Penn State graduate student Sonja Christensen cradles a young Assateague Island sika stag prior to putting a radio collar on him. Photo courtesy of PSU

Maryland's famous Assateague Island is best known for its feral horses. Recently, though, it has been the exotic sika deer and their effect on the island's fragile habitat that have worried National Park Service officials. So they asked a Penn State University researcher to evaluate the sika population and its impact.

"Although the sikas are small in stature, their effect on Assateague's sensitive ecosystem is large," says Duane Diefenbach, adjunct assistant professor of wildlife at PSU. "We are studying the park's white-tailed deer, which are native to the island."

"We plan to capture between 30 and 40 deer, both sikas and whitetails, and put radio collars on them," he adds. "We will monitor their movements by radio telemetry through December 2007. We have captured and radio-collared nine deer already."

Sika deer are small members of the elk family introduced into Maryland in 1916 by private citizens. They are 2 1/2 feet high at the shoulder, weigh 50-100 pounds and originate in Asia. Their coat is dark brown to black. Some sikas have faint white, parallel spots on their backs. They also have white rumps. Males are larger than females and have antlers similar in shape to those of elk, though much smaller. Males also have dark, shaggy manes on their necks.

In recent years, sika numbers on Assateague grew while the whitetail population had declined, but those trends might have leveled off. Feral horse numbers are controlled with the use of a contraceptive drug administered by dart guns.

"The park has liberalized hunting seasons for sikas to control their numbers, but not for whitetails," says Diefenbach. "The question is, how do we best manage the species to limit effects on the island's plant community with so many deer? Visitors and hunters love sika deer. But they are an exotic species that may be upsetting the ecosystem."

The Assateague Island whitetail at right wears a radio collar affixed by PSU researchers to help them study deer impact on the national park's habitat. Photo courtesy of PSU

The first step in Diefenbach's research will be to describe the habitat use and movements of both sikas and whitetails. "One interest is seasonal changes in habitat use," he explains. "We will quantify how these deer use different habitats throughout the year. For instance, during times of the year when biting insects are a problem, deer move out of the salt marshes seeking breezy areas near the beach to get some relief. Also, we are interested in documenting movements during the breeding season and interactions between whitetails and sikas."

PSU graduate student Sonja Christensen, who is pursuing a master's degree in wildlife and fisheries science, will do most of the project's deer monitoring. "It is the damage deer are doing to endangered plants in the interdune meadows that most concerns park officials," she says. "They worry that rising sika deer numbers are resulting in overbrowsing in this very sensitive area."

Mark Sturm, an ecologist with the National Park Service, has been experimenting with exclosure fences to study the effect of deer and horse foraging on vegetation. "We have exclosures that keep out horses but let deer freely enter and exit, and we have exclosures that keep out all deer and horses," he says. "We have already documented several responses by the plant community within these different exclosures. Some plant species that are rarely otherwise seen on the island have even appeared within these sites where ungulates (horses and/or deer) have been excluded.

"We see this Penn State research helping the National Park Service to better understand habitat use by deer, and that should guide future decisions about how to best manage the deer populations."