White-tailed deer are an important and highly visible part of woodlands and savannas in the Forest Preserves. But in some areas, there are more deer than a preserve can support. This deer overabundance threatens the ecological health of the Forest Preserves and leads to less healthy deer populations. The Forest Preserves has a deer management program to help restore the balance in these impacted areas.
On this page:
- Why Manage Deer?
- How Does Deer Management Work?
- Measuring Results & Research Benefits
- Frequently Asked Questions
Why Manage Deer?
Deer have long been part of woodland and savanna ecosystems of northeast Illinois. Like many large mammals in our region, rapid development and hunting pushed deer populations to near zero in the late 1800s and early 1900s. Intentional repopulation of deer started in the 1930s and has been a resounding conservation success. But other large mammals—like wolves and cougars—that would naturally keep deer populations in balance are still mostly absent from Illinois.
By the 1980s, biologists, botanists and visitors to the Forest Preserves began raising concerns about deer overabundance in some locations, noting a decline in flowering spring ephemeral plants and the presence of clear browse lines—a distinct boundary with normal plant growth above and little or no plant growth below—at the height a deer can reach while eating. When deer consume nearly all of the flowers, seeds, branches and stalks they can reach across a site, native plants cannot successfully reproduce, grow and spread.
During a 1983-89 study of deer abundance in Busse Woods and other areas in northeast Illinois—with partners including the Illinois Natural History Survey, Illinois Department of Conservation, Lake County Forest Preserves, Chicago Zoological Society/Brookfield Zoo, Illinois Department of Transportation and O’Hare International Airport Department of Aviation Safety—it became clear that the Forest Preserves needed to implement a deer management program.
While the deer management program has evolved since the 1980s, the reason for the program has remained the same: Deer overabundance threatens the ecological health of the Forest Preserves and leads to less healthy deer populations.
- Ecosystem health: Massive losses of native vegetation cause changes to the structure of local plant communities, aide the spread of invasive plants that deer find unpalatable, and increase soil erosion and stormwater runoff.
- Deer health: Disease outbreaks become more widespread when the deer population exceeds the environment’s ability to sustain it. Large populations of deer are more vulnerable to disease and malnourishment, particularly if the number of deer browsing overwhelms the area’s natural capacity to produce food.
- Threatened and endangered plant species: According to the Prairie Research Institute, 22% of Cook County’s threatened and endangered plant species are faced with challenges from deer overabundance.
- Wildlife impacts: Deer-related reductions in the diversity and abundance of plant species harm insects, birds and other wildlife that depend on these plants for food, shelter and reproduction.
In Cook County, the leading cause of deer mortality is deer-vehicle collisions. Coyotes will prey on fawns or weakened adult deer, but do not provide a meaningful check on deer populations. With no natural predators to keep deer populations in balance, the Forest Preserves deer management program is critical to sustaining our ecological restoration efforts and fulfilling our mission to “…restore and manage lands for the purpose of protecting and preserving public open space… now and in the future.”
How Does Deer Management Work?
Staff from the Forest Preserves and U.S. Department of Agriculture survey vegetation at a preserve to document the extent of deer damage and determine if there is a need for deer management. These biologists and ecologists measure the damage to native indicator plants and compare the results to what would be expected at a similar site (based on size, habitat type and other factors) with a sustainable deer population.
If the damage exceeds established thresholds, the Forest Preserves can request a permit through the Illinois Department of Natural Resources (IDNR) to manage deer at that preserve. The number of deer that can be removed each year at a specific preserve is directly related to the extent of the damage observed.
Each winter, public access is restricted at preserves with deer management permits. Following strict safety guidelines from the IDNR, professional sharpshooters station themselves at these sites after dusk to remove deer, continuing to visit until they have reached the limit prescribed in the IDNR permit.
The initial population reduction stage at a new deer management site generally lasts three to five years. A maintenance stage then follows with adjustments based on vegetation monitoring and deer population counts. In some years, management may not be warranted at specific sites based on population trends.
Measuring Results & Research Benefits
At sites where deer populations are managed, the results are both obvious and significant. The native plants and animals of these areas are in much better condition than areas where management efforts have not been used, and the deer are healthier in these areas.
The Forest Preserves monitors vegetation at sites that have undergone deer management, determining if the return of native vegetation is of a quantity and quality that indicates that the local deer population has been reduced to a more sustainable level. Visual indicators of success include the reappearance of native shrubs, wildflowers, grasses and tree branches growing below six feet. This is measured using meander surveys or vegetational plots.
The deer management program also contributes valuable data used for monitoring the transmission of zoonotic disease—diseases that can be transmitted from wildlife to pets or humans—some of which can only be sampled from deceased animals. This ongoing research helps inform public health advisories.
Frequently Asked Questions
Are there non-lethal options?
To date there are no safe or practical non-lethal methods available to natural resource agencies for managing deer overabundance in free-ranging deer populations. Contraception methods either require repeat captures of individuals, which is impractical, or large doses of contraceptives that would pose a health risk if a treated animal were somehow eaten by humans, dogs, or coyotes. While some studies have shown successful use of contraception methods in isolated deer populations, these results have not been replicated on free-ranging deer populations.
Moving the wild deer from Forest Preserves land to another location is not practical. No enclosed facilities are available to handle an influx of new deer and moving deer and releasing them in the wild is prohibited by the State of Illinois because of documented high levels of subsequent deaths and the potential for disease transfer.
We continue to monitor the available science and work with other land managers and research institutions—including the IDNR and Prairie Research Institute—to keep up-to-date on potential non-lethal approaches to deer management.
Why can’t you manage deer through hunting?
Gun hunting is not allowed in Cook County and, due to high human population densities, unlikely to ever be allowed. Bow hunting has only been shown to be effective in managing deer populations in very limited situations and would pose its own safety issues in Cook County.
Can’t you feed the deer to prevent damage to natural areas?
State law prohibits the feeding of deer in the wild, and this would not reduce deer populations. Learn why feeding wildlife is harmful.
What happens to the deer that are removed from the Forest Preserves?
The deceased deer are moved to a Forest Preserves facility, where blood and tissue samples are taken from some deer to aid ongoing deer disease research. All disease-free deer are processed and donated to a food pantry, in compliance with state law.