Monitoring Deer Impacts on Natural Vegetation in Ann Arbor:
A Pilot Study of Red Oak Seedlings as Experimental Indicators of Deer Browse Intensity
Across 10 city parks
November 2015 – October 2016
Jacqueline Courteau, Ph.D.
Consulting Biologist/Ecologist, NatureWrite LLC
October 17, 2016
This pilot experimental study, in which red oak seedlings were planted and monitored in 10 city parks (and in a separate study at the Arboretum), found that deer are browsing 20–90% of tree seedlings, a level that exceeds the 15% recommended in existing scientific literature as allowing for sustainable tree regeneration (Blossey 2014). Overall, deer alone were responsible for 76% of the browse-damaged seedlings, with an additional 11% browsed by both deer and small mammals.
A total of 9% of seedlings were browsed by small mammals only; including seedlings also browsed by deer, 20% of seedlings showed evidence ofsmall mammal browse.
Deer biology, the impact of too many deer, and deer management options are the topics topics of this February 2017 report. Links to data collected by the City of Ann Arbor and research from major institutions and scientists provide documentation.
Ann Arbor has too many deer as shown by 1) scientific studies of our natural areas, 2) a city survey of citizen experiences, 3) the increase in the number of deer-vehicle collisions, 4) the observations of a professional deer biologist and 5) annual aerial surveys. These metrics and the report from the deer management contractor will be used to inform future decisions about the deer management program.
The City began a 4-year deer management program in 2016 with a cull of 63 deer using professional sharpshooters. In 2017, the city added a second method– the surgical sterilization of 54 does in two neighborhood areas — in addition to the culling of 96 deer in natural areas in the city.
The Ann Arbor deer management program is based on science. Here’s a summary of deer biology, the impact of too many deer, and deer management options.
- Deer populations grow rapidly.
- Deer evolved as a prey species, so they reproduce rapidly. Deer populations double every 2 years. In year 1, a doe bears one fawn. In years 2-15, she has two or three fawns each year. In a U of M study in Washtenaw County, 4 does and 2 bucks increased to 222 deer in 7 years.
- In Ann Arbor, there are no natural predators (hunters, wolves, cougars, bears); only cars limit the population growth.
- Rapid population growth means that delays in response increase the problem and increase the cost to resolve it.
- Removing deer from healthy populations like Ann Arbor’s does not increase reproductive rates.
- Deer are “the” major herbivore of forest ecosystems.
- Deer are browsers; they eat leaves, buds, and green stems as high as they can reach (unlike cows and other grazers that eat grass). They prefer native plants and many garden plants. Deer eat 5-10 pounds of greens daily.
- At high population density, they have disproportionately large impacts on biodiversity and forest dynamics.
- Deer can prevent forest regeneration (stopping the next generation of trees and shrubs), endanger native plants, and facilitate the establishment of invasive species.
- Their impact cascades through the food web, impacting small mammals, birds, reptiles, insects, and amphibians, as well as stream quality.
- In Ann Arbor, two studies have shown significant deer damage to 10 city parks, the U of M Arboretum. Due to the damage to U of M property,t U of M has joined the City’s effort.
- Deer thrive in suburban landscapes like Ann Arbor’s because of the ideal habitat with ample food and no hunting or wild predators.
- Deer flourish on edges since edges provide more food than fields or mature woods. Ann Arbor’s yards create much more edge habitat than naturally occurs, helping to provide the 5-10 pounds of food that a deer eats daily.
- In Ann Arbor, aerial surveys show that deer density varies across the city, with most in wards 1 and 2. Deer are present and increasing in all wards.
- Deer social groups affect deer impact and management methods.
- Females live in social groups and stay and raise offspring in a natal home range learned from their mother. Suburban female deer have home ranges less than 1/2 mile squared, about 100-200 acres.
- Deer have no way to know whether another area is available and have no urge to leave – they are faithful to their natal home range. Removing deer in one area does not cause other deer to move in; they do not “act as gas molecules,” spreading to fill empty space.
- Deer-vehicle collisions (DVC’s)
- Collisions increase as the number of deer increase.
- Collisions decrease when deer are removed.
- DVC’s are an indicator of the deer population.
- In Ann Arbor, from 2004-2015, DVC’s nearly tripled from 31 to 90, while all crashes went up only 6%.
- Lyme disease
- Deer are hosts for blacklegged ticks. These ticks can carry Lyme and other diseases that can cause severe headaches, arthritis, heart palpitations, inflammation of the brain, memory loss, and more. Lyme diseases is very difficult to diagnose and treat.
