Changes in the Composition and Standing Crop of Salt-Marsh Communities in Response to the Removal of a Grazer

Abstract

SUMMARY (1) Changes in species composition, live standing crop and plant litter have been measured in a salt marsh at La Perouse Bay, Manitoba in the presence of a grazer (the lesser snow goose, Anser caerulescens caerulescens) and when the grazer is excluded. (2) The composition of vegetation in exclosures (5 m x 5 m) changed rapidly. After five years sixteen species of higher plants were present in the oldest exclosure compared with six species in an adjacent (control) grazed area. Dicotyledons, such as Potentilla egedii and Plantago maritima, increased in frequency in the absence of grazing, and Carex subspathacea replaced Puccinellia phryganodes as the dominant graminoid. (3) New species, which appeared in the exclosure, were present in ungrazed areas elsewhere in the marsh. However, the frequency of these species in the exclosures was different from that in ungrazed areas. (4) The grazer is a generalist in its selection of forage species. The removal of geese leads to extensive clonal growth of species, such as Potentilla egedii, which are unable to tolerate heavy grazing and trampling, unlike Carex and Puccinellia. (5) Within a season the standing crop of vegetation was highest in exclosures erected one or two years previously. Litter accumulated in exclosures. The ratio of maximum standing crop to litter fell from >4: 1 to <2: 1 as communities aged in the absence of grazing. (6) Cessation of grazing leads to rapid changes in the structure and composition of the community. The vegetation within an exclosure was poorly grazed when the fence was removed after two years.