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 im pact on the vegetation dynamics of a salt marsh when they grub for bel ow-ground resources. The Ems Dollard Estuary is a traditional haunt fo r migrating Greylags. After 1975, the maximum number during fall migra tion increased more than tenfold in the estuary, thus allowing assessm ent of goose impact on the marsh vegetation. The number of wintering G reylags was negatively related with the winter severity; up to 4000 bi rds 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 rhi zomes and winter buds of Spartina anglica and tubers of Scirpus mariti mus. Both plant species were mapped in a few study sections in 1983 an d 1991. The area where S. anglica was (co)dominant decreased by more t han 60% during this period, Exploitation of juvenile plants by geese p revented new establishment in seemingly suitable habitat. Scirpus mari timus was dominant at the edges of the marsh at the beginning of the 1 980s, but decreased dramatically. At the edges of one marsh sector, Gr eylags peaked in three consecutive, very mild, winters, during which t hey apparently depleted the food stocks, Greylags removed especially t he shallow-buried tubers of S. maritimus. The tuber biomass needed two years to recover to a level typical for an exclosed stand, Recovery t ime was even longer when the accessibility of tubers to geese was enha nced by an artificial gully, It is suggested that the apparent overexp loitation of S. maritimus by Greylag Geese might be related to the spe cies' population response to the increased availability of agricultura l food resources in western Europe.