Are Sika Deer Outcompeting and Replacing Maryland’s Whitetails?

July 10, 2024 By: Matthew McBride and Dr. Jacob Bowman

I had no idea that nearly 20,000 short, stocky sika deer roam the marshes of Maryland’s Eastern Shore until I watched a MeatEater episode filmed in Dorchester County. A few years later, I jumped at the chance to study these charismatic little deer as I earned my master’s degree at the University of Delaware. 

As you probably know, sika deer (Cervus nippon) are not native to the Delmarva Peninsula or North America. Ultimately, we can probably hold Lord Powerscourt responsible for afflicting Maryland, the United Kingdom and New Zealand with sika deer. Lord Powerscourt had a thing for bringing hoofed mammals from around the world and “acclimatizing” them to Ireland. 

Acclimatization didn’t work out so well for nilgai, sambar deer, or eland, but the temperate climate was ideal for the little Japanese deer that Lord Powerscourt purchased some time around 1860. He bought a male and three females who flourished, increasing to around 100, by 1884. Powerscourt referred to sika deer as “a most satisfactory little deer,” described their meat as excellent, and gave or sold sika deer to other deer parks throughout the U.K. 

Five sika deer from one of these parks, located at Woburn Abbey, made their way to Clement Henry in Cambridge, Maryland. In 1916, Henry released these sika deer onto James Island, which used to sit about a half mile off the coast of Dorchester County. The island is gone now, but sika deer had already dispersed off the island and colonized the western third of Dorchester County by the 1960s.

Sika Deer in America

Today, sika populations on the Delmarva peninsula (excluding the population on Assateague Island) occur across an area of approximately 714 square miles. In Maryland, sika occur throughout the forests and marshes of Dorchester County, south to at least the northern third of Somerset County, and west of the city of Salisbury. Sika deer also occur in the southwest corner of Sussex County, Delaware, in forested areas along the Nanticoke River.

The western third of Dorchester is mostly low-lying marsh dominated by invasive phragmites reeds and spartina grass. Sika deer have become almost synonymous with marsh in this part of the world. Sika deer do use marsh elsewhere, in the U.K. for example, but more typically, sika are considered a forest deer. They consume a diverse diet that often reflects what is available, including forbs, coniferous and deciduous browse, and grasses. Sika deer will also eat bark and will kill trees by girdling them. 

This trail-cam photo of a whitetail buck and a sika hind (female) shows that sika are generally shorter than whitetails but stockier. Their coat is darker. Adult sika deer maintain a few white spots, mainly down their back.

Sika deer are native to Eastern Asia, from Russia to northern Vietnam, and Taiwan, and Japan –though they have been extirpated throughout much of this range. They have also been introduced to New Zealand, the British Isles, continental Europe and Texas. They are super adaptable and have flourished in a variety of forest types as well as marshes. Harvest data from Maryland indicates that the sika deer population is growing and spreading.

A New Study of Sika Deer

To successfully manage a species, especially an exotic, it is critical to understand the habitat requirements and potential distribution of that species. What is the potential distribution of sika deer across the Delmarva peninsula? We used trail-cameras to survey for sika across Dorchester, Wicomico and Somerset counties in Maryland to try and answer this question. 

The Delmarva Peninsula is a pretty simple landscape: it’s flat, there is plenty of water, there are very few predators, and the climate is mild. Forest type varies, but we know sika deer use deciduous, mixed, and coniferous forests in other parts of the world, so we felt comfortable treating all forest types as the same. Forest type might influence sika deer populations in other ways but probably does not influence sika deer occurrence. We developed our study design to focus on three basic questions. Are sika on the Delmarva Peninsula dependent on marsh, are they in both forest and marsh, or are they everywhere, kind of like whitetails are?

We found that sika deer are not dependent on marsh. Sika deer occupied landscapes with relatively high proportions of natural cover, whether it was forest or marsh. Compared to white-tailed deer however, sika were much less likely to occupy woodlots or narrow belts of riparian forest if the surrounding landscape consisted of a high proportion of agriculture. To put a number on it, if a 400-acre area had more than 238 acres of natural forest or marsh cover (anything a sika might feel nice and safe in), it was likely to have sika deer in it. 

Sika deer stags generally have a dark, shaggy mane running down their neck, and their antlers sweep backwards rather than forwards like a whitetail’s. According to Maryland DNR, a 6-point stag is a trophy, with 8-pointers being extremely rare.

Whitetails vs Sika Deer

Much of the peninsula south of the Delaware state border is heavily forested and is likely highly suitable for sika. Large blocks of forest and marsh also exist in Delaware, but heavy fragmentation and reduced connectivity may prevent these areas from becoming colonized by sika. As sika continue to spread into heavily forested areas with established whitetail populations, there is concern that competition between the two species will have negative impacts on whitetails.

