S04-1 The impact of goose grazing on arctic and temperate wetlands

Geese are large, herbivorous birds that graze in huge flocks in ways that may have a considerable impact on vegetation. This is exemplified best in two subspecies of snow geese, the lesser and the greater, both of which have increased dramatically in numbers in recent decades. In arctic coastal salt marshes, moderate goose grazing on Puccinellia phryganodes enhances plant production, but if it intensifies beyond a certain threshold it destroys the plant cover, leading to hypersalinity, soil erosion and little revegetation for long periods. In freshwater tundra wetlands dominated by Dupontia fisheri, Eriophorum scheuchzeri and brown mosses, grazing changes plant composition and reduces production of Eriophorum. Grazing may also favor mosses at the expense of grasses and sedges because mosses short-stop most of the nitrogen released from goose faeces. In temperate salt marshes, damage to the binding plant Spartina alterniflora from goose grubbing has been locally severe and has led to the devegetation of large areas. In temperate brackish marshes, geese heavily grub the rhizomes of Scirpus pungens. Their grubbing depresses Scirpus production, alters plant species composition, and influences marsh dynamics by enlarging ice-made depressions which are then colonized by other species. Grazing and grubbing in arctic and temperate freshwater wetlands apparently leads to a low-level production equilibrium between geese and the plants, but not in salt marshes.