The R* rule (also called the resource-ratio hypothesis) is a hypothesis in community ecology that attempts to predict which species will become dominant as the result of competition for resources.
[1] The hypothesis was formulated by American ecologist David Tilman.
A large number of experimental studies have attempted to verify the predictions of the R* rule.
[3] There are fewer tests of the R* rule in communities of larger organisms, in part because of the difficulty of creating a situation in which only a single resource is limiting.
[3][4] However, some studies have used the R* rule with multiple resources to predict which groups of plants will be able to coexist.
We will assume that each species competes for a single resource, and ignore the effects of interference or apparent competition.
[1] This method has been extended to analyze more complex models, such as species with a Type II functional response.
Under many additional circumstances, the above result still holds: the species who can survive at the lowest resource levels will be the competitive dominant.
[3] Understanding the differences between the R* theory and its major alternative the CSR triangle theory is a major goal in community ecology for many years.
[6][7] Unlike the R* theory, the CSR theory predicts that competitive ability is determined by relative growth rate and other size related traits.
The R* theory assumes that competition is size symmetric (i.e. resource exploitation is proportional to individual biomass), the CSR theory assumes that competition is size-asymmetric (i.e. large individuals exploit disproportional higher amounts of resources compared with smaller individuals).