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Managing deer is critical to protect and improve our natural resources. This chapter reviews deer impacts and management strategies.
Deer are a natural part of the region’s ecosystem, but long ago lost their primary natural predators (other than humans). A lack of natural controls coupled with previously strict hunting limits has resulted in populations much greater than natural lands can sustainably support.
Deer thrive on the edge habitat that a fragmented, suburbanized landscape provides. However, in overabundant numbers they consume the young trees, shrubs, and wildflowers that make a natural area healthy, beneficial to wildlife, and self-sustaining. Selective browsing of native plants, which are preferred by deer compared to introduced species, can cause a decline in native cover and an increase in invasive plants across natural cover types (forest, meadow, shrubland, etc). The loss of native species diversity and structural diversity in the understory reduces habitat for local and migratory wildlife. Overabundant deer populations can also cause significant damage to agricultural crops and ornamental plantings, spread Lyme disease, and increase the number of vehicular accidents.
A healthy forest can sustain roughly one deer per 64-128 forested acres without compromising ecological integrity. Heavily browsed vegetation, lack of shrub and tree regeneration, and decreased biodiversity indicate an unsustainably large deer population. According to the PennState Extension, the statewide deer density in 2007 averaged 30 deer per forested square mile, three to six times the desired density of 5-10 per square mile. In some Pennsylvania suburban areas, populations have risen above 100 per square mile. Deer densities at this level threaten the sustainability of forest communities, which depend on the ongoing establishment of tree seedlings and saplings in sufficient numbers to occupy the gaps that are created by periodic natural or human disturbance, as deer overbrowse seedlings, saplings, and shrubs.
In forests that have been subjected to overbrowsing for many years, the deer density will probably need to be lowered even further than the eventual optimal level for a period of time to allow the forest to regenerate. The subsequent section on estimating deer impact provides guidelines developed by Penn State University and the U.S. Forest Service for visually assessing deer impact on a forest community.
The decision to restore any forest must start with the goal of reducing and maintaining deer density at an appropriate level. Unless this goal is achieved first, the management of other stressors becomes a short-term lesson in futility that ultimately ends with the demise of the current canopy trees—and by definition, the forest itself—through natural decline or the next major wind event.
Overly abundant deer populations are also a concern for the health of the deer herds. Malnutrition is one issue with high deer populations as the deer first consume the best available and most abundant food sources, leaving poor quality food sources to survive on. Beyond malnutrition, communicable diseases also become more problematic when deer populations become overabundant.
Two diseases that are becoming more common in regions within the state are chronic wasting disease (CWD) and hemorrhagic disease. CWD is a contagious disease that affects the brain in deer and is always fatal. The disease was first found in wild Pennsylvania populations in 2012. In addition to causing mortality for deer, biological material infected with CWD can spread through other species such as scavengers and ticks and infect other species. At this time, there is no evidence that people can be infected with CWD. However, it is similar to other diseases such as mad cow disease that can infect people, and as such, the Pennsylvania Game Commission has enacted regulations to reduce spread of CWD. More information about CWD and restrictions can be found on the Pennsylvania Game Commission website.
There is one main virus that causes hemorrhagic disease in Pennsylvanian deer—epizootic hemorrhagic disease virus. This virus causes outbreaks among deer populations that result in high mortality. The virus is spread through midges that bite deer. According to the Pennsylvania Game Commission, there were outbreaks in Pennsylvania in 2002, 2007, 2018, and 2023. More information on hemorrhagic diseases can be found on the Pennsylvania Game Commission website.
Wherever deer are present on natural lands in Pennsylvania, there is a strong likelihood that the population is higher than the ecosystem can sustain without substantial damage. This section is designed to help a land manager determine which option or options are most appropriate for their property.
No deer management is an option if natural factors (predators, disease, famine) and human activities (current hunting levels, car accidents) within the area are maintaining the deer population at a level that does not adversely affect important natural or cultural resources. However, these conditions are rarely met in Pennsylvania, making no management an unsupportable option.
