SCIENCE LITERACY WORKING WITH DATA
The following graph shows the numbers of Kaibab deer on the isolated Kaibab Plateau in northern Arizona. It was declared a National Game Preserve in 1906, and predator removal was encouraged to protect the deer. Between 1907 and 1937, more than 800 mountain lions were removed. Wolves were exterminated by 1926, and more than 7,000 coyotes and 500 bobcats were also removed. The deer population increased, and by 1915, there was significant damage to the grasses, shrubs, and trees that were being eaten by the deer.
Interpretation
Question 9.15
With the initial level of predators, what was the probable carrying capacity (K) of the Kaibab Plateau for deer at the start of this story?
30,000 deer. Without predators as a resistance factor affecting K, the primary limit on the population was the critical factor of available food.
Question 9.16
Once the population had passed its probable K, would it be able to sustain itself at the higher numbers? Explain your answer.
No. K is based on the ability of a population to sustain itself for a long period. Once a population increases beyond K it begins to affect the environment. In this case, the deer began to damage the environment, leading to a lowering of K as the damaged plants were now less able to grow and support the deer.
Question 9.17
In what 2- to 3-year period did the deer population have the highest reproductive growth rate?
1920-1923 — the part of the line that is the steepest
Advance Your Thinking
Question 9.18
Explain the factors—growth, resistance, and density-dependent and density independent factors—that accounted for the changes in population numbers for the deer in 1905, 1915, and 1930.
In 1905 the resistance factor of predators (density dependent) was being removed, which allowed more deer to survive and reproduce. The growth factor of food (density-dependent) was the primary reasons for the rapid increase of deer; favorable weather also probably played a role (density-independent). These two factors continued to play an important role in 1915, as the population continued to expand. Around 1915 the population exceeded the capacity of the environment to support it sustainably (density dependent). By 1930 the lack of food (density-dependent growth factor) had caused massive starvation and reduced the population dramatically.
Question 9.19
Did Kaibab deer behave like an r-selected species or a K-selected species? Justify your answer.
Kaibab deer only have a few offspring at one time, and are fairly long-lived, but they can have a fawn or two every year for many years, and given the rapidity of the population increase, and its increase far beyond K and subsequent crash, it behaved like an r-selected species.
Question 9.20
Before the predators were exterminated, the habitat of the Kaibab Plateau consisted of meadows and forests with a wide diversity of grasses, shrubs, and trees. How do you think it probably looked in 1935, after 30 years of damage?
By 1935 many to most of the grasses had been eaten to the roots and killed, the shrubs were nearly gone, and the tree leaves had been eaten to above the height that a hungry deer could reach. The bark was also stripped from trees up to the browse height, killing many of the trees.