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Whooper Swan Grazing Cut Wetland Plant Biomass Nearly in Half in Finnish Experiment

Whooper swan family in a Finnish wetland
Research · Ecology

Whooper swan (Cygnus cygnus) grazing substantially reduced wetland vegetation in a field experiment across ten boreal wetlands in Finland. The study, published on 5 March 2026, compared fenced plots that excluded swans with nearby open plots exposed to normal grazing.

The experiment ran through the 2023 breeding season, from April to August. Researchers measured plant biomass, height, cover and species richness and also sampled aquatic macroinvertebrates to test whether changes in vegetation were accompanied by effects at another trophic level.

Paired open control and fenced whooper swan exclosure plots at Vitikankorpi, Finland
Paired control (left) and swan-exclosure (right) plots at Vitikankorpi, one of the study wetlands with the highest whooper swan density, photographed from the air on 15 August 2023. Figure 3 from Kjeller et al. (2026), Aquatic Ecology. Photo: Gunnar Gunnarsson. CC BY 4.0. Source.

Protected plots held far more vegetation

Aboveground plant biomass was 84% higher inside the swan exclosures than in the open control plots, averaging 52.2 compared with 28.4 g dry weight. Vegetation cover was 86% higher inside the protected plots, and vegetation height and plant-species richness also increased when swan grazing was excluded.

The strongest effects were found in water horsetail (Equisetum fluviatile) and graminoid vegetation. Horsetail biomass, cover and height were all significantly greater inside exclosures, while graminoid biomass and cover were also significantly higher.

Macroinvertebrates changed much less

The researchers had expected the extra vegetation inside exclosures to support more aquatic macroinvertebrates. That part of the prediction was not supported. They found little evidence that excluding swans affected either the total abundance or the taxonomic richness of aquatic macroinvertebrates, and none of the commonly analysed invertebrate groups differed significantly between treatments.

The authors therefore conclude that the clearest ecological effect detected over the study season was on the vegetation itself rather than a measurable cascading effect on the aquatic invertebrate community.

Even breeding pairs can have a clear local effect

Large flocks were not present at the study wetlands. Most sites held only a territorial breeding pair, sometimes with cygnets. The results therefore show that relatively low densities of whooper swans can still exert substantial grazing pressure when the same birds repeatedly use a wetland through the breeding season.

Sources and image credit

  1. Kjeller, E., Djerf, H., Holopainen, S., & Gunnarsson, G. (2026). Trophic interactions in boreal wetlands: Whooper swan Cygnus cygnus grazing and its effects on vegetation and macroinvertebrates. Aquatic Ecology, 60, 35. https://doi.org/10.1007/s10452-026-10273-7Primary open-access research article published 5 March 2026 under CC BY 4.0. Paired exclosures and open control plots in ten Finnish boreal wetlands during the 2023 breeding season showed 84% greater aboveground biomass and 86% greater vegetation cover where whooper swans were excluded, while macroinvertebrate responses were weak.

Image: Simo Räsänen (Ximonic) — whooper swans (Cygnus cygnus) on Aarnilampi at Luosto, Sodankylä, Lapland, Finland, photographed 17 September 2021; Wikimedia Commons; representative Finnish wetland image, not birds from the ten study wetlands · CC BY-SA 4.0 · Source

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