Dams are ubiquitous features of rivers that disrupt upstream-downstream connectivity and shift patterns of community assembly. Conversely, dam removal – an increasingly common restoration strategy – is expected to promote the re-establishment of biotic communities and river ecosystem function. However, our understanding of the ecological impacts of dam removal is still in its early stages, particularly for lowhead, run-of-river dams. Here, we synthesize the responses of three river-riparian consumer communities – benthic macroinvertebrates, fish, and spiders - in two reaches previously impounded by a lowhead dam on the Olentangy River, Ohio, USA. Following dam removal, one reach was subjected to additional, active channel restoration activities including substantial channel reconfiguration, construction of floodplain wetlands, and riffle development. The other reach was allowed to passively adjust. We assigned macroinvertebrates and fish to consumer-specific functional-trait modalities (e.g., trophic guild, voltinism, attachment morphology, dispersal), and compared the relative abundance of these traits for 3 years following dam removal. These patterns were compared to a reference reach. We used stable isotope analysis to evaluate how fish foraging guilds and riparian spiders that rely on benthic invertebrates in larval or adult-stages interacted with post-dam removal changes to benthic and riparian habitat, and re-established upstream-downstream connectivity.
Macroinvertebrate communities were functionally similar between post-dam removal restoration strategies (active vs. passive), and the relative abundance macroinvertebrate traits changed comparably between the two reaches following dam removal. Relative abundance of sedentary taxa with armoring and erosional rheophily increased, whereas that of multivoltine, poorly armored, and depositional taxa decreased following dam removal. Winged-adult flying strength and female dispersal distance showed negligible changes following dam removal. Trophic guilds shifted from primarily collector-gatherers to broader representation by filterers, herbivores, and predators. Shifts in macroinvertebrate traits and their abundance are likely related to patterns of predation susceptibility (armoring, mobility), secondary production (voltinism), and detrital vs. algal energy transfers to consumers (collector-gatherers vs. herbivores). Marcoinvertebrate shifts were concurrent with coarsening of benthic substrate, and increased relative abundance of insectivorous fishes vs. large-bodied omnivores. While emergent insect communities were comparable between reaches, riparian spiders relied more on aquatic energy sources (e.g., periphyton) and showed short-term density declines, implying that dam removal and functional changes to benthic consumer communities may affect aquatic-to-terrestrial nutritional subsidies. Collectively, our findings show multiple consumer communities may respond differently to dam removal restoration, and emphasize the need for long-term dam-removal restoration studies that evaluate interactions among multiple consumer groups.