Foraging Geese, Vegetation Loss and Soil Degradation in an Arctic Salt Marsh

The North American mid-continent population of Lesser snow geese (Chen caerulescens caerulescens L.) has increased by ca. 7% per year, largely as a result of geese feeding on agricultural crops in winter and on migration. We describe the long-term effects of increasing numbers of geese at an arctic breeding round (La P6rouse Bay, Manitoba) on intertidal salt-marsh vegetation. Between 1985 and 1999 goose grubbing caused considerable loss of graminoid vegetation along transects in intertidal marshes. Loss of vegetation led to bare sediment with a plant cover of less than 2%. Changes in vegetation could not be described by simple linear, geometric or exponential functions; most losses occurred between 1988 and 1990 and losses were staggered in time between individual transects, some of which had all vegetation removed. Between 1979 and 1999 the standing crop in July in remaining intact heavily-grazed swards of Puccinellia phryganodes and Carex subspathacea fell from 40-60 g m-2 to 20-30 g m-2. Intense grazing on remaining patches of sward has restricted growth of these clonal forage plants and hypersalinity of bare sediments has precluded re-establishment of vegetation. Between 1989 and 1993 numbers of faecal droppings in grazed plots reached maximum values of 15-22 droppings m-2 wk-1. Since then peak values have remained at less than 13 droppings m-2 wk-'. The loss of vegetation and changes in soil conditions have resulted in the establishment of an alternative stable state (hypersaline bare sediment). Introduction Herbivores strongly influence plant community dynamics (Harper 1969; Crawley 1983; Huntly 1991; Sinclair 1995; Olff et al. 1999). Besides their effects on the abundance and standing crop of different species, herbivores can also influence the physical structure of the environment, particularly the soil-plant continuum (Graetz 1991; DeAngelis 1992; van de Koppel et al. 1997; Jefferies 1999). The effects of herbivores and changes in physical processes result in irreversible transitions in community assemblages leading to discontinuous multiple stable states (Holling 1973; Noy-Meir 1975; May 1977; Westoby et al. 1989). Abrupt and rapid changes between stable states are often caused by positive feedbacks (Maruyama 1963; DeAngelis et al. 1986; Oksanen 1990). For example, limited vegetation loss can be amplified into large-scale desertification by positive feedbacks between plant mortality, soil degradation processes and inability of plants to re-establish (Graetz 1991; Srivastava & Jefferies 1996). The initial vegetation loss that triggers such changes of state is frequently linked to increasing herbivore populations. Herbivore populations are often kept in check by negative feedbacks linked to food sources (resource limitation) or to predators (density-dependent predation). When predators are absent, herbivore populations can increase dramatically, resulting in a trophic cascade and a sustained reduction in the biomass of primary producers that lead to changes in plant and animal assemblages (Paine 1969; Lawton 1989; Power 1992; Strong 1992; Polis & Strong 1996). The same effect also may occur if consumers increase in numbers as a result of an external food subsidy without, necessarily, a concomitant increase in predators (Polis 1999). In this paper we use the term 'apparent rophic cascade' to describe the cascading effects of a burgeoning population of snow geese on intertidal vegetation and soils that have led to ecosystem changes. In the absence of increased predation this