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

Factors affecting the distribution of the grasses Spartina alterniflor a and Distichlis spicata in a mid-Atlantic salt marsh were examined. A series of eight shallow wells, four in patches of each grass type, wa s used to describe physicochemical conditions known to limit the distr ibution of both grasses. Tidal amplitude, surface and subsurface salin ity, and subsurface oxidation-reduction potential were found to be wit hin range, but suboptimal, for both grasses. Evidence of selective gra zing upon S. alterniflora was found through examination of grazing sig n within the grass patches, observations of feral horse feeding behavi or, and examination of feral horse feces for grass epidermal fragments . An exclosure experiment simulated preferential grazing and showed th at S. alternifora responded negatively while D. spicata responded posi tively to simulated preferential grazing of S. alterniflora. These res ults point to a competitive relationship between the grasses under sub optimal conditions for dominance of either species. Selective herbivor y (analogous to predation) upon S. alterniflora was shown to be a plau sible factor impacting the competitive relationship to favor D. spicat a. Location along physical gradients, interspecies competition, and he rbivory are discussed in relation to salt marsh plant communities.