Road effects on benthic macroinvertebrate assemblages in boreal headwater streams

Roads constitute a worldwide network of ecological barriers traversing countless streams and rivers. A large fraction of the land area lies in close proximity to roads. Ecological effects of roads likely extend well beyond the road network, suggesting wide ranging impacts on lotic ecosystems. Road impacts are multifaceted including fragmentation, changing hydrology, sedimentation and pollution. Yet, the ecological impacts are incompletely documented. We examined the effects of roads traversing lotic ecosystems on the structure and function of benthic macroinvertebrates in small temperate headwater streams. Ecological effects differed between reaches upstream and downstream of roads compared to upstream and downstream reaches in control streams. Total macroinvertebrate density and mayfly density were lower downstream of roads than upstream of roads, but they were similar at downstream and upstream reaches of control streams. Species density, but not species richness, tended to be lower downstream than upstream of roads, likely due to the lower macroinvertebrate densities at downstream sites. There were no comparable effects in control streams. Species composition and species abundances differed between road impacted streams and control streams likely because streams that were traversed by roads selected for a different set of species compared to control streams as indicated by checkerboard distribution of species in streams that were traversed by roads. Functional impacts included a greater prevalence of predators in control streams, and a higher proportion of grazers and shredders in streams that were traversed by roads. The study is inconclusive regarding the mechanisms mediating the ecological impact of roads. The ecological effects are likely caused by a combination of factors including fragmentation, pollution and hydrological change among others. Given the vast global road network, the quantitative significance of road effects driving freshwater biodiversity loss may be highly underestimated especially in smaller headwater streams comprising the major part of fluvial ecosystems.