Sex and Parthenogenesis in Sparse Populations

Abstract

A model is developed to predict the minimum population density required (critical density) for successful sexual reproduction in animals. The model predicts that: (1) Hermaphroditic reproduction requires only half the critical density of bisexual reproduction; (2) a linear increase in mortality results in an exponential increase in critical density; and (3) increasing the time interval between clutches reduces critical density but makes a population more sensitive to external mortality. The probability of encountering a mate is dependent on the distance at which a male can detect a female. If predators can also detect females, there is an optimum conspicuousness for a female, which is dependent on the densities of males and of predators. A female should be very conspicuous only when both predators and males are rare. In an application of the model to zooplankton species, the smallest zooplankters are predicted to require relatively high densities for sexual reproduction. These animals reproduce with cycli...