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Friday, July 22, 2011

Deer Herd Management

Deer have the ability to reproduce and expand their numbers at an almost exponential rate. A classic example of deer herd growth potential is documented at the George Reserve in southern Michigan.This area is a 1,200-acre tract enclosed by an eleven-foot deer-proof fence. In 1928, six deer (2 bucks and 4 does) were released inside the area. Six years later, a drive count yielded a minimum population of 160 deer (Hickie 1937). The growth of the George Reserve herd reflects a mathematical model known as the logistical equation (Caughley 1977).
This model is characterized by an S-shaped curve reflecting how factors such as reproductive success and mortality affect a population. During the early stages of population growth, deer numbers are low and quality forage is abundant. Consequently, mortality is low and reproductive output is high. As the population increases, so does competition for quality forage and other habitat components. This increased competition leads to lower reproductive output and fawn survival. The fawn recruitment rate eventually reaches a point where it equals the mortality rate and the population stops growing. Physical condition of the herd is usually poor and disease problems may be chronic. A deer herd at this point has reached absolute carrying capacity or CC K.
The population level at absolute or CC K consists of the maximum number of animals the habitat can support. At any level above CC K, plants in the habitat are utilized at a rate greater than they can sustain. In terms of deer management, the term reasonable carrying capacity (RCC) may more accurately describe the maximum number of animals acceptable relative to herd quality, habitat integrity, and other social constraints. RCC is reached at a population level that is lower than CC K. At RCC, the population level is at the upper limits of the habitat’s capacity to sustain the population in good condition throughout the year. RCC takes into consideration seasonal fluctuations in habitat quality, impacts to other wildlife species, and human considerations.
The example of the George Reserve relays the importance of controlling deer population levels. Deer managers should be cautioned that maintaining a deer population at carrying capacity is a risky and often costly proposition. The levels at which deer populations should be maintained depend on land use objectives, human dimensions, and overall herd management objectives. It may be desirable to keep deer numbers low in order to reduce problems associated with crop damage, disease, and accidents. Independent of these considerations, deer populations should always be managed to meet some goal relative to management objectives, herd health, and the protection of habitats and ecological integrity.
In the absence of sufficient predator populations, the work of maintaining deer populations at appropriate levels has shifted to the modern hunter. The most effective way to regulate deer populations is through hunting. Failure to control deer numbers always results in overpopulation and habitat degradation that affects not only deer, but also many other animals. For example, many species of neotropical migrant birds are impacted by excessive deer herd densities and the resulting overbrowsing of important food and nesting flora. Proper regulation of deer populations ensures critical habitat components are protected for numerous wildlife species. Hunting should be appreciated for the cultural and societal benefits it provides, as well as the effective management tool it has become.
As with any tool, hunting can be applied improperly or inefficiently. It is the job of state wildlife agencies and biologists to provide the regulatory framework and, along with research universities, the management information that ensures the most efficient application of hunting as a management tool. In the years since subsistence hunting largely disappeared, there have been numerous advances in the field of scientific wildlife management. There also are numerous approaches to the management of deer through legal hunting. Some of these approaches have served the public and the deer herd very well, while other methods have resulted in poorly managed and unnatural deer herds. Today the principles of proper deer management are well defined and effective. Implementing these management techniques often entails overcoming the obstacles of popular deer lore, people’s resistance to change, and user groups with conflicting objectives.

The primary objective of deer managers should be to maintain a deer population within the bounds of the reasonable carrying capacity. Beyond this, management objectives may include the production of mature bucks, balancing adult sex ratios, or maintaining a maximum deer harvest. Deer harvest data is most effective in determining whether a population is within the reasonable carrying capacity. If harvest data indicates too many deer for a unit of habitat, an aggressive harvest of deer—especially does—should be implemented to reduce the population to a more compatible level. Continued monitoring of harvest data will assist in determining when the population has been reduced to the appropriate level.
Often, an aggressive doe harvest is the fastest and most efficient method to reduce overall herd densities. In addition to simply removing excess deer numbers, harvest of female deer limits reproductive output and works to balance adult sex ratios. In cases of gross overpopulation, greater numbers of deer should be removed regardless of sex. Once a population has been reduced to a level within reasonable carrying capacity (RCC), approximately one-third of the herd must be harvested each fall to maintain this population level. Within the annual one-third harvest, at least half of the deer taken should be females. Any significant departure from this basic harvest regime will result in population growth and herd densities that exceed RCC.
Deer populations respond to varying mortality rates by decreasing, increasing, or remaining stable. A deer herd will continue to grow with annual mortality rates of less than 35 percent. The rate of growth will depend on how far below 35 percent the annual mortality rate actually is. For example, an annual mortality rate of 20 percent allows for rapid population growth while at 30 percent, population growth may be more gradual. With pproximately 35 percent annual mortality, a population will generally remain stable. With a 40 percent annual mortality rate, the total population will decline; at rates greater than 40 percent this decline becomes more pronounced. The effect annual mortality has on a population also depends on how the population is structured with respect to adult sex ratio and on how the annual mortality is distributed between both sexes. Reproductive output and recruitment also influences the net effect of annual mortality rates.
Population models have shown deer herds produce the greatest sustained yields when maintained at approximately 40 to 80 percent of the estimated carrying capacity (Downing and Guynn 1985). At these herd densities, the highest harvest rates may be achieved without compromising habitat integrity. Reproduction and recruitment will exceed natural mortality significantly at these levels, thereby providing the optimum range for a sustained annual harvest of deer. Studies have shown peak harvest rates are achieved at deer densities of 50 to 60 percent of carrying capacity (Downing and Guynn 1985). At this level, allocation of resources in habitat and fawn production/recruitment is maximized. All deer in the herd will develop to the potential of the habitat’s nutritional ceiling. In areas of suboptimal habitats with inherently lower carrying capacities, this level may be too low to provide acceptable hunting satisfaction because deer sign and sightings may be reduced.
-By Chris Cook and Bill Gray, Wildlife Biologists

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