CASE STUDY: MULE DEER POPULATION DYNAMICS

IN THE KAIBAB PLATEAU, ARIZONA

The environment may be altered by forces within the biotic community as well as by relationships between organisms and the physical (abiotic) environment. The carrying capacity of an ecosystem is the maximum number of organisms that an area can support on a sustained or continuing basis. The population density (number of individuals per unit area) may produce such profound changes in the environment that the environment becomes unsuitable for the continued survival of that species. Humans can also interfere with natural interactions of species with their environments with either positive, negative, or neutral effects. This activity will show how these interactions affect a population of mule deer in Arizona.

OBJECTIVES:

  1. to produce a line graph using data on the Kaibab deer population of Arizona from 1905~1939
  2. to analyze the results and consequences of human interventions responsible for the changes in the deer population
  3. to be able to define terms such as: carrying capacity, range deterioration, predator, prey, biotic, abiotic, growth curves

BACKGROUND ON THE KAIBAB PLATEAU:

John Wesley Powell, the famous explorer of the Colorado River, was the first in written record to apply the term "Kaibab" to the plateau. "Kaibab" is a Paiute Indian word meaning "mountain lying down." Most of the Kaibab Plateau was withdrawn from the public domain in 1893 as part of the Grand Canyon Forest Reserve. In 1906, President Theodore Roosevelt created the Grand Canyon Game Preserve. This area included lands on both sides of the Grand Canyon and essentially all of the North Kaibab. In 1908, the Forest Reserve north of the Grand Canyon, including the Game Preserve, was renamed the KaibabNational Forest. In 1919, Grand CanyonNational Park was created from the Forest Service lands surrounding the Grand Canyon. In 1934, the TusayanNational Forest south of the Grand Canyon was consolidated into the KaibabNational Forest, forming the present Forest boundaries. The Forest area north of the Canyon became the North Kaibab Ranger District. The Kaibab Plateau is an "island" of forested lands surrounded by the sage and grasslands of lower elevations. With elevations up to 9000 feet, it is bordered on the south by the Grand Canyon; on the east and west by tributary canyons of the Colorado River and on the north by plains that are dissected by tiers of uplifted cliffs.

In 1905, the mule deer population on the Kaibab Plateau in Arizona was estimated to be about 4,000 individuals on 300,000 hectares of vegetated land (or range). Scientists estimated the total average carrying capacity of the Kaibab plateau was about 30,000 deer. On November 28, 1906, President Theodore Roosevelt created the Grand Canyon National Game Preserve to protect what he called the "finest deer herd in America." The first step in protecting the deer was to ban all hunting in the area. Then, in 1907, the Forest Service tried to exterminate the natural predators of the deer, killing approximately 800 mountain lions, 20 wolves, 7400 coyotes, and 500 bobcats between 1907 and 1939.

Signs that the deer population was out of control began to appear as early as 1920. The most important sign was severe and rapid deterioration of range grass and abundance and quality. Unfortunately, by this time, the Kaibab forest area was severely overgrazed by livestock such as sheep, cattle, and horses originating from surrounding ranches. Most of the tall, perennial grasses had been eliminated in the area. The forest service then reduced the number of livestock grazing permits to allow more grass for the deer. By 1923, however, the deer were reported to be near starvation and the range conditions were described as "deplorable."

A Kaibab Deer Investigation Committee recommended that all livestock not owned by local residents be immediately removed from the range and that the number of deer in the herd be reduced by 50 percent (culling) as quickly as possible. Deer hunting was reopened and during the fall of 1924, about 675 deer were killed. These deer represented only 10 percent of the number that had been born that spring!

Today, the Arizona Game Commission carefully manages the Kaibab area with regulations geared to specific local needs. Hunting permits are issued and predators are protected to keep the deer in balance with their range so that the herd size

does not exceed the carrying capacity.

CASE STUDY: MULE DEER POPULATION DYNAMICS IN THE KAIBAB PLATEAU, ARIZONA

Year / # of deer
1905 / 4000
1910 / 9000
1915 / 25000
1920 / 65000
1924 / 100000
1925 / 60000
1926 / 40000
1927 / 37000
1928 / 35000
1929 / 30000
1930 / 25000
1931 / 20000
1935 / 18000
1939 / 10000

MATERIALS:

  • graph paper
  • colored pencils
  • ruler
  • population data

DATA:

PROCEDURE

1. In 1906, the U.S. Forest Service began protecting a herd of deer on a 300000 hectare range on Arizona's Kaibab Plateau. In previous years, the Kaibab forest area had been overgrazed by cattle, sheep, and horses. Graph the Forest Service's data. Plot the year along the x-axis and the population size along the y-axis.

2. In 1906, the Forest Service estimated the carrying capacity of the range to be about 30000 deer. Draw a straight horizontal line across your graph(s) beginning at the 30000 -deer level. Label this line “carrying capacity”.

3. Use the information in #1 and #2 above when you construct the graph. Follow these guidelines when constructing your graph:

  • You should have the correct units and labels for each axis. Don’t forget your title.
  • Select the correct scale so that your graph is not too small on the sheet of graph paper. For each axis determine the maximum and the minimum before you start labeling your axis scale, also think about how much each square should be worth. Don’t forget the line for carrying capacity.

QUESTIONS(Using your graph, answer the following questions)

1. Define carrying capacity and population density?

2. Describe the relationship of the deer herd to the carrying capacity of the range in 1915 (Higher or Lower). What do you think the condition of the vegetation was at that point?

3. Describe the relationship of the deer herd to the carrying capacity of the range in 1920. What do you think the condition of the vegetation was at that point?

4. Describe the relationship of the deer herd to the carrying capacity of the range in 1924. What do you think the condition of the vegetation was at that point?

5. In 1907 the Forest Service banned hunting of deer in the park. Also, they began a 32-year campaign to exterminate natural predators of the deer. Thousands of predators (panthers, coyotes, wolves) were killed. Describe the effects these actions had on the deer herd and on the range. Explain your reasoning.

6. In 1920, seeing that the range was deteriorating rapidly (the vegetation in the park was becoming scarce), the Forest Service reduced the number of grazing permits. Cattle and other livestock were restricted from neighboring ranches were prevented from coming into the park to graze. Describe the effects these actions had on the deer herd and on the range. Explain your reasoning.

7. In 1924, the deer population was on the brink of starvation.Deer hunting was allowed again. Deer shot by hunters in autumn represented about one-tenth the number that had been born the previous spring. What do you think the Forest Service learned between 1905 and 1939? List at least 3 things