex-specific, then a given population's growth rate will be influenced not only by its sex ratio, but by the sex ratios of populations from which it receives migrants and by the sex ratio of the immigrating and emigrating individuals. Sex-biased dispersal often occurs when there is competition between members of the same sex for a limited number of mates ('local mate competition'), or where there is competition for a limited resource that is necessary to rear offspring ('local resource competition'). In either case, the gender in excess often exhibits a higher likelihood of dispersal.
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