The potential for competition exists when two species live in the same place and use the same limited resources. In the case of ruminants like deer, food is often the limiting resource that regulates population size. Sika deer will eat just about any kind of vegetation, and this trait has probably contributed to the success of sika deer in many of the places they have been introduced. Sika deer digest cellulose far more efficiently than whitetails. This means sika deer can utilize poorer quality forages like grasses that whitetails can’t digest as efficiently. 

However, sika also eat the browse and forbs that white-tailed deer prefer. University of Delaware researchers found that dietary overlap between whitetails and sika in Dorchester County was high at 55 to 85%. Whitetail populations in areas with sika ate 44% more plant species compared to whitetails elsewhere on the Delmarva Peninsula, yet their diets were 17% less nutritious. Sika deer have a competitive edge over whitetails because they can utilize a broader range of food resources.

Texas Studies Exotic Deer Competitors

In Texas, researchers were curious about competition between exotic ungulates and native white-tailed deer. They built three 96-acre enclosures and paired white-tailed deer with sika deer in one enclosure and with axis deer in another. Each enclosure got two males and four females of each species. The third enclosure contained white-tailed deer only. Populations of both species initially increased in the sika/whitetail pasture, but after 10 years, the whitetails died out while sika deer increased to 62 individuals. 

The fact that sika deer reached 62 individuals is worth highlighting. The population in the whitetail-only enclosure never exceeded 17 and stabilized in the mid-teens. The axis deer population maxed out around 20 before suffering a die-off that dropped the population to 15. White-tailed deer paired with axis deer increased to 11 individuals before also suffering a die-off. No-die offs were noted for sika. 

It’s not just whitetails that seem to be outcompeted by sika. In New Zealand, a red deer herd was replaced by sika deer. In Russia, biologists noted that roe deer populations declined at relatively high sika densities. Given that the mechanism for sika to outcompete white-tailed deer exists, are sika displacing whitetails on the Eastern Shore? Several landowners I talked to suggested displacement occurred or was occurring on their properties. 

Are Sika Replacing Whitetails?

This can be a difficult question to answer because competition between white-tailed deer and sika deer is likely density-dependent. At low densities, sika might not have any impact on white-tailed deer. As sika deer reach densities that are high enough to impact whitetails, it may still take years for whitetails to disappear from the landscape.

We expected to observe white-tailed deer at every site we surveyed. The sites we surveyed were in portions of Dorchester, Wicomico, and Somerset counties where more white-tailed deer than sika are harvested every year. We intentionally did not survey the portion of Dorchester County where sika have been the dominant cervid for decades. We found that we saw whitetails less frequently at sites with higher numbers of sika compared to sites with fewer sika. 

sika deer
Landowners in the Delmarva Peninsula report seeing fewer whitetails as they see more sika deer. The author’s research supported the idea that as sika deer numbers increase in an area, whitetail numbers decline.

On any given day of the camera survey, we had a 65% chance of observing a whitetail at sites with few sika compared to a 35% chance at sites with lots of sika. This may indicate that although whitetails are present, whitetail populations in areas with more sika tend to be lower than areas with fewer sika. Furthermore, we found that whitetails were less likely to be detected at a site at all as sika numbers increased. The probability that a site was occupied by whitetails decreased from around 1.0 to less than 0.5 as sika numbers increased. These two findings taken together, and in the context of the collection of evidence that sika are highly competitive, support the hypothesis that sika deer are outcompeting and displacing whitetails on the Eastern Shore.

All is not lost for whitetails, however. Much of the Delmarva Peninsula’s landscape is dominated by agriculture, and these areas may act as refuges for whitetails. As sika spread into forested areas of the peninsula, land managers should be aware of the potential for sika deer to outcompete whitetails. However, because competition only becomes important when food resources become limiting, land managers may be able to maintain populations of both species by improving habitat quality and keeping densities low.

About Matthew McBride and Dr. Jacob Bowman:

Matthew McBride was born and raised in Tanzania where he developed a passion for wildlife. After finishing undergrad in 2018, he worked on several big game projects around the United States and started earning his master’s at the University of Delaware in 2021, studying sika deer. He is an avid hunter and fisherman. Dr. Jacob Bowman is a professor of wildlife ecology and the chairperson in the Department of Entomology and Wildlife Ecology at the University of Delaware. His research is focused on deer and bear but also carnivores and wild turkey. He has been researching deer for over 30 years. He's a certified wildlife biologist and earned his M.S. and Ph.D. at the Mississippi State University Deer Lab. He is also an avid hunter and fisherman.