Birth functions in stage structured two-sex models

The most commonly used birth function in two-sex demographic models is the harmonic mean birth function. This function treats all individuals of one sex as identical, i.e., stage specific fecundity is not taken into account. In the analysis presented here, the harmonic mean birth function is developed to incorporate size and sex specific fecundities. This structured birth function is compared with the harmonic mean birth function using both a hypothetical population and data for populations of the limpet Patella vulgata. A general expression to calculate the threshold value where the unstructured and the structured birth functions coincide is also provided. Using the structured birth function resulted in changes in population dynamics, growth rate, proportion of males and reproductive output. In conclusion, the choice of birth function is important due to its effects on the deterministic population characteristics, which in turn may have consequences for the managements of endangered or vulnerable species.