From local to cumulative: How barriers and reservoirs affect river macroinvertebrate traits

The cumulative effects of barriers and reservoirs (B&R) on ecological processes remain understudied. Invertebrate traits provide a mechanistic understanding of responses to both natural and human-induced stressors. I analysed trait responses to a wide range of natural and anthropogenic variables across 474 river sites in France, encompassing a wide variety of watershed size and environmental contexts. The cumulative impact of reservoirs, particularly the influence of upstream reservoir density, was comparable in intensity and pattern to other anthropogenic pressures. These effects were evident on traits related to water quality, substrate and current affinities, and, perhaps more unexpectedly, invertebrate dispersal modes. Barriers, which in my dataset were mostly small culverts, strongly impacted only dispersal modes, while they exhibited positive effects on other traits, especially those associated with cold-water taxa. At the local scale, the influence of B&R was very weak. Therefore, although reservoir removal is a necessary step, it represents a long-term restoration strategy. To ensure river restoration is both effective and timely, efforts must be more ambitious, targeting other sources of water degradation, to enhance the ecological integrity of both rivers and reservoirs.