Project data

Increases in species richness with habitat area (species-area relationship, or SAR) and increases in ecosystem function with species richness (biodiversity-ecosystem functioning, or BEF) are among the most widely studied ecological patterns. Incorporating functional trait analysis into assemblage datasets may help clarify interpretations of SAR and BEF relationships in natural ecological systems. For example, life history theory can be used to make predictions about what species are most important in generating ecosystem function given a certain set of environmental conditions. We used a quantitative assemblage data set for freshwater mussels at 9 sites in western Alabama, USA to test for SAR and BEF relationships. At each site we calculated species richness, mussel assemblage density, and two fundamental metrics of ecosystem function: biomass and secondary production. We also tested whether the proportional biomass and production contributions from species belonging to each of three life history strategies—<i>opportunistic strategists</i> adapted to unstable or frequently disturbed habitats, <i>periodic strategists</i> adapted to habitats subject to predictable large-scale disturbances, and <i>equilibrium strategists</i> adapted to stable habitats—varied longitudinally with stream drainage area, a proxy for habitat area. Species richness increased with stream size (SAR), and both biomass and production increased with species richness (BEF) and mussel density. There were few longitudinal changes in the proportional contributions of the different life history strategy classifications that we used, but the invasive clam <i>Corbicula fluminea</i> contributed proportionally more biomass and production at sites that had smaller drainage areas. This study provides further evidence for a clear longitudinal SAR in stream-dwelling taxa. It also suggests apparent BEF relationships for biomass and secondary production in natural assemblages but underscores the importance of assemblage density in BEF studies that use observational field data. Variation in proportional biomass and production contributions by different life history strategies was likely limited by the size of the environmental gradient (stream size) in our study, as contributions were uniformly high for species with life history traits better adapted to stable and productive habitats such as mid-sized to large rivers with low or predictable hydrologic disturbance frequencies. This highlights the need to understand how organisms’ functional traits govern their relationships to the environment at different scales.