Consequences of Roads and Traffic-Related Stressors on Freshwater Biodiversity

Road development has expanded globally, with growing recognition of the impacts on freshwater systems. We synthesized findings from 426 publications to assess (1) where and for which freshwater taxa road effects have been studied; (2) the types of roads and stressors examined; (3) the study designs and biological responses measured; and (4) the reported road effects on freshwater taxon. Most studies were conducted in North America (62.1%), with the overall evidence base focusing on amphibians (36.1%), fish (14.8%), and reptiles (10.9%) in ponds/wetlands, laboratory settings, and streams. Research primarily assessed the impacts of road density or proximity (20%), traffic (18.4%), and salt/de-icers (17.9%) stressors, mostly using impact-only (51.0%) and control-impact (46.5%) study designs, restricting causal inference. Demographic, diversity, and mortality responses were the most frequently measured outcomes. Road effects were predominantly negative, underscoring widespread road impacts on freshwater taxon. Major knowledge gaps included understudied taxa (avifauna, mammals, crustaceans, molluscs, insects, plants), life-stages (eggs, juveniles), and several stressors. Future research should focus on addressing the knowledge gaps and apply comparative approaches to improve understanding and management.