The only lasting truth is change: multiple dimensions of biodiversity show historical legacy effects in community assembly processes of freshwater fish

How communities are structured is a fundamental ecological question. Community structure, while constrained by the regional species pool, may be altered by changes in climate and other environmental stressors. Changes in patterns of functional and phylogenetic dispersion over time can illuminate the temporal dynamics of the processes structuring communities. We quantified temporal changes in taxonomic, functional and phylogenetic diversity of stream fish assemblages in the southern plains of the U.S. across four decades to assess how climatic change has influenced community patterns. We also explored how the use of three different functional trait categories (life history, environmental tolerance and trophic level) and all traits combined affected the response of functional diversity to environmental drivers. We found, for all diversity indices, that assemblages with low historical richness had high contemporary diversity, while assemblages with high historical richness had low contemporary diversity. Functional richness based on life history traits, trophic traits, and all traits combined decreased in diversity over time, while functional richness based on environmental tolerance traits showed the opposite pattern. Phylogenetic dispersion of both over- and under-dispersed communities shifted toward randomness. Changes in fish diversity patterns were influenced by changes in temperature over time, though impacts were metric dependent. Overall, we found that while community structure has changed, specific changes were more strongly predicted by the historical richness of the community than by regional climate change.