The Coexistence of Plankton Species with Various Nutrient Conditions: A Lattice Simulation Model

In aquatic ecosystems, species diversity is known to be higher in poor nutrient conditions. The enrichment of nutrition often induces the loss of biodiversity. This phenomenon is called the paradox of enrichment, since higher nutrient levels can support more species. Furthermore, the species diversity is usually high in most natural communities of phytoplankton. However, the niches of planktonic algae seem almost identical in apparently homogeneous, aquatic environments. Therefore, the high species diversity of phytoplankton is incomprehensible and called the paradox of plankton. Mathematical studies show that local coexistence of competitive species is rare. In a competitive community, the most superior species eliminates all the inferior species in the long run. Experimental results using chemostats also support this theoretical prediction. Thus we have no sound explanation for the local coexistence of many planktonic species in low nutrient conditions. Here we build a lattice model of ten planktonic species. All ten species are under competition for space in a relatively large lattice space. We report a few cases of simulation run. Simulation shows that, in an ecological time scale, coexistence of many species is observed when all species have low identical birth rates. We also show that, when the average birth rates are high, the most superior species exclude all the inferior species immediately. Our results suggest that competition for space does not function among species, when the densities of species are extremely low. The results of current simulation experiments may be related to the paradox of enrichment as well as that of plankton.