Species diversity often changes along environmental productivity gradients. While ecological processes have been frequently invoked to explain the relationship between productivity and diversity,  it remains largely unknown whether evolutionary processes operating on relatively long time scales may produce similar patterns. Here we study evolutionary branching of phytoplankton and zooplankton along productivity gradients using a simple nutrient-phytoplankton-zooplankton (NPZ) model that incorporates size-dependent metabolic rates.

Results/Conclusions

This model makes the following predictions. 1) In the absence of zooplankton, evolutionary branching does not occur for phytoplankton and a single phytoplankton species dominates at all productivity levels. 2) In the presence of zooplankton, evolutionary branching of phytoplankton may occur, followed by evolutionary branching of zooplankton, potentially resulting in multiple coexisting phytoplankton and zooplankton species in spatially unstructured systems. This indicates the importance of species interactions for sympatric speciation. 3) Evolutionary branching only occurs after environmental productivity reaches a minimum threshold; above this threshold, the number of phytoplankton and zooplankton species increases with productivity. These results provide a simple evolutionary explanation for the positive relationship between environmental productivity and species diversity in planktonic systems.