Without deer management, trees and shrubs need to be protected with fencing or tree tubes until above browse height and mature enough to withstand deer rubbing. The land manager also needs to accept the likely loss or severe degradation of the native herb layer.
Active methods to control deer overbrowsing can be grouped into two categories: those that restrict or deter deer access to desired vegetation and those that reduce deer population within a tract of land. Barriers, repellants, and lethal removal are currently used to modify white tailed deer behavior and decrease populations. Contraceptives and the trap-and-transfer technique are often discussed, but they are infeasible and are in most cases prohibited. The following table summarizes possible options; the following section goes into each option in more details.
Barriers physically restrict deer from interacting with vegetation in the treated area. Options under this method include tree shelters, netting, and deer exclosure fencing. Tree shelters and netting protect individual trees or shrubs; fencing excludes deer from all the vegetation in a specific area. Physical barriers have proven to be effective in protecting trees and shrubs in formal landscapes and forest vegetation although they can be expensive if used over a large area.
Tree shelters are useful to protect seedlings until they reach 6 feet in height and are above the maximum browsing height of deer. Tree shelters require periodic monitoring and maintenance as they are attractive to deer as rubs and to rodents for habitat. Vegetation around tree shelters should be periodically cut to prevent the creation of habitat for rodents. Tree tubes will likely need to be replaced, reset, or re-staked due to weather events. More information can be found in the Tree Planting section.
Fencing can be useful as a deer management tool, but it involves significant up-front expenses and frequent monitoring to ensure the integrity of the fence. Deer fencing around significant areas of land should be 8–12 feet high and constructed of steel wire, plastic mesh, or electrified wire (shorter fencing can be effective if the enclosed area is very small or narrow, such as a vegetable garden). In addition to its high initial cost, fencing requires constant monitoring to quickly repair any breaks caused by falling limbs or vandals and restricts not only deer movement, but also the movement of several other animal species.
Fencing can also be used as an instructional and monitoring tool. Small (10 × 10 meters to 20 × 20 meters) deer exclosures can be built at a relatively low cost to be monitored and compared to the existing forest. These study and demonstration areas provide a picture of the forest’s potential when browsing impacts from deer are removed. They also provide a feasible, more understandable, and far more useful barometer of deer overabundance than estimating deer density, which is inaccurate, and in any case, largely misses the point, which is deer population impact, not deer population numbers (but see Estimating Deer Population Numbers, below). The state of the forest within the exclosure can guide deer management outside.
Repellents create unpleasant sensory experiences that discourage deer from physically interacting with vegetation in the treated area. Repellents include periodic loud sounds, bright lights, or foul-tasting foliar sprays, often with a base of capsaicin, the fiery alkaloid in chili peppers. Repellents can be effective in small areas where the goal is to reduce browsing damage to tolerable limits.
The main drawbacks to repellents are cost and their short-term effectiveness. Deer, particularly those in dense populations, quickly adapt to these tactics. The manager must be committed to continually monitoring application needs and experimenting with new products as deer adapt. Although foliar sprays may be useful for landscape and other special plantings (though they have to be reapplied after rain), repellents are usually impractical for natural lands.
Immunocontraceptives and hormonal contraceptives for deer have been tested, but they require further study. Immunocontraceptives “vaccinate” an animal against egg proteins. Hormonal contraceptives work primarily by preventing ovulation in does. Contraceptives are not currently a feasible option for control and management as they are currently illegal in Pennsylvania.
Trapping or darting deer (requiring a permit from the Pennsylvania Game Commission) and moving them to another location is the most expensive, difficult, and ineffective deer control method. It is an option fraught with problems, the greatest of which is finding a location willing to accept more deer. This problem has become more difficult with the spread of chronic wasting disease (CWD) to nearby states. Attracting well-fed deer into baited traps is the next challenge. Finally, survival rates of transported deer have been low. At present, the Pennsylvania Game Commission has a policy of not issuing permits for trapping and transferring deer.
