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: hermaphroditic reproduction requires only half the critical density of bisexual reproduction; a linear increase in mortality results in an exponential increase in critical density; and 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 cyclic parthenogenesis, and the timing of sexuality may be a response to high population densities and high probabilities of encounter between mates. Comparing the predicted critical densities of zooplankton with numerous field studies, parthenogenetic zooplankton, cladocerans, and rotifers, experience large population fluctuations above and below the predicted critical densities of equivalent sexual populations, while sexually reproducing copepods are found above their predicted critical densities. Parthenogenesis may be adaptive for habitats with unpredictable cycles of disturbance and resource abundance. This is supported by a comparison of large and small lakes, which shows that a smaller proportion of zooplankton is parthenogenetic in great lakes.