May 2010
A female white-tailed deer, with characteristic tail color, at Snowshoe Mountain, West Virginia. Photo: Moxfyre
White-tailed deer (Odocoileus virginianus) rank among the most charismatic wildlife in North America. Emblematic of the countryside, a deer sighting signifies we left the busy city behind. Spotted fawn sightings evoke images of Bambi, and such sightings serve as harbingers of summer. In recent decades, deer numbers have grown. In some places, deer are so numerous that they are degrading their own habitats and causing conflicts with people. Consider:
There are an estimated 30 million deer in the United States today. Under optimal conditions, deer populations will double every two years.1 They can reach densities of over 1 deer per hectare (100 deer per square kilometer).2
Over 10 million people spend nearly $6 billion to hunt deer in the United States each year.3 Over 6 million deer are killed during the hunting season, based on state agency records. If each deer provides an average of 22 kg of meat, this amounts to 132,000 metric tons. An estimated 12 million fawns are born a few months after hunting season.
In 2008, over one million deer collided with cars and motorcycles in the United States. According to estimates by the Insurance Institute for Highway Safety, this resulted in the death of 150 people, injuries to 29,000 others, and an estimated $1.1 billion in vehicle damage.4
An average deer will eat nearly 500 kg of vegetation each year.5 The entire population of deer in the United States will eat the equivalent of 15 million metric tons of vegetation per year, which is greater than the combined weight of all aircraft carriers in the U.S. Navy!
One recent study of 13 northeastern states revealed deer caused $248 million damage each year to agricultural crops, nurseries, and landscaping.6
In parks and other natural areas, deer consume wild plants. Over time, spring wildflower displays diminish. Plants highly prized by deer decline or disappear entirely. In some places, deer are responsible for the disappearance of over half of all plant species—in just a few decades.7,8
Deer play a role in the transmission of tick-borne infections to humans, including Lyme disease.9
It is quite possible that deer provide more economic and aesthetic benefits to people than any other mammal in North America. At the same time, they may cause more harm and injuries to people than any other North American mammal. Confronted with the mixed blessing of white-tailed deer, wildlife biologists are broadening the scope of the discipline of wildlife management.
The problems posed by white-tailed deer overabundance are by no means unique. There are several cases of overabundant populations of moose (Alces alces)10 and elk (Cervus elaphus)11 in parts of their native range in North America that lack native predators. Sika deer (Cervus nippon) have caused extensive damage throughout parts of Japan.12 Both native and introduced deer have caused extensive damage to woodlands and plantations throughout Europe, and introduced deer and elk have reshaped temperate rainforests in Canada, Chile, and New Zealand.13
The fall and rise of white tails in America
With an estimated 30 million white-tailed deer in the United States today, it is difficult to believe these animals nearly became extinct just over a century ago. The United States was undergoing a profound transformation, facilitated in part by a rapidly expanding railroad network. In some ways, environmental changes benefited deer. The bounty hunting of gray wolf (Canis lupus) and cougar (Felis concolor), and the subjugation of indigenous peoples, reduced predation pressure on deer. At the same time, market hunting, weak enforcement of game laws, and habitat loss conspired to drive deer to dangerously low numbers by the late 1800s. By the early 1920s, deer were extinct in some states—including Kansas and Indiana. They were endangered in many others.14
Active conservation efforts, including hunting restrictions, conservation law enforcement, predator control, game refuge creation, and deer reintroduction were used to restore endangered or extirpated deer populations. These conservation efforts were initially financed by governments and were later financed in part by deer hunters through state licensing revenues. Deer populations responded by increasing 1–2 orders of magnitude throughout their range over the next 100 years. Even today, the recovery of white-tailed deer is a premier example of successful wildlife recovery and management in the United States.15
Initially, deer hunters and state wildlife agencies were partners in this recovery. Hunters provided funding and political will, while the state provided increasing numbers of deer. Over time, deer populations grew to the delight of both hunters and state wildlife agencies15; however, new stakeholders emerged:
- Animal rights groups objected to sport hunting.
- Farmers wanted more control over the deer on their properties that were eating their crops.
- Park managers wanted more say in the numbers of deer on the lands they managed.
