The physical, chemical and biological environment of aquatic habitats is strongly affected by vascular plants. Any factor that affects the species composition or biomass of aquatic macrophytes will thus have indirect effects on the other components of the food web. Many species of waterfowl are dependent on aquatic plants during the winter. Because the reproductive success and annual survival of waterfowl is influenced by food availability on the wintering grounds, it is important to understand the factors that regulate the standing stock and diversity of vascular plants in wintering habitat.

To assess the impact of waterfowl on wintering habitat, I set up waterfowl exclosure experiments in emergent, semi-emergent and submerged vegetation communities at Mattamuskeet National Wildlife Refuge, NC. At the emergent site, used by snow geese and dabbling ducks, belowground plant biomass was significantly higher in exclosures than in grazed references over the study period. The consumption of belowground biomass by the birds in the winter had profound impacts on the biomass and diversity of plants during subsequent growing seasons. In the semi-emergent site, used primarily by dabbling ducks, aboveground plant biomass was significantly lower in grazed areas than exclosures one year after the onset of the experiment, most likely due to the consumption of seeds by wintering ducks. Invertebrate abundance was also significantly higher in exclosures than in grazed plots.

At the submerged site, diving ducks and swans significantly reduced the number of Vallisneria americana tubers. Unlike the emergent and semi-emergent habitats, the effect of this removal was not manifested in differences in aboveground plant standing stock. Mesocosm experiments with Vallisneria suggest that intraspecific competition during the growing season is responsible for the insensitivity of aboveground biomass to belowground abundance. Therefore, moderate intensities of herbivory by waterfowl on tubers may be compensatory with respect to aboveground biomass.

These results suggest that waterfowl are depleting aquatic food resources. Even though several species have adapted to alternative foraging habits (e.g., agricultural fields), species such as diving ducks are not able to exploit alternative habitats. Therefore, management strategies aimed at the less-adaptable species must include wintering habitat conservation.