Published: Saturday, November 17, 2007
Deer harvest is based on biological factors
Sustaining populations subjected to hunting pressure requires species capable of rapid and consistent reproduction. Deer qualify. Over the last century, studies of deer biology and its response to hunting pressure have resulted in thousands of research papers, books, symposia and countless graduate degrees in wildlife biology.
Two classic cases, familiar to every wildlife biologist, stand out as the foundation of deer management in North America.
From 1906 through 1923, government hunters and trappers removed predators from the Kaibab Plateau along the north rim of Arizona's Grand Canyon. During that period, agents removed 781 mountain lions, 30 gray wolves, 4,889 coyotes and 354 bobcats.
The Kaibab mule deer population responded in spectacular fashion. When predator control began, the mule deer population was estimated to be 4,000. Under greatly reduced predator pressure, the deer herd exploded to more than 100,000 in less than 20 years. During the winter of 1924-25, the population crashed. Ever since, wildlife biologists consider "Kaibab Plateau" to be synonymous with the dangers of predator control as a way to protect game populations.
Rarely, however, is population biology so simple. In 1970, Graeme Caughley suggested that the mule deer population explosion on the Kaibab Plateau was due more to the recovery of the range than to the disappearance of predators. It turns out that while predators were being removed from the area, so were domestic livestock. At least 200,000 sheep and 20,000 cattle were removed at the same time the predators were being removed.
Caughley argues that deer flourished not because predators disappeared, but because the recovering range provided a virtually unlimited food source. We'll probably never know which factor was more important, but a case can be made that both predator control and unlimited food can trigger a population explosion.
Just a few years after the Kaibab "experiment," another classic field study unfolded. In the 1920s a Detroit industrialist, Col. Edwin S. George, purchased 12 adjoining farms, a total of 1,146 acres, in southern Michigan. In 1930 he donated the property to the University of Michigan for use as a natural laboratory, though he maintained one house and 40 acres for his personal use. After George's death in 1940, his heirs gave complete ownership and control to the university.
In the 1920s there were virtually no deer in southern Michigan. In 1927 Colonel George erected a 7.5-foot-high fence around the entire site, and the following year, he released six white-tailed deer, two males and four females, inside what today is known as the George Reserve. The deer were trapped on Grand Island in Lake Superior, and the does were thought to be pregnant.
The George Reserve deer herd thrived. Six years later in 1933, biologists conducted the first annual deer drive census to assess the population. They counted 130 deer; in six years the population grew from six to 130, an astounding rate of population growth.
Since that initial deer drive census, this deer population has been intensively studied and manipulated. Various rates of hunting pressure enabled biologists to study and evaluate white-tailed deer population growth under a variety of conditions. Dale McCullough, a University of Michigan professor, published the results of decades of research in "The George Reserve Deer Herd" (1979, University of Michigan Press).
The results of the Kaibab and George Reserve field experiments demonstrated that deer populations, when provided with quality habitat and minimal predation, can grow explosively. Biologists call this exponential population growth. Theoretically, all species grow exponentially until they reach limits imposed by the environment (habitat quality, food availability, space, predator pressure, etc.). Reproductive characteristics such as gestation period, litter size, litters per year, and age at first breeding determine how quickly such a population explosion can occur.
The innate reproductive capacity of deer and other game species is the biological basis for annual harvests. By monitoring population numbers and habitat quality, biologists can adjust hunting pressure on game species annually as populations fluctuate.
Clearly, wildlife management is complicated. This is probably the best reason politicians, unschooled in population biology, should resist the urge to meddle and allow professional biologists to do their jobs.
Send questions and comments to Dr. Scott Shalaway, R.D. 5, Cameron, WV 26033 or via e-mail to firstname.lastname@example.org.