Lethal removal is the most frequently used deer population reduction and maintenance method commonly available to landowners and land managers. The two options are creating a managed hunting program and implementing a cull using professional sharpshooters, if approved by the Pennsylvania Game Commission. The latter is the fastest and safest way to reduce an overabundant population. All lethal means of deer management should focus on reducing the number of does by mainly targeting antlerless deer. Removing bucks has almost no effect on the year-to-year rate of population increase, decrease, or maintenance.
The following options can be used in combination to boost effectiveness.
The purpose of a cull is to quickly reduce the deer population. Under this option qualified professional sharpshooters are hired to euthanize a large number of deer within a property. This requires a special permit from the Pennsylvania Game Commission. The process is very rigorous and requires the landowner to make a convincing case that hunting within current game laws is not a viable option for managing the deer population at desired levels. However, this is probably the safest removal option as removal is usually done at night using infrared sighting scopes, over isolated baiting stations located where sharpshooters direct shots into the ground using rifle silencers (crossbows may be used where firearms are prohibited or infeasible due to safety zone restrictions). It is also the most effective option for reducing the deer population in the shortest time. The cash outlay is relatively high, but the time demands on the land manager can be considerably lower than that required to run a controlled hunting program (see below). The venison is donated to charitable food banks or government-run institutions. Repeated culls are likely to be needed until the deer population within a property reaches a sustainable level. They may then be needed periodically to keep the population low. Once the deer population is reduced and overbrowsing impacts are alleviated, a controlled hunting program—if it is permissible or feasible—may be adequate to maintain the desired deer population density.
Culls can be an issue with community members who want to hunt the property. However, they are compatible with and can be complemented by regulated hunts by municipal staff or qualified members of the public (see Managed Hunting Program below). Regulated hunts can help maintain the deer population at a sustainable level by keeping the population from rebounding following the cull, thereby reducing the frequency and expense of population control. Alternating culls with a regulated hunting program can keep the deer at low levels and open up the property for more hunting by staff or community members.
A regulated hunting program allows permitted hunters to hunt on a site, following the Pennsylvania Game Commission’s regulations for seasons and equipment and any additional rules determined by the land manager. Based on the desires of the land manager, the program can be open only to a designated group such as a hunting club or law enforcement personnel, or it can be open to the broader community. The number of hunters allowed depends on the size of the property, safety zone restrictions, and estimated deer population levels. For land open to recreation, a managed hunting program should focus on ensuring the safety of visitors and hunters, as well as enhancing the natural resources of the property.
Hunters should be informed of all program regulations and tested for proficiency with their sporting arm before receiving a permit to hunt. Property-specific regulations should clearly indicate permitted hunting hours, reporting requirements, and safety procedures. An example of a set of property-specific regulations is included in the subsequent case study section.
Managed hunting programs that utilize only archery can be beneficial in some scenarios, particularly on small properties or properties with numerous residential structures on its borders. This would expand the hunting area (the safety zone for archery is 50 yards; firearms require a 150 yard safety zone) and extend the hunting time during the year by several weeks.
A managed hunting program should require relatively little staff time to administer and operate once it is established, and the benefits should outweigh the costs. In addition to deer control, permitting hunters on the property can reduce unregulated hunting and other unwarranted uses. In some situations, it is more efficient to engage a local hunting club or professional agency to implement a deer population control program. They can handle all program administration, including proficiency tests, the scheduling of hunting times, and data collection on the separate harvest rates of does and bucks. The group should provide proof of insurance and be in close contact with the property landowner or manager to avoid conflicts with other activities in hunting areas.
A regulated hunting program can effectively complement a culling program. In most cases a managed program alone will not effectively address the issue of deer overabundance. Culling alone also cannot keep the deer population under control unless frequently repeated. A combination of culls and a hunting program can greatly reduce populations quickly and keep them stable over time.
One option for expanding the number of deer harvested each year is enrollment in the Pennsylvania Game Commission’s Deer Management Assistance Program (DMAP). This program provides additional permit applications (coupons) to landowners that they can then give to hunters. One coupon is granted for every 5 acres of farmland and every 50 acres of other land cover (forest, meadow, successional). Additional permits above the standard formula are available if the landowner submits a management plan with their request.
