University Park

Researcher evaluating Assateague Island's deer numbers, habitat

University Park, Pa. -- Although Maryland's famous Assateague Island is best known for its feral horses, recently 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 researcher to evaluate their population and impact.

"Although the sikas are small in stature, their effect on Assateague's sensitive ecosystem is large," said Duane Diefenbach, adjunct assistant professor of wildlife. "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 added. "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 two and a half feet high at the shoulder, weigh 50 to 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 a white rump. Males are larger than females and have antlers similar to elk. Males also have dark, shaggy manes around their neck.

In recent years, sika numbers on Assateague grew while the whitetail population had declined, but those trends may 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," said 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 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 explained. "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."

Graduate student Sonja Christensen of Bemidji, Minn., 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 said. "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 said. "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," he said.

A sika deer wears a tracking collar. Credit: Penn StateCreative Commons

Last Updated November 18, 2010