Functional trade-offs increase species diversity in experimental plant communities

Functional trade-offs have long been recognised as important mechanisms of species coexistence, but direct experimental evidence for such mechanisms is extremely rare. Here, we test the effect of one classical trade-off a negative correlation between seed size and seed number by establishing microcosm plant communities with positive, negative and no correlation between seed size and seed number and analysing the effect of the seed size/number correlation on species richness. Consistent with theory, a negative correlation between seed size and seed number led to a higher number of species in the communities and a corresponding wider range of seed size (a measure of functional richness) by promoting coexistence of large- and small-seeded species. Our study provides the first direct evidence that a seed size/number trade-off may contribute to species coexistence, and at a wider context, demonstrates the potential role of functional trade-offs in maintaining species diversity.