Sexual Reproduction and Stable Coexistence of Identical Competitors

Current competition theory predicts that species must be ecologically distinct in order to stably coexist in the same community. This prediction is based on the implicit assumption that the consequences of sexual reproduction can be ignored. If anything, it is generally assumed that sex will only add problems, such as failure of insemination and inbreeding depression to species that have become rare, thus hastening competitive exclusion. Here we suggest that sexual reproduction may also bring rare species advantages in terms of the rate of per capita population growth. The key to our argument is that species-specific density dependence in population growth can occur when sexual reproduction is explicitly considered. We show that density-dependent sex ratio, sexual conflict, and sexually transmitted diseases can all promote species coexistence without separate niches.