Monitoring vegetation indicators is a practical way to assess the effect of deer on forested areas. Vegetation can be assessed by two methods: (1) qualitatively comparing the overall influence of deer browsing on existing vegetation to an established index or (2) quantitative sampling. The U.S. Forest Service and Penn State University have developed a five-level deer impact index to visually assess the level of deer influence on forest health:
Deer Impact Index 1
Very low: No deer browsing. Occurs only within a well-maintained deer exclosure.
Deer Impact Index 2
Low: Species composition and height of regeneration is determined mainly by available light, nutrients, and seed source. There is a well-developed shrub layer and native wildflowers are abundant and grow to their full size.
Deer Impact Index 3
Moderate: Evidence of browsing is common with a greater reduction in height and abundance of the most-preferred species than of the least-preferred species.
Deer Impact Index 4
High: Species preferred by deer are sparse or absent and all plants are nearly the same height as a result of browsing. Vegetation in the shrub layer is sparse except for the least-preferred species (e.g., spicebush, American beech, exotic invasive shrubs).
Deer Impact Index 5
Very high: A pronounced browse line is evident with virtually no vegetation below the browse line except for two rhizomatous fern species, hay-scented fern and New York fern, or exotic invasive herbaceous species such as Japanese stiltgrass and garlic mustard.
Please note that these impact levels apply to later successional stages, particularly maturing, mature, and old-growth forests. Young forests (up to approximately 30 years old) typically have a dense canopy that prevents sunlight from reaching the forest floor. In this stage—called the pole stage—the understory is largely free of shrubs and herbs due to heavy shade. As the forest matures and the canopy thins from the death of weaker trees, there is sufficient light to support a shrub layer on which deer can browse.
The deer impact index is a qualitative measure; its utility for detecting change over intervals as short as one or two years is weak and its usefulness depends heavily on the level of experience and knowledge of the evaluator on food-plant preferences of deer, expected maximum sizes of various plant species under a variety of habitat conditions, and how to distinguish signs of deer browsing from plant damage by other animals and causes other than herbivory.
Quantitative sampling is more time-consuming and requires specialized expertise, but its interpretation involves less judgment. A quantitative approach could include periodic surveys along a transect or cataloging vegetation change within fixed quadrats. The latter are most effectively used in conjunction with the construction of deer exclosures, with sampling quadrats both inside and outside for comparison. Exclosures can be built to exclude deer while allowing free passage of all other herbivore species. Methods need to be scientifically rigorous if the results are to be sufficiently credible to serve as the basis for labor-intensive and potentially costly deer management procedures. For example, the protocol should include:
The data gathered within sampling plots or along transects may include:
An even more rigorous method of assessing deer impact over time separately from all other effects on vegetation is to compare identical plantings of native woody and herbaceous species in adjacent plot pairs in which one plot is inside an exclosure and the other is outside. In one such study (Latham 2019), adjacent 10 meter × 10 meter plots were matched in soils, topography, and sun exposure and cleared of existing vegetation; 95 plants of four native tree and shrub species and five native herbaceous species were planted in identical, evenly spaced arrays in each plot. After two growing seasons, survival, flowering/fruiting status, stem count per plant, and height of the tallest stem were recorded for each planted individual, and each spontaneously growing (unplanted) species was recorded, along with its estimated category of abundance in the plot (1-10, 11-50, 51-100, 101-500, 501-1,000, 1,001-5,000, 5,001-10,000). The four metrics of each planted species were compared between fenced and unfenced plots. For unplanted species, species diversity (native species richness and total species evenness separately) and relative estimated population sizes among native and nonnative invasive species were compared between fenced and unfenced plots. Periodically all vegetation is destroyed within each plot and the process repeated, to track ongoing progress in reducing deer impacts. This study was done by a municipal Environmental Advisory Council inexpensively using volunteer labor (several Eagle Scout candidates and their teams) to clear existing vegetation, build the exclosure, and plant the arrays, and the data collection and analysis was done by a professional ecologist, also as a volunteer. The results are there for all in the community to see; this transparency has done much to quell local skepticism and resistance against the need for deer management and its implementation. The U.S. Forest Service has developed assessment procedures for determining the current level of deer impact specifically on forest tree regeneration, as well as the level of competition from invasive species and other plants (e.g., ferns, mountain-laurel) that might interfere with the establishment and growth of tree seedlings after a timber harvest. A copy of this assessment procedure is outlined in the Forest Service General Technical Report NRS-11, Managing Timber to Promote Sustainable Forests: A Second-Level Course for the Sustainable Forestry Initiative of Pennsylvania (Finlay et al. 2007; see Outside Resources, below).
