SELECTIVE HERBIVORY AND PLANT COMMUNITY STRUCTURE IN A MID-ATLANTIC SALT-MARSH

Factors affecting the distribution of the grasses Spartina alterniflora and Distichlis spicata in a mid-Atlantic salt marsh were examined. A series of eight shallow wells, four in patches of each grass type, was used to describe physicochemical conditions known to limit the distribution of both grasses. Tidal amplitude, surface and subsurface salinity, and subsurface oxidation-reduction potential were found to be within range, but suboptimal, for both grasses. Evidence of selective grazing upon S. alterniflora was found through examination of grazing sign within the grass patches, observations of feral horse feeding behavior, and examination of feral horse feces for grass epidermal fragments. An exclosure experiment simulated preferential grazing and showed that S. alternifora responded negatively while D. spicata responded positively to simulated preferential grazing of S. alterniflora. These results point to a competitive relationship between the grasses under suboptimal conditions for dominance of either species. Selective herbivory (analogous to predation) upon S. alterniflora was shown to be a plausible factor impacting the competitive relationship to favor D. spicata. Location along physical gradients, interspecies competition, and herbivory are discussed in relation to salt marsh plant communities.