Modeling species fitness in competitive environments

•A model to simulate population fitness in competitive environments is proposed.•Consumption, accumulation and use of resources over a period of time are followed.•Resource competition lowers fitness even in case of intraspecific competition.•Multiple limiting resources can improve fitness.•Multiple limiting resources can enable interspecific coexistence.

Using a model of resource acquisition, we studied species competition in a case where resources limit population growth. Our model is based on calculations of the distribution of individuals of single or multiple species over consumed resources. Calculations show that, as equilibrium is reached in purely resource competitive systems, the density of resources is lowered to the lowest sustainable level, directly leading to low levels of fitness among species. In the case of competition between species with different lowest sustainable levels, the density of the more successful must be limited by some cause other than the resource in question for all species to coexist. We explore two cases of such coexistence.