Simulation and management implications of feral horse grazing on Cumberland Island, Georgia

Abstract

Cumberland Island National Seashore, Georgia, is inhabited by a population of feral horses that intensively graze the island's salt marshes. Based on 18 months of experimental grazing studies, a carbon flow simulation model was developed for a medium height Spartina alterniflora marsh and used to estimate an acceptable population size of feral horses. Five-year simulations indicated a threshold of 2,700 kg/ha aboveground Spartina biomass below which the system did not recover if intensive grazing continued. The difference between this threshold and annual peak biomass of ungrazed Spartina was used to estimate horse densities that would not cause marsh degradation. Results suggest the horse population should number between 49 and 73 horses if excessive damage to the salt marshes is to be prevented. Thus, the current population of 180 horses should be reduced.