Context is key: divergent ecological impacts of swan versus goose herbivory in freshwater wetlands

While populations of large herbivorous waterbirds such as geese and swans have increased dramatically in recent decades, several other waterbird species have shown declining trends. These opposing patterns raise concerns about whether the increase of large waterfowl may be contributing to declines in other species, for example by habitat degradation through their grazing activities. To evaluate grazing effects of large herbivores on wetland ecosystems, we used results from two field experiments conducted during the breeding season. In both studies we used paired fenced exclosures and open control plots to assess direct grazing effects on vegetation and potential indirect effects on aquatic macroinvertebrates. The first study was conducted in ten eutrophic agricultural wetlands in southern Sweden used by geese. The second was carried out in ten mesotrophic boreal wetlands in southern Finland, focusing on the whooper swan (Cygnus cygnus). In terms of vegetation, goose exclusion led to a significant increase in aboveground biomass only, whereas swan exclusion resulted in significantly greater biomass and vegetation cover. Furthermore, significant interaction effects suggested that negative grazing effects on vegetation cover and richness intensified with increasing swan density, a pattern not observed in the goose study. Aquatic macroinvertebrate taxon richness and diversity were unaffected by herbivore exclusion in both cases. However, total abundance of aquatic macroinvertebrates showed contrasting trends; it was significantly lower in exclosures in the goose study, while swan exclusion led to increased abundance at sites with high swan densities. Our results suggest that moderate grazing pressure in productive wetlands may enhance macroinvertebrate abundance by promoting habitat heterogeneity, whereas higher grazing pressure in less productive, boreal systems may hinder vegetation recovery and negatively affect macroinvertebrate communities. Overall, these findings challenge the assumption that large waterbird herbivory consistently degrades wetland ecosystems. Rather, effects appear context-dependent, shaped by grazer density and environmental conditions such as nutrient state. Future studies should aim to identify thresholds at which herbivory shifts from being potentially beneficial to detrimental, thereby offering valuable insights for wetland and wildlife management across diverse landscapes.