- Recent work on the role of deer in Lyme disease shows a correlation between deer densities, the abundance of blacklegged ticks that carry the disease, and cases of Lyme disease. Once established, Lyme disease can only be reduced if deer densities are brought very low.
- Lyme is not yet endemic in Ann Arbor, but there are cases in Washtenaw County.
- Options for dealing with over-abundant deer
- Scientists have studied deer management extensively and concluded that, in areas where sharpshooting is legal and safe, culling is the most effective and cost-efficient method.
- Where there are access issues, scientists recommend surgical sterilization of does. Other contraceptives have problems with effectiveness, require multiple applications, and negatively affect buck behavior. All non-lethal methods of population control take time to be effective – the population is not reduced immediately.
- Methods that don’t control deer populations do not have long-lasting effects. Deer quickly become accustomed to smells, noise, water, and other deterrents. Fences must be at least 8 feet high to be effective and only shift the deer to other areas. “Deer-resistant” plants aren’t resistant when deer are hungry enough.
- Moving deer is not legal because of high mortality and danger of spreading disease.
SOURCES FOR MORE INFORMATION
- The City’s deer management website, which includes, along with other information:
- Information about the 2016 and 2017 deer management programs, including the City Council resolutions, the lethal and sterilization programs, and measures of success.
- Summaries of meetings between City staff and citizen groups.
- Documents relating to the deer management contractor, the permit application, and response from the MDNR.
- Washtenaw Citizens for Ecological Balance (WC4EB) educational website:
- Information on deer biology, overabundance, and management, especially in urban areas.
- Links to scientific reports, news, magazine articles, interviews, and more, along with a link to sign up for updates.
- “An Integrated Approach for Managing White-Tailed Deer in Suburban Environments: the Cornell University Study” by Jason R. Boulanger, et. al. , 2014.
- “Help for Communities Grappling with Abundant Deer Populations” from Cornell University.
Prepared by Washtenaw Citizens for Ecological Balance, February 2017
U-M biologists support Ann Arbor deer cull
Michigan News, Jan 14, 2016
A University of Michigan evolutionary biologist says he and many of his U-M colleagues support the city of Ann Arbor’s plans to kill up to 100 deer this winter, calling the cull “a positive step toward ecological sustainability.”
U-M botanists have long noted declines in native plants that deer favor, Dick said. In a 2015 study, an ecological team surveyed browsing impacts in Ann Arbor’s Bird Hills Nature Area and found browsing damage in 80 percent of the tree saplings.
U-M Biologist Says Ann Arbor’s Deer Cull Is Necessary
WEMU, Jan 14, 2016
He says losing too much vegetation will harm other wildlife and put the ecological health of Ann Arbor at risk.
The above map is a combination of 5 different maps showing and describing deer sightings, dead deer, deer accidents, deer damage, fawns seen, and incidentals, like coyotes seen. We have created a new map for 2016. To report information for this new map, please contact email@example.com. Provide good location coordinates and description of what you are reporting.
Federal judge rejects motion to halt deer cull in light of pending lawsuit
Michigan Daily, Jan 13 2016
A legal motion aiming to halt the city’s deer cull was shot down by a federal judge after a court hearing Monday afternoon.
Judge says Ann Arbor’s deer shoot can go ahead, for now
NPR, Jan 11, 2016
The hearing did not go well for the plaintiff’s attorney, Barry Powers. Judge Tarnow repeatedly criticized Powers, saying he hadn’t prepared, done his research, listened to the other side – nor did he convince the court, Judge Tarnow said, that anyone’s lives or federal rights in Ann Arbor were in imminent danger.
Deer impacts on vegetation and communities in the Leonard Preserve, Manchester, MI
Jacqueline Courteau, Ph.D., Sept 2015
Presence of deer is correlated with significant reductions in plant species composition, abundance and flowering, with the potential to reduce populations of spring and fall wildflowers in both forests and grasslands, and to reduce oak recolonization in old fields. 13 out of 15 indicator species in forest and grassland plots were both more abundant and more likely to flower/fruit in areas protected from deer. Although species composition and abundance data alone do not demonstrate that deer has caused the declines, the combination of data on indicator species (including clear signs of deer browse damage) strengthens the case. Effects ripple to communities and tropic levels beyond plant populations, however, because the affected species provide key resources for a range of insect, small mammal, and bird species.
Deer Browse at the Leonard Preserve Sept 13, 2014
You can see the difference between fenced in area and that available to deer.