Species size and distribution jointly and differentially determine diatom densities in U.S. streams.

Among the most studied relationships in ecology are those of population density with (1) body size and (2) species distribution. The first relationship, in conjunction with metabolic rate, determines the energy flows through species communities, whereas the second relationship shows how local communities are influenced by the species history of dispersal and establishment. Traditionally, these two relationships have been examined separately. Here, I explored how diatom density was affected by cell size (biovolume) and species distribution in benthic and planktonic stream habitats all the way from individual localities and hydrologic systems (regions) to the entire United States. At all scales, density was predominantly a negative function of biovolume and a positive function of distribution. Biovolume was more strongly related to density in the benthos than in the phytoplankton. Partial regressions revealed that biovolume, by itself, explained a substantially higher percentage of the variance in density at local than at regional and continental scales. Conversely, species distribution was a much more important descriptor of density at larger scales and a slightly better predictor than biovolume at local scales. At large scales density was explained primarily by distribution and, to a lesser extent and only in the benthos, by the covariance of distribution and biovolume, whereas biovolume was a marginal predictor in all habitats. This discovery suggests that the strong relationships between density and body size, reported for populations ranging from unicellular algae to mammals, may be less direct than previously thought but mediated by large-scale species distributions.