Approximately 25 % of the trait variation in plant communities worldwide occurs within species. This is a striking pattern with largely unexplored but likely important consequences for species coexistence. Compelling intuitive arguments suggest that intraspecific variation should promote species coexistence, but these arguments have rarely been evaluated with quantitative theory, or with empirical work that links intraspecific variation to the demographic determinants of coexistence. Here we first incorporate intraspecific variation into common models of competition to develop default theoretical predictions for the effects of this variation on coexistence. We then quantify how empirically measured intraspecific variation in competing plant species affects coexistence outcomes in the field. Finally, we explore the consequences of the central unknown in this area of research – the functional form of the relationships between functional traits and the demographic determinants of species coexistence.
Results/Conclusions
We identify two major pathways by which intraspecific variation affects coexistence: 1) because the dynamics of competing populations are often nonlinearly dependent on the demographic and competitive rates of individuals, variation changes dynamics via nonlinear averaging; 2) because demographic and competitive rates are themselves nonlinearly dependent on underlying traits, intraspecific variation in functional traits changes the mean strength of competition also via nonlinear averaging. Our work demonstrates that both mechanisms are just as likely to prevent species coexistence as to promote it. More generally, because of the pervasiveness of nonlinear relationships in nature, our work suggests that coexistence is often unlikely to be predictable based on mean trait values. If the effects of intraspecific trait variation are to be accurately accounted for, field assessments of functional traits need first to describe the potentially nonlinear relationship between those traits and the demographic determinants of species coexistence.