It is often repeated that a density of 1½-2 deer per 64 forested acres is a maximum level allowing minimal advance tree and shrub regeneration, and a density of ½-1 per 64 forested acres is needed to sustain a high diversity of native species, including native herbaceous plants. These numbers come from a small set of studies in large forest tracts of a single forest type in northwestern Pennsylvania, in which deer density was known because the research areas were fenced, emptied of deer, and then restocked with exactly the numbers of deer needed to achieve specific densities. This was excellent research, but we also know that deer density interacts with a host of other factors in determining the level of browsing impact on various forest ecosystem functions. Those other variables include forest type, understory species composition, landscape context, soil type, soil moisture regime, forest-floor light level, length of growing season, alternative food sources, historical land use, patterns of seasonal movement by deer, and legacy effects of prolonged high deer numbers (e.g., depleted seed bank, scarcity of live seed sources within dispersal distance, and disproportionate abundance of non-preferred understory plant species). These interactions are complex, unpredictable, and severely constrain the potential usefulness of deer density alone as a predictor of ecosystem impacts.
Where resources are limited, which is certainly the case for most managers of natural lands in Pennsylvania, it is cost-ineffective to divert time and money from vital tasks, including deer management, to estimating deer density. Keeping careful track of the number of does killed each year by hunters or sharpshooters, in combination with rigorous monitoring of ecosystem indicators (see previous section) is sufficient in most cases for tracking progress in achieving and maintaining a deer density that allows forest ecosystems to be sustainable.
However, if the resources are available, a land manager may wish to monitor deer abundance to make certain that management actions intended to reduce or maintain deer populations actually do so. Making a full count of any animal species in the wild is nearly impossible, but several methods have been developed to estimate the relative abundance of white-tailed deer from year to year on the same piece of land.
Relative abundance survey methods can be classified into two general types: indirect methods based on monitoring deer signs (e.g., tracks, fecal pellets) and direct methods that require capturing or observing deer. Direct methods may deliver more accurate and precise relative abundance estimates but they tend to be prohibitively expensive and risk inadvertent injury or death of animals during capture.
A direct method that does not involve capture is the infrared aerial survey. It has the advantage of covering large areas quickly and easily. Infrared aerial surveys were previously done by hiring pilots and renting aircraft, which is an expensive endeavor; today the use of drones for surveys make this a more viable option, particularly in high-priority areas. The main problem with using infrared aerial surveys for white-tailed deer in this region is visual obstruction by vegetation. Although the region contains a predominantly deciduous forest and infrared aerial surveys are performed only in winter, a large percentage of deer can be obscured by evergreen trees and shrubs, topographic features, and even the trunks and branches of deciduous trees and shrubs. Surveys are ideally done on very cold winter nights, when the thermal contrast is greatest. Sources of error include counting two or more deer lying or standing together as one and missing deer that are partially obscured by evergreen foliage, tree trunks, or topography. Surveys can also be done by traversing a site at night, typically in vehicles where feasible, using thermal imaging cameras. This is often done as part of the deer culling process to estimate how many deer are present prior to a cull, and can be used after a cull to estimate how population numbers have changed.
