Deer Damages

Auto Collisions

4% of Virginians are involved in vehicle collisions with deer each year.  In Fairfax County on the order of 2,000 to 5,000 deer are killed per year by collisions compared to 1,280 (average 2005 – 2009) that are taken by sharpshooters and hunters. Vehicle repair costs average $3,350 (2009 State Farm average payment of $3,100 per claim plus nominal $250 deductible).  In Fairfax County injuries to people occur in 10 to 20 collisions with deer per year and several fatalities occur per decade. 

As the principle cause of deer mortality in Fairfax County, collisions probably prevent deer densities that would totally devastate our forests and result in deer starving.  However, this is not only the most dangerous to humans and expensive way to control deer populations, but also the least humane to deer.  

Lyme Disease

Lyme disease is caused by a bacteria that is transmitted from rodents to other animals, including humans, by the black-legged tick (Ixodes scapularis), also known as the deer tick, primarily during its nymphal stage.  Deer are not a reservoir for Lyme disease.  However, adult deer are the preferred source of the blood meals that ticks require prior to laying their eggs.  The more deer, the more ticks are able to reproduce.  The more ticks, the higher the likelihood that people and rodents will get bitten by ticks infected with Lyme disease bacteria.  Recent data for Fairfax County show that a quarter of black legged ticks carry the bacteria.  Reported human cases of Lyme disease in Fairfax County were 208 in 2007, 191 in 2008, 257 in 2009, 227 in 2010, 148 in 2011, 150 in 2012, and 260 in 2013.  An unknown number of cases are treated without laboratory testing to confirm the diagnosis and are unreported. For more information on Lyme disease  and other tick borne diseases, see the brochure “Ticks and Tick Borne Diseases in Fairfax County”.  Also see other Lyme disease notes on Fairfax County Health Department’s web site.

People are not the only large mammals that get Lyme disease.  Dogs and horses also get it.  The test that detects heartworm in dogs also detects Lyme disease.  There is a vaccine for dogs that is usually, but not always, effective.  Nevertheless, anecdotal reports are that a much greater percentage of dogs get Lyme disease than people do.

Studies show that elimination or significant reduction in deer to 10 deer per square mile may result in reduction in Lyme disease.  Other studies conclude that reduction in deer density alone may not reduce Lyme disease – reducing white-tailed mice and deer ticks would also contribute.  So deer population reduction is a necessary but not sufficient remedy. It is not known yet whether reducing deer populations to such a low level is actually feasible in productive habitats like those found in Fairfax County.

Horticulture

Vegetable gardens, farm crops, and fruit trees are favored deer food sources in season.  Landscape plants are eaten year round. In fall, bucks injure woody plants by rubbing them with their antlers to remove their antler velvet. To suburban homeowners, horticultural damage is probably the most familiar type of deer damage. 

Deer Health and Reproduction

When there are more deer than their habitat can provide food for, they are less able to live through winter and does give birth to fewer fawns.  The figure below illustrates how deer reproduction (number of fawns added to the population or recruited per square mile per year) increases with deer density (all deer per square mile) until the maximum sustained yield of deer in a particular habitat is reached. Deer reproduction per square mile each year then falls even as deer density increases further.  Changes in animal and plant populations begin well below the maximum sustained yield for deer.

Deer become more subject to infectious diseases as density increases. Transmission of epizootic hemorrhagic disease, a viral disease fatal to deer but not harmful to people or pets, likely is enhanced by greater densities of deer.  VDGIF has investigated a bacterial infection in deer in Northern Virginia in recent years that appears to be related to general malnutrition and parasitism, likely related to dense deer herds. Chronic wasting disease has been detected in Frederick County, Virginia west of Fairfax County.  Feeding of deer is discouraged because it increases the risk of spreading communicable diseases among deer.

Invasive Plants

During the growing seasons, deer prefer the native herbaceous plants they co-evolved with over many alien plants.  The result is that excessive browsing gives alien plants a competitive advantage over native plants, thereby contributing to the invasiveness of some alien plants.  Deer also carry the seeds of invasive plants, accelerating their spread.  Our forests suffer from a variety of invasive aliens such as garlic mustard and stilt grass, to name a few. 

Forest Regeneration

When deer are overabundant, the entire understory of herbs, shrubs and tree seedlings may be eliminated producing “browse lines,” the most noticeable effect of over-browsing.  In time, the browse line grows upward, and the mid-story trees are not replaced resulting in cathedral forests with only a canopy of mature trees.  Inhibition of forest regeneration becomes apparent above 20 deer per square mile.  Over-browsing in the long term allows trees that deer dislike to become dominant, at the least.  At worst, complete failure of forests to replace themselves naturally is possible. In addition, invasive vines such as kudzu, oriental bittersweet, Japanese honey suckle, mile-a-minute weed also climb and kill tree seedlings and saplings that deer miss, putting even greater pressure on forest regeneration.

The image below shows how deer can impact the natural succession of forests.  Trees had been logged from this site several years before the photo was taken.  Deer were excluded from the area on the left by the fence.  Only ferns and other plants that are unpalatable to deer grow on the right. 

Habitat destruction

Elimination of herbaceous plants and shrubs of the understory layer also eliminates food and shelter for insects, reptiles, birds, and other mammals.  If all those animals that have disappeared could do so, they might ask how humane people were to allow such high deer densities.