Species composition of local riffle beetle (Coleoptera: Elmidae) assemblages in small coastal streams of the Gulf of Mexico: the influences of local and regional factors

Abstract

Using Partial Mantel tests to isolate distance and environmental effects from each other, we examined species composition of riffle beetle assemblages (Coleoptera: Elmidae) of 22 local streams from the Gulf of Mexico, as a function of dispersal and environmental constraints. In addition, we examined how different stream topologies (Euclidean, Network) influenced dispersal constraints on species composition. In general, when the influences of local and regional influences were isolation from each other (Partial Mantel test), community dissimilarity increased between sites as differences in both stream conditions and distance between sites increased. However, these trends varied over the scale of study (within and across water basins) and stream topology. For example, at the scale of the watershed, overland dispersal did not appear to be a limiting factor in the formation of local elmid assemblages. The relationship between local species composition and distance was not evident in a distance–decay regression, nor was an environmental similarity versus community similarity regression significant. At least within the limits of our study, it was clear that if local and regional influences were not isolation from each other (Partial Mantel test), their associations with local assemblages were not evident. We also argue the utility of restricting study to a single family of insects when attempting to clarify the causal mechanisms responsible for local species assemblages.