Influences of river regulation and environmental variables on upland bird assemblages in northern Sweden

Abstract

AbstractMost large rivers in Sweden are regulated to produce hydropower. This transformation from free‐flowing rivers to chains of elongate run‐of‐river impoundments has been shown to have consequences for aquatic, riparian and adjacent upland environments, and for the emergence patterns of aquatic insects that are important for terrestrial consumers. In this study, we investigated bird assemblages in upland‐forest environments along seven large rivers (three heavily impounded and four free flowing) in northern Sweden. Bird densities were assessed by point counts in the breeding and post‐breeding seasons. While we observed no significant differences in bird species richness between regulated and free‐flowing rivers, cumulative densities of two feeding groups of birds (those feeding on seeds and/or large insects and those feeding on small insects) were higher along free‐flowing rivers than along regulated rivers in the breeding season, consistent with known differences in aquatic‐insect emergence. Further, ordination analyses showed seasonal shifts in bird assemblage structure, and that these shifts differed between regulated and free‐flowing rivers and between the two feeding groups. However, the variables explaining the most variance (11–28 %) in bird assemblage structure were related to a gradient of agricultural‐to‐forest land use. River regulation contributed to the model in the post‐breeding season, but was of relatively low importance. Nevertheless, the observed contrasting seasonal shifts in upland‐forest bird assemblage structure between regulated and free‐flowing rivers suggest that regulation‐induced modifications of aquatic‐insect emergence and subsequent changes in prey availability to the birds are also important considerations.