Local Mate Competition Promotes Coexistence of Similar Competitors

Limited dispersal of individuals (or gametes if they are the dispersive units) imposes constraints on mating patterns in most natural populations. This aspect of neighbourhood interaction, however, has not received the attention that it fully deserves in the field of competition and coexistence. Hamiltonʹs local mate competition theory (1967) predicts that as a species in a spatially structured habitat tends to become rare, a female-biased sex ratio would result. Such an effect of spatial structure means that a species at low density may gain an advantage in per capita population growth, facilitating coexistence of similar competitors. Thus, while other mechanisms of competitive coexistence that exist require some degree of difference between species, this “local mate competition” hypothesis of coexistence can permit identical species to stably coexist, invalidating the competitive exclusion principle.