Effects of community richness and competitive asymmetry on protozoa evolution in Sarracenia purpurea leaves

Predicting evolution in natural systems will require understanding how selection operates in multispecies communities. We predicted that the amount that traits evolve in multispecies mixtures would be less than the amount that would be predicted from the additive contributions of the pair-wise interactions and that subordinate species will be more likely to evolve in competitive systems than dominant species. We conducted an experimental test of these predictions using a guild of protozoans found in the water-filled leaves of the pitcher plant Sarracenia purpurea. The response to selection did not significantly change as we increased richness from monocultures to two- and four-species mixtures. In accordance with our second prediction, subordinate species demonstrated greater growth in competition after selection than before, while dominant species generally showed no response to selection. Monod-type experiments to determine minimum resource levels found that the dominant species had much higher resource requirements than the subordinate species and that the minimum resource requirements evolved to be higher in the subordinate species. Importantly, these results suggest that subordinate species evolve to become more similar to dominant species, which may involve resource-use convergence. Our findings and other recent works suggest that community diversity can affect evolution in surprising ways that warrant further investigation.