Mutualism or cooperation among competitors promotes coexistence and competitive ability

In the Lotka-Volterra competition model and similar competition models, species can only coexist if between-species competition is weaker than within species. Prior modeling and field studies have shown that coexistence can be promoted by reducing the competitive ability between species through spatial or temporal niche differentiation. Colonization, disturbance, aggregations in patchy habitats, resource transport and supply rates, predation, sex ratio mediation have been suggested to promote species coexistence. The role of mutualism in promoting species coexistence has never been quantitatively studied. In this study, by starting from the traditional Lotka-Volterra competition model, a new general model with introduction of mutualism between two competitive species is proposed. We hypothesized that the interaction of one species to the other is flexible instead of always negative; the zero growth isoclines of the competing species are parabolic. Three conclusions are drawn from this modeling study. First, mutualism is a new way of promoting coexistences of two species. Second, mutualism often increases the carrying capacities of both species, and then promotes their competitive abilities. Third, the mutualism-competition model is also appropriate to describe the dynamics of cooperation and competition between individuals or groups within species, and mutualism between plants and animals. The behind mechanism lies in mutualism or cooperation reduced the severity of competition at low density. Inferior competitor, if cooperative to superior competitor, is also possible to survive. Since mutualism or cooperation is commonly seen among competitors or between prey and predators, its role in shaping social or community structure is worth to explore. (C) 2003 Elsevier Science B.V. All rights reserved.