Fecal analysis and direct grazing observations in salt marshes and pens indicated that feral horses on Assateague Island, Maryland selectively graze Spartina alterniflora and avoid Distichlis spicata. A simulated grazing experiment was then conducted to determine the vegetative response of salt marshes on this Island to selective grazing of S. alterniflora, non-selective grazing of S. alterniflora and D. spicata, and no grazing. Selective grazing of S. alterniflora benefited D. spicata and allowed this unpalatable species to expand in grazed salt marshes, whereas non-selective grazing and no grazing did not benefit either species. These results suggest that overgrazing of the preferred S. alterniflora by feral horses can alter salt marsh competitive relationships and species composition, which may result in the dominance of salt marshes on Assateague Island by D. spicata.
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