- Public health officials expressed concern about the relationship between deer abundance, tick-borne diseases, and human health.16
Such disparate views confounded wildlife managers. Serving the public became a lot harder when there were multiple competing interests.17
Historically, deer densities varied from 2–6 animals per square kilometer (km-2).18 Today, white-tailed deer inhabit a range of natural, semi-natural, and anthropogenic environments, and they can reach densities in excess of 40 km-2, in the absence of hunting or predation.19 Even where deer are managed through sport hunting, populations typically exceed 10 km-2, even though negative impacts to vegetation and fauna become apparent at lower densities.20 Recreational hunting remains the primary mechanism that limits white-tailed deer throughout most of their range.21 This leads us to an obvious question: why are deer densities increasing? There are several reasons:
A stately male white-tailed deer is on the alert at Quivira National Wildlife Refuge. Kansas. Photo: Jerry Segraves.
Absence of predators.22 Historically, wolves and cougars preyed on deer year-round. Wolves and cougars are now absent from most of the eastern United States. Other predators of deer—black bears (Ursus americanus) and coyotes (Canis latrans)—typically prey on fawns but not adult animals. Additionally, unlike wolves and cougars, deer are not their primary prey item.
Declining numbers of hunters.21 The number of hunters in the United States has been declining since the 1970s. Fewer hunters in the woods during deer season will translate into larger deer populations over time.
Expansion of “no hunting” zones.21 Where deer hunting is prohibited, deer populations can grow and spread to surrounding areas. Parks, open green space, small towns, and in many cases, suburban developments are de facto deer sanctuaries. In rural areas, private landowners that once welcomed deer hunters are increasingly turning them away, due in part to legal liability concerns.
- Habitat changes. The agricultural fields and pastures, orchards, fertilized and well-watered suburban landscapes, as well as parks provide a greater abundance of high quality food for the deer than the eastern deciduous forest they replaced. Greater habitat productivity leads to greater numbers of deer.23
These factors combined create conditions favorable for the maintenance of high deer densities for the near future.
How many deer is too many deer?
In an ideal world, biologists would have a simple common practice to guide deer management. In recent years, some biologists have regarded 8 deer per square kilometer as the maximum number a habitat can support long-term. This is complicated greatly by habitat quality, however. It is likely some habitats could support much more than 8 deer km-2, whereas other habitats could not support 4 deer km-2 in the long-term. Biologists instead try to determine whether a deer population is overpopulated in a particular region. From a management perspective, deer populations can be considered overpopulated if any of the following six conditions are met24-25:
Deer negatively impact vegetation structure and composition, local fauna, or soils or other physical features of the environment.
Deer populations exhibit a poor average body condition in terms of body mass, reproductive rate, or trophy scores.
Individual deer have unusually high parasite loads or infectious disease prevalence.
Deer are transmitting disease to humans, livestock, or other species.
High deer population densities cause significant economic losses in agriculture, horticulture, forestry, or residential gardens and landscaping, as well as other property damage, like vehicle damage from deer collisions.
Deer population densities are associated with a significantly increased risk of injury or death to humans—primarily via deer-vehicle collisions.
How are deer impacts and deer populations managed?
There are two approaches to mitigate deer impacts:
- reducing damage, and
- reducing the population
Fences, motion gear, and other systems can reduce damage done by deer.
With the first approach, managers focus on reducing damage caused by deer. For example, plants are protected with fencing or repellents. Unwelcome deer are startled by motion-activated sprinklers, flashing lights, or noise. Homeowners and builders landscape with deer-resistant plants. Road signs warn drivers that deer-vehicle collisions are common. Animal detection systems warn drivers when deer approach roadways. Large right-of-ways are cleared along roadsides to improve the sight distance for drivers, as well as to reduce browse and cover plants close to roads. This type of mitigation focuses on deer impacts, not on deer numbers.
With the second approach, managers focus on reducing deer numbers. Deer population management can be classified as non-lethal or lethal. Non-lethal management includes natural regulation and trap and relocation. In urban and suburban communities, non-lethal management is often perceived by some members of the public as the safest and most humane option. Under natural regulation, the deer population is not manipulated.25 Individuals affected by deer populations are left to take countermeasures. So for example, gardeners might build deer fences around their plants, drivers slow down in, or avoid driving through, areas with where vehicle collisions are likely, and park visitors take extra care to avoid ticks. Deer impacts are lessened and tolerated.
