Understanding the distribution of species across altitudinal and latitudinal gradients and how species coexist across space have been two of the most frequently posed questions in ecology. Since Whittaker (1956) and Connell (1961), numerous studies have extensively described community composition across environmental gradients. While this pioneering work greatly influenced the field of community ecology, a mechanistic understanding of species distributions across gradients is still lacking. Here we argue that, to mechanistically understand the processes important for species distributions and co-existence, it is necessary to analyze community structure on the basis of species-specific function. Further, as plasticity in function also likely influences the distribution and co-existence of species, we argue intra-specific gradients in species function across geographic ranges should also be quantified. In this study we quantify inter- and intra-specific variability in tropical tree species leaf function along an elevational gradient in the Luquillo Mountains, Puerto Rico. We test the hypothesis that the range and co-existence of species can be predicted on the basis of inter- and intra-specific variability in functional trait values.

Results/Conclusions

The results of the study show that the inter- and intra-specific variability in function is non-randomly distributed across the elevational gradient. Specifically, we find similar directional trends in leaf function within and across species, but the magnitude of this change in the function generally varies non-randomly along the gradient. This is ramified as directional changes in the functional range or volume within and across species along the gradient. The work provides one of the, if not the, first explicit analyses of variability in both inter- and intra-specific function across multiple tropical tree communities. The data are used to show that functional variability changes non-randomly within and across species along the environmental gradient. We argue that if we are to gain deeper insights into the mechanisms promoting species distributions and co-existence in diverse communities on the basis of functional traits, we must quantify both inter- and intra-species variability in functional traits.