A resource ratio theory of cooperation.

Resource ratio theory predicts that two species may coexist in the presence of two limiting nutrients provided that each species is limited by the resource it is least able to deplete. We modify this classical competition model to allow interspecific cooperation through trading. We show that resource trade expands the realm of stable coexistence, and that optimal trading partners competitively invade and exclude any other trading or non-trading strategy. We show that natural selection favours evolution towards establishment of a trading relationship so long as partners can establish long-term associations even though cooperation may result in a decrease in abundance of one species. This theory substantively expands traditional applications of resource competition models and suggests additional empirical experimentation.