Trophic interactions in boreal wetlands: whooper swan Cygnus cygnus grazing and its effects on vegetation and macroinvertebrates

Abstract

The rapid population growth of whooper swan Cygnus cygnus in Fennoscandia raises concerns about their ecological impact on freshwater wetlands. As large herbivores, swans can potentially alter wetland vegetation through intensive grazing, which in turn suggestively affects other trophic levels, including aquatic macroinvertebrates. However, studies examining both direct and indirect effects across trophic levels remain limited. To address this gap, we conducted a field experiment across ten boreal wetlands in Finland during the breeding season (April–August) of 2023, using fenced exclosures to prevent swan grazing, paired with open control plots. We expected that swan exclusion would result in higher vegetation aboveground biomass, cover, height, and species richness, which in turn would support greater abundance and taxon richness of aquatic macroinvertebrates. Our results indicate that swan exclusion led to increases of all vegetation variables, particularly aboveground vegetation biomass. Among vegetation types, the strongest grazing effects were observed for water horsetail Equisetum fluviatile and graminoid species. However, there was weak support for indirect effects on macroinvertebrate abundance and taxon richness. These results suggest that swan herbivory may shape wetland ecosystems primarily by altering vegetation. Further research is needed to particularly assess the long-term consequences of swan herbivory on wetland ecosystem functioning and community composition.