High compositional dissimilarity among small communities is decoupled from environmental variation

Communities composed of small populations are predicted to be strongly influenced by stochastic demographic events and, thus, less affected by environmental selection than those composed of large populations. However, this prediction has only been tested with computer simulations, simplified controlled experiments, and limited observational data. Here, using multiple datasets on fish abundance in 541 streams we tested 1) if communities composed of small populations are more spatially variable and 2) less related to the environment variation than communities composed of large populations. We used process-based simulations to identify beta-diversity metrics and community-environment measures that were appropriate to investigate the role of assembly processes along a gradient of community size. We show that variation in species composition among small communities is higher than among large communities and that the strength of community-environment relationships is weaker in small communities. Our results indicate that community size affects the strength of ecological drift and environmental selection in metacommunities. We thus suggest that further declines in the size of populations and ecosystems can make spatial variation in biodiversity more unpredictable.