Control of water plants by large grazers

Posted on July 2, 2014

While studies have shown that the establishment and survival of aquatic plants are strongly influenced by abiotic conditions, the potential impacts of grazing on freshwater macrophytes have been somewhat neglected. Working in ten nature reserves in the Netherlands, researchers compared the survival and growth of two macrophyte species in experimental 3 x 6m plots at the edges of wetlands. Both species – the emergent plant Phragmites australis and the floating form Stratiotes aloides – were introduced to the plots as one-year-old plants. In the experimental design, exclosure plots, where herbivore entry was blocked by mesh fences and anti-bird ropes, were paired with open control plots. Summer grazing significantly reduced the biomass growth of both species – Phragmites by 25% and Stratiotes by 60%. Although the herbivores responsible were not identified, the exclosures denied entry to large fish and muskrats as well as birds such as geese, swans, ducks and coots. Macrophyte grazing is likely to
be on the increase in Europe and North America as a result of expansions in mute swan and geese populations.

Reference: Veen, G.F. et al. 2013. Aquatic grazers reduce the establishment and growth of riparian plants along an environmental gradient.
Freshwater Biology 58, 1794–1803.