Data from field studies was used in a simulation model to determine the number of feral horses (Equus caballus) that can graze Spartina alterniflora in the salt marshes of Cumberland Island without causing damage to this plant community. The model indicated that damage to the salt marsh occurred when biomass of S. alterniflora was reduced below a threshold of 2700 hg/ha (dry weight). Biomass of S. alterniflora produced above this threshold would be able to sustain between 49 and 73 horses when horses consumed S. alterniflora as 40 - 60% of their diet. Therefore, the horse population will need to be reduced from approximately 180 horses to this level if forage production and the health of the salt marsh plant community are to be maintained.
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