NPS photo

Figure 1. Symbol of Assateague Island National Seashore, feral horses grew in number from 28 in 1968 to a high of 179 in 2002. A successful contraceptive program has reduced the population to 134 individuals in 2008.

Ungulates are a threat to many of the natural resources at Assateague Island National Seashore (Maryland and Virginia). Feral horses (Equus caballus) (fig. 1, above), Asiatic sika deer (Cervus nippon) (fig. 2, below left), and native white-tailed deer (Odocoileus virginianus) (fig. 3, below right) roam the barrier island, competing for resources and disrupting natural processes that maintain habitat for a diverse array of plant and animal species of conservation concern. These include the piping plover (Charadrius melodus), a shorebird, and seabeach amaranth (Amaranthus pumilus; see sidebar, fig. 4), a fleshy-leaved plant. Though these species are threatened with extinction, management of ungulates to help protect them is a complex challenge owing to historical and ecological factors and multiple management jurisdictions.

B. Emerson (left); NPS photo (right)

Figures 2 and 3. Population estimates indicate that nonnative sika deer (left) outnumber native white-tailed deer (right) by about three to one on Assateague Island. Both deer species occur throughout the island and are frequently observed in marsh, forest, shrub, and dune habitats.

Assateague Island is located on the highly developed mid-Atlantic coast of the United States. The Maryland portion of the island, 21.7 miles (35 km) long, is managed primarily as Assateague Island National Seashore, while a roughly 1.8-mile (3 km) section is managed by the Maryland Department of Natural Resources as Assateague State Park.

Feral horses have continuously occupied Assateague Island for more than three centuries. Though the origin of the horse population is unclear, by the time the national seashore was established in 1965 the horses had long since become an important regional icon, and they remain very popular among visitors today. Given the horses’ historical and cultural significance the National Park Service identified them as a desirable species in 1982 and manages them as wildlife. However, in order to control the size of the population, the park employs a contraceptive program developed for the national seashore (Kirkpatrick 1995) that has reduced the species’ numbers from a high of 179 in 2002 to its current size of 134.

Sika deer were introduced to the island in the 1920s by a private landowner (Flyger 1960). Since then, this “deer” species, which is actually a small Asiatic elk, has become naturalized on the island. Both nonnative sika and native white-tailed deer are subject to a congressionally authorized hunting season to control their abundance. Despite considerable annual hunting pressure, sika deer have continued to maintain a sizable population, and at this time their elimination from the island may not be feasible.

For more than two decades scientists have been studying the effects of the three ungulate species on island vegetative communities and natural processes. This article summarizes that program of research and its implications for effective ungulate management strategies.