Indirect methods do not attempt to count all the deer, or in most cases any deer at all; they count items that serve as indices of relative abundance, considered useful for detecting changes over time within a particular area. For example, counts of the abundance of deer trails, deer footprints, deer sightings per kilometer walked on foot, abundance of fecal pellet groups, and number of deer killed on roads have all been used as indices of abundance. All of the deer abundance index methods assume that potential sources of variability in the index (e.g., deer defecation rates, hunter effort, movement by deer across the landscape) are constant in a specific area over time so that the changes in the index over time reflect changes in population size alone.
Counting fecal pellet groups has been the most widely applied means of indirectly estimating deer relative abundance. However, pellet group counts are subject to many sources of error. They include observer skill and fatigue in detecting pellet groups, choice of plot shape, habitat (vegetation) influences on detection of pellet groups, and decay rate of pellets. The most sophisticated surveys apply the technique of “distance sampling” to account for differential detection among habitats, factor in the decay rates of pellet groups, and use a statistically based sampling design. However, even the most careful surveys are based on a number of questionable assumptions, including a constant defecation rate and no variation in decay rates among habitat types. Research on defecation rates indicates that they vary among seasons (presumably because of dietary changes) and among age and sex classes and that pellet decomposition rates differ according to habitat type.
Once again, even if there were a perfectly accurate method for counting deer, it would still be an indirect and inaccurate measure of what matters to the land manager, namely the ecosystem impacts of deer. If we could know even the exact number of deer in an area of land, it would be a relatively poor predictor of ecosystem impact, for the reasons given in the first paragraph above under Estimating Deer Population Numbers. Land managers are encouraged to stick to the direct measurement of deer impacts using measures of vegetation taken inside and outside of exclosures.
At Natural Lands, an environmental non-profit that manages over 23,000 acres of land in southeastern Pennsylvania and southern New Jersey, the goal is to preserve and enhance the plant communities within its preserve system to maximize wildlife benefits. With that goal in mind and based on an understanding of the requirements of the state wildlife code, Natural Lands has instituted a deer management program that focuses on reducing and maintaining deer populations to a level that allows tree regeneration and survival of native herbaceous species. While Natural Lands employs small exclosures to protect certain plants and for demonstration purposes, controlled hunts have been employed over the majority of its lands to reduce the numbers of deer.
The rules that hunters must adhere to reflect an overriding concern for safety, not only for the participants of the management program, but also for other preserve users such as walkers and birders. A mandatory proficiency test ensures that hunters are familiar and competent with their sporting arm and a flagged map locates hunter positions for the preserve manager and other hunters. Participants wear bright Natural Lands armbands that allow preserve managers as well as others to tell from a distance if a hunter has permission to hunt. The rules place due emphasis on removing does from the population because it is almost exclusively the doe removal rate that influences population size. Preferentially harvesting does is capable of bringing populations to tolerable levels far more quickly than would a random removal strategy; preferentially harvesting bucks has almost no effect on birth rates and therefore will not control the population size.
Operating the program requires relatively little staff time to administer. In fact, staff time expended in administration is readily made up through time saved by the reduction in staff patrolling time during the hunting season. Permitted hunters monitor unwarranted access to the preserve during the hunting season, enabling managers to attend to other responsibilities.
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Finley, J.C., S.L. Stout, T.G. Pierson, and B.J. McGuinness. 2007. Managing timber to promote sustainable forests: a second-level course for the Sustainable Forestry Initiative of Pennsylvania. Gen. Tech. Rep. NRS-11. U.S. Department of Agriculture, Forest Service, Northern Research Station, Newtown Square, Pennsylvania. 47 pages. (research.fs.usda.gov/treesearch/12471, as of 2024)
Latham, R.E. 2019. Is the deer population above forest ecological carrying capacity in Rose Valley, Pennsylvania, wildlife sanctuaries? Rose Valley Borough Environmental Advisory Council, Rose Valley, Pennsylvania. 26 pages. (rosevalleyeac.org/rose-valley-2019-deer-impac.pdf, as of 2024)
Pennsylvania Game Commission: Healthy Forests, Healthy Deer (pgc.pa.gov/HuntTrap/Hunting/Pages/HealthyDeer.HealthyForests.aspx, as of 2024)
