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nearshore dune vegetation. Figure 2. American beachgrass colonizes bare sand and promotes natural dune development and stabilization. The U.S. Geological Survey (USGS), National Park Service (NPS), and National Aeronautics and Space Administration (NASA) are jointly investigating horse-grazing impacts on American beachgrass and resulting effects on natural dune establishment along Assateague Island National Seashore (ASIS). American beachgrass is a pioneer dune-building plant as well as a major staple in the feral horse diet at Assateague Island. Using NASA Airborne Topographic Mapper (ATM) elevation surveys, color-infrared photography, and field data, scientists are investigating the changes in topography and vegetation cover in experimental plots to determine the impacts of horse grazing on geomorphology at ASIS. The horses (Equus caballus Linnaeus) of Assateague Island are a non-native species that have inhabited the barrier island for several centuries. Early colonial settlers introduced horses onto Assateague Island in 1669 to avoid the king’s taxation on fencing and livestock by allowing the horses to graze freely (Bearrs, 1968). Today, the northern two-thirds of the 57-kmlong island is managed by the NPS as Assateague Island National Seashore. Within its boundaries, the Maryland Department of Natural Resources manages a 3.2-km-long region as Assateague State Park. Upon the establishment of ASIS in 1965, the horses inhabiting the Maryland portion of the island have remained free roaming (Kirkpatrick, 1995). In 1965, the ASIS horse population consisted of 21 horses: nine stallions and 12 mares (Kirkpatrick, 1995). With no natural predators and despite the harsh environment, the ASIS herd had grown to 180 individuals by the spring of 2001. The significant growth in the feralhorse population has increased grazing pressure on native vegetation at ASIS. FromApril through September when insect populations flourish in the salt marshes and inland areas of the island, the horses migrate toward the beach to avoid the infestation. During these months, horses feed intensively on American beachgrass (Ammophila breviligulata Fernald) growing near the Atlantic shoreline (Figure 1). American beachgrass colonizes bare sand and plays an important role in the natural bioengineering process of dune

USGS, NPS, and NASA investigate horse-grazing impacts on Assateague Island dunes using airborne lidar surveys