Community assembly refers to relatively short term ecological processes that affect how patterns of biodiversity change in response to mortality events and habitat alteration over contemporary time scales. Deterministic (niche-based) hypotheses of community assembly posit that environmental constraints select functionally similar species to co-occur at a site, yet some intra-site heterogeneity is necessary to create the niche separation needed to allow species to coexist. Stochastic (dispersal-based) models of community assembly have demonstrated that dispersal limitation in unpredictable landscapes can diminish the accuracy of niche-based predictions, and still produce non-random aggregations of organisms. The use of patterns in among-site variation in community composition (beta-diversity) to distinguish between niche-based and dispersal-based hypotheses has led to controversy about whether investigators can identify the influence of different community assembly processes from diversity patterns, which hypotheses are the most parsimonious, and whether investigators are even focusing on ecologically relevant measures of diversity.

Results/Conclusions  

Here we show beta-diversity patterns in community composition, when supplemented with functional trait information, can be used to diagnose assembly processes. Using simulated data, we show how incorporating both taxonomic and functional-trait composition into beta-diversity provides a measure of how communities respond to environmental variation at different spatial scales. We then use these synthesized patterns as a diagnostic template to show that an in situ metacommunity of benthic macroinvertebrates is likely sorted by habitat and resource availability at regional and local scales, respectively. The framework developed here provides a way for ecologists to create testable models based on specific community assembly processes by building from the most parsimonious starting point (a trait-neutral model) one niche dimension at a time. We intend this analysis as a template for investigators to create focused hypotheses from the growing archives of publicly available data (e.g., USEPA Environmental Monitoring and Assessment Program, USGS National Water-Quality Assessment).