Ecoregion boundaries affect distance decay of similarity in aquatic communities

The decay in community similarity with distance (distance decay) has been reported for many taxa in a variety of geographic settings.  However, the majority of these studies have been at large geographic scales, where identification of underlying mechanisms is difficult because these large scales tend to span more than one ecosystem.  Distance decay is expected to be influenced by regional changes, and only by conducting studies at these intermediate geographic scales can such signals be identified.  Here, we examine the distance decay pattern in both aquatic benthic macroinvertebrate and fish assemblages at an intermediate scale (i.e., 2260 km²) using both stream network distance and linear geographic distance within the Patuxent River basin, Maryland, USA.  The area of study contains two distinct ecological regions divided by a geological fall line.  Significant decay patterns were observed both within and among regions; however, trends differed between assemblages and among scales suggesting a sub-region/assemblage effect on distance decay.  We suggest that examining species patterns at intermediate scales that include environmental transitional zones will further our understanding of the mechanisms behind species patterns.