Goose and swan populations have increased concurrently with environmental degradation of wetlands, such as eutrophication, vegetation losses, and decrease in biodiversity. An important question is whether geese and swans contribute to such changes or if they instead benefit from them. We collected data from 37 wetlands in southern Sweden April - July 2021 to study relationships between geese, swans and other waterbird guilds, macrophytes, invertebrates, as well as physical and water chemistry variables. Neither goose nor swan abundance was negatively correlated with other trophic levels (abundance, richness, or cover). On the contrary, goose or swan abundances were positively related to abundances of surface and benthic feeding waterbirds, cover of specific macrophytes, and to invertebrate richness and abundance. Moreover, invertebrates (number of taxa or abundance) were positively associated with abundance of several waterbird guilds and total phosphorous with surface feeders, whereas water colour was positively (surface feeders) or negatively (benthic feeders) related. We conclude that waterbirds are more abundant in productive wetlands and that geese and swans do not show clear deleterious effects on other trophic levels included in this study. However, patterns may be masked at the species level, which should be addressed in further studies, complemented with experimental studies of grazing impact.

In recent decades, interest has grown in how increasing populations of herbivorous geese and swans (Anseriformes: Anatidae: Anserinae) affect macrophyte communities in wetlands, especially because many water-bodies are simultaneously subjected to stressors like eutrophication and biodiversity declines. Here, we review the literature on methods applied in grazing experiments that have been conducted in aquatic ecosystems. We also investigate and how different macrophyte characteristics may respond to waterfowl herbivory. Results indicate that both research methodology and responses of macrophytes differ widely among studies. While most experimental studies on grazing pressure employ a ‘paired plot design’ with exclosures and open control plots, the structure, size, and placement of plots vary among studies. Commonly sampled macrophyte variables are biomass (of either above- or below-ground plant parts), density, height, plant cover, and community composition. The literature provides support that geese and swans significantly affect several of these variables, but the outcome depends on additional factors, e.g. waterfowl density, water depth, and timing (within or between seasons/years). Because of the persisting conservation threats to aquatic ecosystems, more knowledge is needed about potential direct and indirect consequences of waterfowl herbivory in these environments.

De senaste årtiondena har intresset ökat för hur svanar och gäss påverkar växtligheten i våtmarker. För detta finns flera skäl. Ett är att flera arter av dessa fåglar har ökat kraftigt i antal, vissa också i utbredning. Många våtmarker hyser därför avsevärt fler av dessa stora växtätare än förut, i vissa fall fler än någonsin tidigare. Därtill är våtmarker föremål för andra förändringar, såsom eutrofiering, klimatförändringar, brunifiering och negativ utveckling hos delar av deras biologiska mångfald.I denna artikel ger vi en översikt av den vetenskapliga litteraturen om svanars och gäss bete på vattenvegetation. Dels har vi undersökt vilka undersökningsmetoder som vanligen används i experimentella studier, dels hur olika vegetationsvariabler svarar på bete. Vi finner att både undersökningsmetoder och växternas svar skiljer sig avsevärt mellan olika studier. Många experimentella studier av beteseffekter bygger på en kombination av uthägnade provytor och öppna kontrollytor i samma våtmark, alltså likartade ytor utan respektive med bete. Likväl är det ofta svårt att dra generella slutsatser av dessa studier eftersom provytornas antal, storlek och placering skiljer sig mycket åt. Än mer problematiskt är att användningen av olika mät- och provtagningsmetoder skiljer sig åt, vilket också försvårar jämförbarheten mellan studier. Vanliga vegetationsvariabler i jämförande studier är biomassa (ovan eller under markytan/bot-ten), täthet, höjd, täckningsgrad och artsammansättning hos växtsamhället. Tidigare forskning stöder uppfattningen att svanar och gäss på ett signifikant sätt kan påverka alla dessa variabler negativt. Graden av påverkan beror dock på olika andra faktorer, såsom vattendjup, tid på året och tätheten av växtätande fåglar. Med tanke på den tidigare och pågående kraftiga antropogena på-verkan på våtmarker är det viktigt att ytterligare studera direkta och indirekta effekter av svanars och gäss bete i våtmarksekosystem. 

During recent decades, increased populations of geese have raised concerns about their potential impact on the abundance and composition of shoreline vegetation through grazing, and how this may affect other organisms relying on the same vegetation for food or refuge.We investigated the direct effects of goose grazing on shoreline vegetation and the potential indirect effects on aquatic macroinvertebrates, assessed through an exclosure experiment. The study was carried out over one growing season (March–August) in ten eutrophic wetlands with varying goose density, all located in agricultural landscapes in southern Sweden. We predicted that exclosures protected from grazing would have higher vegetation aboveground biomass, height, cover and species diversity (H′), as well as greater macroinvertebrate total abundance, taxon richness and taxon diversity compared to control plots.Aboveground biomass was 27% higher in exclosures. However, goose density correlated positively with vegetation cover (%) and species diversity, but negatively with vegetation height. Aquatic macroinvertebrate total abundance was significantly lower in exclosures, whereas neither taxon richness nor diversity differed between controls and exclosures. Moreover, the total abundance of aquatic invertebrates was generally higher in wetlands with moderate goose density. No significant associations were found between goose density and taxon richness or diversity of macroinvertebrates.In conclusion, predictions regarding direct grazing effects on vegetation were confirmed only for aboveground biomass. However, in some sites, dense stands of reed and cattails may have reduced grazing pressure by physically restricting accessibility. Additionally, substantial differences in vegetation species composition among study sites contributed to high data variability, which potentially could have masked treatment effects. The results add to a growing number of studies suggesting that geese do not invariably cause or contribute to the degradation of aquatic ecosystems, at least in highly productive wetlands and when goose density is moderate. Our study highlights the need for more experiments investigating the grazing effects of large herbivorous waterbirds in aquatic ecosystems in different environmental settings. Identifying thresholds at which grazing effects begin to impact the ecosystem would provide a valuable tool for wildlife and wetland management.