- The establishment of riparian plants is determined by abiotic conditions and grazing, although it is usually presumed that the former are most important. We tested the impact of aquatic grazers on the survival and growth of establishing riparian plants and whether the impact of grazing interacts with abiotic conditions.
- We conducted an experiment across 10 Dutch wetlands, covering a large range of water depth and nutrient availability. We introduced 1-year-old plants of an emergent (common reed, Phragmites australis) and a floating (water soldier, Stratiotes aloides) species in individual enclosures (n = 5 per site) that excluded predominantly waterbirds, which were the most abundant grazers, and on adjacent unprotected plots. Survival and growth were measured during one growing season.
- Grazing reduced growth (as biomass) of Phragmites and Stratiotes by a mean of 25 and 60%, respectively. Grazing decreased survival of Stratiotes, but not of Phragmites. Shallow water, water-level fluctuations, eutrophic conditions and enough light favoured both growth and survival of Phragmites. Growth of Stratiotes was unaffected by these factors, but they reduced its survival. For both species, grazing effects on biomass were consistent across environmental conditions, but for Phragmites, grazing effects on survival were influenced by abiotic conditions.
- We conclude that aquatic grazers significantly reduce the establishment and growth of macrophytes in the riparian zone over a wide range of environmental conditions.