The impact of grubbing by Greylag Geese (Anser anser) on the vegetation dynamics of a tidal marsh

This study shows that staging Greylag Geese may have a considerable impact on the vegetation dynamics of a salt marsh when they grub for below-ground resources. The Ems Dollard Estuary is a traditional haunt for migrating Greylags. After 1975, the maximum number during fall migration increased more than tenfold in the estuary, thus allowing assessment of goose impact on the marsh vegetation. The number of wintering Greylags was negatively related with the winter severity; up to 4000 birds wintered in the area in the 1980s and early 1990s. The geese feed on above ground biomass of grasses and dicotyledons, as well as on rhizomes and winter buds of Spartina anglica and tubers of Scirpus maritimus. Both plant species were mapped in a few study sections in 1983 and 1991. The area where S. anglica was (co)dominant decreased by more than 60% during this period, Exploitation of juvenile plants by geese prevented new establishment in seemingly suitable habitat. Scirpus maritimus was dominant at the edges of the marsh at the beginning of the 1980s, but decreased dramatically. At the edges of one marsh sector, Greylags peaked in three consecutive, very mild, winters, during which they apparently depleted the food stocks, Greylags removed especially the shallow-buried tubers of S. maritimus. The tuber biomass needed two years to recover to a level typical for an exclosed stand, Recovery time was even longer when the accessibility of tubers to geese was enhanced by an artificial gully, It is suggested that the apparent overexploitation of S. maritimus by Greylag Geese might be related to the species' population response to the increased availability of agricultural food resources in western Europe.