Background/Question/Methods: Modern coexistence theory aims at understanding why species coexist in natural communities, yet most applications only focus on one trophic level and do not include explicitly model resource or predator dynamics. We simulated a multispecies phytoplankton community and included explicit competition for nitrogen, phosphorus and light, the three main resources for phytoplankton growth. The consumption and growth rates were drawn from empirically measured phytoplankton species to obtain realistic parameter values. We investigated which resources are most important for coexistence by simulating communities competing for all three resources, or competing for only two resources. Similarly, to understand the effect of community parameters on persistence we simulated communities in which all species have identical resource uptake traits, or identical half-saturation constants. Additionally we computed niche and fitness differences for all multispecies communities to understand which resources and community parameters are most important for coexistence.
Results/Conclusions: According to theory at most three species can coexist on three limiting resources at steady state. We found that in most communities the number of coexisting species was lower than that, 1.5 species on average. Light was the least important limiting resource for coexistence. Eliminating competition for light had little effect on the number of coexisting species, and niche differences were not significantly reduced. Competition for nitrogen and phosphorus were equally important: when species only competed for light and one nutrient 1.2 species coexisted on average. The most important parameter for coexistence is the resource uptake trait. In communities where all species have identical resource uptake traits only 1.2 species coexisted on average, as no niche differentiation was possible via the nutrient resources. The outcome is then the same as for communities competing for only one nutrient. The other parameters (half saturation constant and intrinsic growth rate) had no effect on persistence.
