pproximately 170 feral horses graze on the dunes and marshes of Assateague Island National Seashore. American beachgrass (Ammophila breviligulata) is a primary forage grass for the horses and is also the major dune stabilizing grass. With the increase in the horse population from 21 in 1965 to 170 today, concern over destabilization of the dunes due to overgrazing and the subsequent erosion has become a concern. To assess grazing along a 13 mile stretch of foredunes on the Maryland portion of the island, 18 exclosure plot pairs, i.e. fenced and unfenced plots, were established. Plant response to grazing in the fenced and unfenced plots was determined over a two-year period. Plant cover and vegetative spread were significantly greater in the exclosures than in the unfenced plots, as was the percent of culms flowering. Reducing the photosynthetic surface by grazing and forcing stored reserves to be used to rebuild the canopy decreases seed production. This reduces the number of potential propagules for colonization of blow-out sites or sites of recent sand deposition.

Funding Source: National Park Service

The ponies graze on American Beachgrass growing on the sand dunes.

Dr. Denise Seliskar
American beachgrass is important in controlling erosion. It holds the sand in place with its extensive root system.

Plot pairs
In order to study the effects of grazing by horses on American beachgrass, plot pairs consisting of fenced plots (which exclude horses) and unfenced plots are delineated at 18 sites along 13 miles of the Maryland portion of Assateague Island National Seashore.

Ungrazed  fenced plots Plant growth, spread, and flowering were monitored in the ungrazed fenced plots and compared to that in the unfenced plots where grazing was permitted.

Assateague horses In the summertime the horses often enjoy the sea breeze along the ocean beach and will venture into the surf for short periods of time when the biting flies and mosquitos are unbearable.