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Keywords:

  • Annual plants;
  • coexistence;
  • competition–colonisation trade-off;
  • functional diversity;
  • manipulation experiment;
  • neutral theory;
  • null models;
  • seed size;
  • species diversity

Abstract

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.