Urbanization Drives Riparian Scavenging Avian Communities and Salmon Carcass Consumption

Linkages between terrestrial and aquatic ecosystems offer important routes through which energy and marine-derived nutrients flow. These linkages serve to maintain ecosystem structure and function, and are essential aspects of the maintenance of the health of terrestrial ecosystems. The carcasses of anadromous salmonids are dispersed to terrestrial areas after spawning, and the nutrients in these carcasses are particularly important in the Northern Hemisphere. Terrestrial vertebrates, such as mammals and birds, mediate much of the nutrient transport through salmon carcasses and are therefore essential for maintaining the link between terrestrial and aquatic areas at the watershed scale. Urbanization has a strong impact on riparian communities, which interact with both terrestrial and aquatic ecosystems. However, few studies have been conducted on the effects of urbanization on these linkages with terrestrial vertebrate consumers that disperse energy over large areas. We investigated the effects of urbanization on scavenging avian communities and their total salmon carcass consumption in a salmon spawning river. We selected 17 study sites along 12 salmon spawning rivers in western Hokkaido, northern Japan. We recorded the number of scavenging birds of each species, and calculated the total salmon consumption by birds. We also calculated salmon carcass biomass, proportion of the urban area, channel width, and the distance from the coast. Based on these factors, we clarified the relationships between the abundance and species richness of scavenging birds, carcass consumption, and factors, using piecewise structural equation models. We found that both scavenging bird abundance and their total salmon consumption had unimodal responses to urbanization intensity. Our results suggest that urbanization has wide-scale effects on the strength of the linkages between ecosystems, including at the watershed scale. Suburban or peri-urban areas may also be important for maintaining local ecosystem functions.