Waterbird impacts on widgeongrass Ruppia maritima in a Mediterranean wetland: comparing bird groups and seasonal effects

Abstract

We studied the effect of waterbirds on the submerged macrophyte Ruppia maritima in eleven fish ponds within Don˜ ana Natural Park (SW Spain). Separate exclosure designs allowed us to exclude flamingos or all waterbirds from 3 x3 m plots within the ponds and compare them with control plots. Four experiments were conducted for three month periods at different points of the annual cycle with varying bird densities. Flamingos and wildfowl (ducks and coot) had significant negative additive effects on the presence of aboveground (leafs and shoots) or belowground (roots) parts of Ruppia at all times of the year. For plots where Ruppia was present, aboveground biomass was significantly higher in all-bird exclosures than in controls or flamingo exclosures. Presence and biomass of this annual plant varied significantly between seasons as did the density of seeds in sediments. Seasonal changes in seedbank densities were consistent with consumption by birds. There were no significant treatment xseason interactions for Ruppia presence, aboveground biomass or seeds. This is the first exclosure study to compare the effects of waterbirds on submerged macrophytes at different times throughout the annual cycle, and the first to compare simultaneously the effects of different bird groups. Our findings refute previous suggestions that major effects of waterbirds are limited to temperate regions and to periods of early growth or when major concentrations of migratory wildfowl are formed in autumn. Flamingos are important in structuring shallow wetlands in the Mediterranean, and possibly many other regionsPeer reviewe