A positive feedback cycle between soil salinity and graminoid growth, triggered by intensive grazing and grubbing by geese, is proposed to account for the loss of graminoid vegetation in an arctic salt marsh (La Pérouse Bay, Manitoba, Canada) based on the results of experimental field studies. Decreases in above-ground biomass, caused by goose herbivory, increase soil salinity. High soil salinities further reduce graminoid growth, both in the salt marsh and under controlled conditions, causing additional decreases in above-ground biomass. Graminoid growth is also reduced by intraspecific competition. The implications of this positive feedback on vegetation dynamics and the size of the goose colony at La Perouse Bay are discussed.