Contrasting Impacts of Dams on the Metacommunity Structure of Fish and Macroinvertebrate Assemblages

Impoundments are common features of many rivers that often impact abiotic conditions and the organisms that inhabit impacted reaches. As dams alter both local environmental conditions and the ability of organisms to move throughout the habitat, it is difficult to know which factors are driving changes in assemblage composition. To separate these effects, we employed a metacommunity approach to evaluate drivers of assemblage composition in fish and macroinvertebrate communities across two impoundments slated for removal in Danville, Illinois. Based on movement ability, we would expect dams to represent barriers to fish populations, whereas macroinvertebrates, with their motile adult phases, should be easily able to move across impoundments. Therefore, we would expect the assemblage structure of these two groups to be driven by different processes. We sampled habitat quality (measures) and both fish and macroinvertebrate communities in a replicated series of sites across two low‐head dams. As expected, the presence of dams resulted in reduced habitat quality as well as changes in fish and macroinvertebrate quality indices. However, fish and macroinvertebrate assemblages were driven by different metacommunity processes. Fish communities showed strong environmental filtering, responding to local environmental conditions, with no effect of distance between habitats. In marked contrast, macroinvertebrate communities were only related to physical distance between sites, with no indication of environmental filtering. These results suggest that when these dams are removed, fish assemblages should change with the removal of dam‐generated habitats. In contrast, dam removal may not alter macroinvertebrate composition, as it did not vary with habitat but appears driven by local dispersal. Received November 17, 2015; accepted July 17, 2016 Published online October 28, 2016