Food web interactions of waterbirds and fish in eutrophic wetlands of Alberta's Aspen Parkland

Studies on competition between fish and waterbirds have typically involved large-bodied fish and oligotrophic aquatic ecosystems. In eutrophic systems, small-bodied fish such as fathead minnow (Pimephales promelas ) may also compete with waterbirds for invertebrate prey, thereby reducing the viability of waterbird breeding habitats like Aspen Parkland. Such wetlands are often disturbed by drought and winter hypoxia, making distributions of fish in pothole wetlands highly variable. Relationships between waterbirds, fish and invertebrates in the Aspen Parkland of North America are largely unknown, despite the importance of these regions for breeding waterbirds. My research showed that abiotic factors such as nutrient status of wetlands were important determinants of waterbird and invertebrate assemblage compositions in Aspen Parkland, and that invertebrate assemblages differed among wetlands with and without fish. An observational study showed that low densities of fathead minnow and brook stickleback (Culaea inconstans), reduced numbers but not biomasses of some invertebrate prey of two waterbird species, Red-necked Grebe and Blue-winged Teal. Unlike previous studies, birds' foraging activities were unaffected by fish, but Blue-winged Teal were generally less abundant on wetlands with fish than fishless wetlands. Experimental addition of fish, followed by natural winterkill of added fish, induced increases in Blue-winged Teal foraging activities that did not reverse when fish were extirpated. Herbivorous invertebrates (gastropods) were also reduced in the presence of added fish, but increased in biomass when fish were extirpated. Fish had no effect on Red-necked Grebe foraging activities or abundance, but stable isotope analysis revealed that grebes obtained resources for egg production at a higher trophic level in the presence of these fish than in their absence. Conversely, American Coot, being more herbivorous than Red-necked Grebe, generally obtained egg resources by foraging at a lower trophic level in the presence of fish. Longer-term studies that monitor Aspen Parkland waterbird-fish-invertebrate relationships during wet years would allow managers to make decisions based on knowing the point at which fish become an important biotic factor in determining waterbird habitat quality. Land-use activities that alter the incidence of winterkill of fish, such as wetland consolidation, should be discouraged.