Non-lethal management
- Trap and relocation. Trap and relocation involves capturing deer in one location and moving them to another. Deer can die from stress during capture or translocation, and there are seldom release sites that need deer. Relocated deer often continue to pose problems in their release environment. Trap and relocation often amounts to exporting the problem and involves extensive collateral damage.26 More significantly, moving deer across state boundaries is no longer allowed, given the recent concerns involving Chronic Wasting Disease [a neurological disorder that affects hoofed animals—similar to Mad Cow Disease in cattle].
- Reproduction control. Reproduction control techniques—like surgical sterilization and contraception—seek to limit deer impacts by controlling deer population fertility. Sterilization is a permanent procedure, while contraception relies on implants or hormones to limit deer fertility. Fertility control is most cost-effective in closed deer populations.27 In other words, the deer population needs to be relatively cut off from larger populations, with few if any new fertile immigrants entering the population. Examples of closed populations include fenced government installations and islands. Additionally, fertility control is a long-term proposition. To stabilize populations, at least 50% of all fertile deer need to be treated each year, and the fertility control program must operate for at least a decade.28 Although free-ranging deer may not be able to reproduce, they may continue to cause conflicts—for instance, they may cause agricultural damage or collide with vehicles. Fertility control efforts that follow an initial period of lethal control could be more successful, less expensive, and easier to administer in the long-term. Although reproductive control has been demonstrated to be effective for individuals, there has yet to be a cost-effective, landscape-scale demonstration for free-ranging deer.
Lethal management
Lethal management includes recreational hunting, controlled hunts, and predator reintroduction. Recreational hunting encompasses most of the deer harvesting that takes place within the confines of a scheduled hunting season. Hunters play a critical role in limiting the growth of deer populations.21 Demographic studies, however, reveal a steady decline in the number of deer hunters in the United States, and this trend is expected to continue.21 Moreover, some hunters are unwilling to adopt hunting strategies that limit deer population sizes (such as harvesting female deer).15,29 Some researchers are starting to question whether deer hunters will be effective in limiting deer population sizes over broad areas in the future.21
Controlled hunts. Controlled hunts are conducted in small areas—such as a park or municipality—and they are usually managed to reduce the number of deer present drastically. This is accomplished using volunteer hunters or paid sharpshooters. Controlled hunts are often very effective in rapidly decreasing deer population sizes,30 as long as the targeted area is small. Attempts to conduct controlled hunts over broad geographic areas (hundreds to thousands of square kilometers) with a mix of public and private lands have been less successful.31
Reintroduction of predators. The reintroduction of predators, such as the gray wolf and the cougar, has not been widely tested. Cougars are still largely absent east of the Mississippi River. Wolves have successfully recolonized northern Wisconsin and Michigan in recent years, but no compelling evidence exists yet to indicate the wolves have significantly reduced deer numbers in those regions. The reintroduction of large predators is controversial,32 and it is not clear if successful reintroductions would reduce deer numbers. Coyote and black bear are important predators of deer fawns,33 and they can account for over 50% of fawn mortality in some years and in certain places.34 It is not clear how this translates to deer population numbers; for example, some parks in the Midwestern U.S. have large coexisting deer and coyote populations.
Unfortunately, there is not a “one size fits all” deer management strategy. Decision-makers often need to forge a path forward, addressing the hopes and fears of stakeholders, while at the same time, staying within a budget. Furthermore, any plan must be within the realm of sound science.
Conclusion
Over the past one hundred years, deer management has changed radically. Initially, wildlife managers focused on “too few” deer. Today, wildlife managers have the problem of “too many” deer in some places, and the right amount of deer in others. Some hunters want more deer; yet, many farmers, foresters, and suburban dwellers want less. Deer population management has become a long-term proposition with high stakes. Hunters, foresters, farmers, property owners, city councils, drivers, naturalists, and animal rights activists need to better understand each others’ interests. To the extent that wildlife management becomes more participatory in the future, these groups will need to work together to devise effective deer management and impact mitigation strategies.
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