Medium vs. short-term effects of herbivory by Eurasian beaver on aquatic vegetation

• Grazing by beavers increases macropyhyte richness and diversity.
• Beaver foraging significantly reduces macrophyte biomass.
• Similar volumes of macrophytes are consumed monthly.
• Beavers feed selectively on macrophyte species and their organs.

It is important to disentangle the effects of physical ecosystem modifications from plant–herbivore interactions to understand how keystone species, such as beavers, influence aquatic ecosystems, especially when populations are reintroduced or non-native. Through dam building beavers have the potential to influence macrophytes indirectly by altering the hydrological regime, but macrophytes also comprise a major component of beaver diet. In water bodies uninfluenced by dams, direct grazing will therefore be the primary basis of beaver-induced effects on macrophytes. Temporal change in macrophyte height, abundance and composition from three habitats within a pond system in Scotland were measured over a nine year period following beaver introduction. From 2003 to 2012, persistent, selective grazing by beaver led to three-fold increases in macrophyte richness and significant turnover in composition, reflecting overall reductions in abundance of the preferred dominant species (e.g. Iris pseudacorus). Within-season herbivory effects were quantified in 2011–2012 using exclosures and by conducting cafeteria-style choice experiments. Significant volumes of macrophyte biomass (mostly rhizomes of Menyanthes trifoliata) were removed at constant rates throughout the growing season. Feeding was highly selective whilst effects on diversity were negligible in the short term. In the medium term, selective foraging by beavers significantly increased alpha and beta diversity of macrophytes. Macrophytes in temperate and boreal regions are potentially widely influenced by beaver grazing, although the positive effects we observed are not necessarily universal and are unlikely to persist within individual sites indefinitely.