Connecting the trophic dots: responses of an aquatic bird species to variable abundance of macroinvertebrates in northern boreal wetlands

Abstract

To evaluate variation in abundance of boreal wetland macroinvertebrates and test for effects of this variation on the diet and habitat use of a bird species that consumes aquatic invertebrates (lesser scaup, Aythya affinis), we collected macroinvertebrates and birds and conducted bird surveys (June–August) at two wetland complexes in northwestern Canada and assessed diet composition using an isotopic approach. At both study areas, for macroinvertebrate taxa reported to be key prey items for scaup, biomass varied intra-seasonally and annually, but patterns differed among taxa and between areas. Macroinvertebrate biomass varied strongly across wetlands within study areas, and isotopic mixing models indicated that local heterogeneity in macroinvertebrate biomass was reflected in duckling diets, which also varied across wetlands, indicating a generalist foraging strategy for this species. Wetland habitats used by brood-rearing female scaup had greater amphipod and gastropod biomasses. Our results show that boreal wetland macroinvertebrate abundances vary considerably across coarse and fine spatial scales. Female scaup and their ducklings appear well adapted to exploit this dynamic food resource, but overall productivity of scaup may depend on the abundances of certain taxa, suggesting that conservation efforts should focus on maintaining abundant populations of key wetland invertebrates.