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
Habitat-population density relationships for white-tailed deer in Illinois
John L. Roseberry and Alan Woolf, Wildlife Society Bulletin, 2008
White-tailed deer have reached population densities in the agricultural Midwest that would not have been predicted 20-30 years ago. To help explain this phenomenon, we inventoried and analyzed potential deer habitat in Illinois, using classified satellite imagery and a proximity-based habitat model. Statewide prehunt deer densities (ca 1992) were estimated at 4-5 deer per km2 of total area and 30-37 deer per km2 of forest based on population reconstruction and modeling. Habitat suitability indices explained 81% of the variation in deer population densities at the county level. The amount and distribution of deer habitat in Illinois was primarily dependent on intensity of agricultural land use, which in turn was dictated by soil productivity and terrain. We found no evidence that relative use of available habitat at the county level was adversely affected either by habitat fragmentation or human presence on the landscape.
White-tailed deer (Odocoileus virginianus) browse damage in Ann Arbor, Michigan;
Bird Hills Nature Area, Winter 2015
Jacqueline Courteau, Ecologist, Moriah Young, University of Michigan, Independent Study Research This survey of 142 tree saplings (less than 2 meters tall) and shrubs in Bird Hills Nature Area shows that 80% have been browsed by deer, and 51% have half or more branches browsed. This level of browsing could interfere with forest regeneration and diminish the flowers and fruit available for birds, butterflies, and bees. Further monitoring would be necessary to track mortality, to reveal whether particular tree and shrub species of concern are browsed in future years, and to assess whether wildflower species are also being heavily browsed.
The deer browse figures compiled in this survey may underestimate actual browse damage in several ways. First, the survey excluded plants that were already dead or lacked live buds for identification. Many of the excluded plants showed clear signs of deer browse, which suggests that browse damage could be contributing to mortality, but estimating browse-related mortality was beyond the scope of this study. Numerous other studies suggest that browse damage over several decades may already have eliminated or greatly reduced populations of deer-preferred species (Côté et al. 2004, Rooney and Waller 2003, Ferker et al. 2014). Second, it is not possible to count how many buds are missing from a plant, so we focused on the number of branches browsed. However, some unbrowsed branches were counted even if they were quite small, while the portions of branches browsed off may have been larger than those that remained. Third, we assessed browse damage on all species, rather than on a set of species known to be preferred by deer; damage on preferred species could be even higher.
Sustainable Management of White-Tailed Deer and White-Cedar
Laura S. Kenefic, Jean-Claude Ruel and Jean-Pierre Tremblay
The Wildlife Society, Oct 5, 2015
Using recent and historical regeneration data from the Forest Service’s permanent sample plots, Larouche and her colleagues compared the dynamics of white-cedar to those of companion species over a 40-year period and found that white-cedar seedlings have not progressed to larger-size classes over time, while seedlings of other species have grown into sapling and merchantable-size classes (Larouche et al. 2010). Furthermore, density of white-cedar has declined in the study area since the 1960s, with close to 90 percent of sampled white-cedar seedlings and large saplings having evidence of deer browsing in 2005.
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.
Long-Term Regional Shifts in Plant Community Composition Are Largely Explained by Local Deer Impact Experiments
PlosOne, Dec 31, 2014
The fact that herbivores and predators exert top-down effects to alter community composition and dynamics at lower trophic levels is no longer controversial, yet we still lack evidence of the full nature, extent, and longer-term effects of these impacts. Here, we use results from a set of replicated experiments on the local impacts of white-tailed deer to evaluate the extent to which such impacts could account for half-century shifts in forest plant communities across the upper Midwest, USA. We measured species’ responses to deer at four sites using 10–20 year-old deer exclosures. Among common species, eight were more abundant outside the exclosures, seven were commoner inside, and 16 had similar abundances in- and outside. Deer herbivory greatly increased the abundance of ferns and graminoids and doubled the abundance of exotic plants. In contrast, deer greatly reduced tree regeneration, shrub cover (100–200 fold in two species), plant height, plant reproduction, and the abundance of forbs. None of 36 focal species increased in reproduction or grew taller in the presence of deer, contrary to expectations. We compared these results to data on 50-year regional shifts in species abundances across 62 sites. The effects of herbivory by white-tailed deer accurately account for many of the long-term regional shifts observed in species’ abundances (R2 = 0.41). These results support the conjecture that deer impacts have driven many of the regional shifts in forest understory cover and composition observed in recent decades.
Deer Browse at the Leonard Preserve Sept 13, 2014
You can see the difference between fenced in area and that available to deer.
Hunting gives deer-damaged forests in state parks a shot at recovery
Phys.org, July 9, 2014
A research team led by Michael Jenkins, associate professor of forest ecology, found that a 17-year-long Indiana Department of Natural Resources policy of organizing hunts in state parks has successfully spurred the regrowth of native tree seedlings, herbs and wildflowers rendered scarce by browsing deer.
“We can’t put nature in a glass dome and think it’s going to regulate itself,” he said. “Because our actions have made the natural world the way it is, we have an obligation to practice stewardship to maintain ecological balance.”
Indiana state parks historically did not allow hunting. But by the 1990s, white-tailed deer populations in parks had swelled to such size that many species of native wildflowers such as trillium and lilies largely disappeared, replaced by wild ginger and exotic species such as garlic mustard and Japanese stiltgrass, plants not favored by deer. Oak and ash tree seedlings gave way to highly deer-resistant or unpalatable trees such as pawpaw.