Deer Overabundance Research
This page links to scientific studies which illustrate the negative impact of overabundant white-tailed deer on ecological health and human well-being.
Impacts on Forest Health
“The Forest Nobody Knows” is a brief summary of severe deer impacts on forest understory condition
and post-forestry regeneration based upon research performed by Dr. Stephen Horsley and Dr.
Susan Stout in Pennsylvania. Their research included large enclosures with pre-determined deer
densities under varying forest canopy clearing levels. The article also provides a brief history of deer
management policy of the Pennsylvania Game Commission. References to original research articles are
provided in the report.
This very comprehensive report was prepared by Audubon Pennsylvania and the Pennsylvania Habitat
Alliance. The report carefully outlines forest health issues and future goals within the sociopolitical
context of Pennsylvania, which includes a significant forestry industry and large numbers of hunters.
The report provides specific recommendations for game code changes that would lead to herd reduction
and subsequent improvements in forest health as determined through careful monitoring.
Negative effects on vegetation became significant at deer impact levels well below those observed in many eastern forests. Moreover, species not browsed or resilient to browsing may have indirect effects on vegetation development through plant-plant interactions and on wildlife habitat quality for small mammals, birds, and deer. Managing these impacts is important as pressures to harvest and fragment eastern forests accelerate.
There has been increasing concern in the scientific and conservation communities regarding the effects of high densities of deer on natural communities. Multiple studies document impacts ranging from severe reductions in tree regeneration to loss of diversity of native forest herbs. Scientists at The Nature Conservancy of New Jersey have made observations of severe impacts at preserves in Northern New Jersey (e.g. limited native vegetation below 5 feet).
Current populations of white-tailed deer in North America far exceed the biological carrying capacity of the land, and natural communities are being extensively altered. What follows is a review of the literature that illustrates the various direct and indirect effects white-tailed deer have on the alteration of forested plant communities and the associated faunal populations.
Facilitation of Invasive Plant Species
Our results suggest that white-tailed deer herbivory can accelerate the invasion of exotic plants and that canopy disturbance can interact with herbivory to magnify the impact. In addition, our results provide compelling evidence of nonlinear relationships between deer density and the impact of herbivory on exotic species abundance. These findings highlight the important role of herbivore density in determining impacts on plant abundance and provide evidence of the operation of multiple mechanisms in exotic plant invasion.
Damage from deer browse coupled with human-related effects… severely impact some of New Jersey’s remaining public and private natural lands. The unintended consequence is the destruction of some of our remaining natural lands.
• Deer directly damage wildlife habitat and can eliminate rare plant communities.
• High numbers of deer find refuge in residential areas or on private land where hunting is
• Over-browse by deer eliminates the native shrub layer, which deprives breeding habitat
for many species, particularly shrub-nesting birds.
• Deer over-browse creates a favorable environment for invasive plants to germinate and
crowd out native species.
• Deer selectively browse on native species, which allows non-native plants to become
established and thrive.
Impacts on Wildlife
It is assumed that deer densities in forests of eastern North America have significant effects on the abundance and diversity of forest birds. The ability of deer to affect forest bird population is tested by monitoring the density and diversity of vegetation and birds for nine years in northern Virginia.
The emergence of Lyme disease as a public health issue is largely related to landscape changes and the increase in abundace of white-tailed deer. Most female “deer” ticks feed on this animal and deer ar the key hosts for tick reproductive success.
A unique study revealing a 90% reduction in questing ticks following the complete
and permanent extirpation of deer from an offshore island has demonstrated the quintessential
role of this host in the maintenance of deer tick populations. In addition, mainland transect
surveys have demonstrated a strong, positive relationship between deer presence and tick
abundance, with few ticks found below deer densities of approximately 15 deer/mi2.
Because the abundance of ticks is directly related to the abundance of deer, herd reduction represents an extremely important and effective method to reduce the risk of Lyme disease, particularly in populated areas where deer ticks are established and local ordinances and posted properties allow deer herds to burgeon.
The scientific literature strongly supports the role of deer as the main determinant of tick density, which is directly related to risk [of Lyme disease]
We conducted sharpshooting programs in 3 suburban communities to reduce deer
numbers and to address rising DVCs [Deer-vehicle collisions]. Annual or periodic population estimates were conducted using both helicopter snow counts and aerial infrared counts to assess population trends.
Management efforts were conducted from 3 to 7 years. Local deer herds were reduced by
54%, 72%, and 76%, with resulting reductions in DVCs of 49%, 75%, and 78%, respectively.
These projects clearly demonstrate that a reduction in local deer densities using lethal methods
can significantly reduce DVCs.
Collisions between large vertebrates and vehicles along roadways are an increasing concern, not only because of ecological consequences, but also because of associated economic and social costs. We used a large-scale, long-term data set comprising several databases from Utah to summarize and analyze these costs. The overall cost for 13,020 collisions from 1996 to 2001 in Utah was approximately $45,175,454, resulting in an estimated average per year cost of about $7,529,242 and a mean collision cost of $3,470. These figures include human fatality costs of $24 million (53% of total costs); vehicle damage costs of $18 million (39%); loss of deer, valued at $2.7 million (6%); and human injury costs of $1 million (2%). Cost-benefit analyses have shown that mitigation efforts, which are prioritized based on road-kill data, can produce positive net economic gains and also increase driver safety.
Abstract: We conducted a survey and literature review to identify affected stakeholders and gauge economic impacts from unwanted deer-human interactions in the northeastern United States. We estimated an annual economic impact from deer-vehicle collisions and deer depredation to select high-value agricultural, grain, and nursery crops, and residential and commercial landscaping for 13 northeastern United States at nearly $640 million. Our results can be used by Extension and wildlife professionals to inform and involve stakeholders participating in deer management decisions, tailor management strategies to mitigate deer-human conflicts, and assist policy makers when weighing the benefits against the negative impacts from deer.
In the 1990’s New Jersey’s agricultural community identified rising losses from crop depredation due to deer as a major problem. Deer damage is a concern of both the agricultural community and the NJDEP Division of Fish, Game and Wildlife (the Division). Yet, incomplete data was available on the extent of farmers’ deer-related damage problems.
Rutgers’ New Jersey Agricultural Experiment Station (NJAES) Center for Wildlife Damage Control designed and conducted a 65-question survey of N.J.’s farmers in 1998 to contribute to a better understanding how deer, and current deer management practices, impact agriculture. This comprehensive opinion survey determined farmers’ perceptions of deer and identified and quantified how current deer management practices impact their farming. Survey results should lead to improved deer management programs that are more responsive to the needs of farmers seeking solutions to crop damage.
Impact on Carbon Sequestration and Greenhouse Gas