Effects of migratory geese on plant communities of an Alaskan salt marsh

Abstract

SummaryWe studied the effects of lesser snow geese (Anser caerulescens caerulescens) and Canada geese (Branta canadensis) on two salt marsh plant communities in Cook Inlet, Alaska, a stopover area used during spring migration. From 1995 to 1997 we compared plant species composition and biomass on plots where geese were excluded from feeding with paired plots where foraging could occur.Foraging intensity was low (650–1930 goose‐days km−2) compared to other goose‐grazing systems.Canada geese fed mainly on above‐ground shoots ofTriglochin maritimum, Puccinelliaspp. andCarex ramenskii, whereas the majority of the snow goose diet consisted of below‐ground tissues ofPlantago maritimaandTriglochin maritimum.Plant communities responded differently to goose herbivory. In the sedge meadow community, where feeding was primarily on above‐ground shoots, there was no effect of grazing on the dominant speciesCarex ramenskiiandTriglochin maritimum. In the herb meadow community, where snow geese fed onPlantago maritimaroots and other below‐ground tissues, there was a difference in the relative abundance of plant species between treatments. Biomass ofPlantago maritimaandPotentilla egediiwas lower on grazed plots compared with exclosed, whereas biomass ofCarex ramenskiiwas greater on grazed plots. There was no effect of herbivory on total standing crop biomass in either community. The variable effect of herbivory onCarex ramenskiibetween communities suggests that plant neighbours and competitive interactions are important factors in a species’ response to herbivory. In addition, the type of herbivory (above‐ or below‐ground) was important in determining plant community response to herbivory.Litter accumulation was reduced in grazed areas compared with exclosed in both communities. Trampling of the previous year’s litter into the soil surface by geese incorporated more litter into soils in grazed areas.This study illustrates that even light herbivore pressure can alter plant communities and affect forage availability.