Mating systems and sex ratios can have substantial effects on population dynamics. This may be of special interest in light of sex-selective harvesting, which can create large and rapid changes in the sex ratio of exploited species. We investigate these effects using a stochastic model which allows for a continuous scale of mating systems, from strict monogamy to extreme polygyny, as well as a continuous range of sex ratios. We assume females to be monogamous, and consider populations with non-overlapping generations. In this model, the number of mated females is the measure that best represents the population dynamics. Other demographic measures, like the total number of males and of females in the population, fluctuate around this measure, in much the same way that population size fluctuates around the total reproductive value in age-structured models. We therefore analyze population dynamics through the number of mated females in a population.

Results/Conclusions

We have found major effects on population dynamics of altering mating systems (degree of polygyny) and sex ratios. There are strong interactions between these two factors, making the combination of mating system and sex ratio the critical measure. In systems where the expected number of females is close to the expected number of male matings, stochasticity leads to a decrease in the growth rate of small populations, compared to the deterministic growth rate. The decrease is more pronounced the smaller the population is. Thus, we observe a type of Allee effect driven by mating system and sex ratio. Also, when the sex ratio becomes too female biased, the demographic variance (including covariance terms) increases dramatically. This may have ramifications for the extinction probability of populations in which the sex ratio is altered. The amount of sex ratio skew that can be tolerated without experiencing the leap in demographic variance depends on the mating system. Mechanisms behind these finds will be discussed, as well as their implications for conservation.