Populations Dynamic Equilibrium
Objective:
This activity points out that populations do not remain stable but tend to fluctuate. Here we will study some of the interrelationship between predator and prey population fluctuation. The concept of carrying capacity and its relation to population controls will be stressed.
Introduction:
There will be three parts to this activity. In the first section, you will deal with the concepts of a population that fluctuates on a regular basis, a population of hares (rabbits). Hopefully you will see very clearly how the cycle (or regular fluctuation) of one population (the rabbits) affects the cycling of a second population (the lynx). The lynx and the hare are very intimately related to one another through a predator-prey relationship. The study of the lynx and hare populations will comprise part “A” of this activity.
Part “B” of this activity will deal with the causes behind the fluctuations of the hare populations and the importance of prey numbers on the equilibrium of an ecosystem.
The data you will work with in Part “C” is taken from a classic blunder in game management. In 1906, the Kaibab Plateau in Arizona was declared a park and all deer hunting was stopped. In addition, a systematic elimination of the predators was undertaken. In 25 years, 781 mountain lions were killed, an addition of 30 wolves, 4,889 coyotes, 544 bobcats, and an unknown number of eagles were slaughtered. As you will see, the data will show the relationship of the ensuing explosion of the deer population and the resulting change in carry capacity of the range. In order to protect range quality and carrying capacity, some populations do not undergo a regular cycling of population numbers. Deer, for example, must be preyed upon or their numbers will increase to a point where they require more food than is available to them. When this happens, the deer will eat everything within reach. The final result is that the vegetation in the area is ruined and the area of range can no longer support a population of the original size. This brings us to the concept of carrying capacity of the number of animals of a given kind that the range will support on a continuing basis. It should be easy to see that if the natural controls on a deer population are removed, the resulting increase in numbers will lower the carrying capacity and eventually there will be fewer deer than there were before the controls were removed.
Populations Dynamic Equilibrium Part “A”
Part “A” Hare and Lynx Populations: The following data was taken from the records of the Hudson Bay Company and is based upon the number of pelts brought in by trappers.
Questions: Please answer the following questions on a separate sheet of paper.
1. Do the population tend to peak at regular intervals? If so, what is the approximate interval?
2. In some cases the lynx population peak comes just after the hare population peak. Explain why this may occur.
3. What reason can you give for there being a higher hare average than lynx average?
4. The carrying capacity is defined as the number of animals that the land can support without harming the land. In most instances, a Wildlife Biologist can approximate the carrying capacity of an area by finding the average population over a number of years under natural conditions. If conditions are natural, the carrying capacity is assumed to be slightly above the average population of the animal. Assume that the carrying capacity is 1000 more than the average for each. In your words, what are some of the factors that would cause the population crashes of the hares? Try to include why the population falls way below the carrying capacity.
5. What are some of the factors that would cause the population crashes of the lynx? Include why the population falls way below the carrying capacity.
6. What would be the probable condition of the vegetation during peak hare populations? During lows in hare population?
7. At what points on the hare population cycle would a newborn hare have the best chances of survival?
8. From your observations of the graph you prepared of the hare, lynx populations, what do you think “dynamic equilibrium” means?
9. What appears to be the main limiting factor of the hare population? of the lynx?
Part “B” A Study of Factors Affecting Fluctuations of Hare Populations
Procedure: Read the story below to answer the questions from this passage.
The snowshoe hare, and important animal in the taiga ecosystem and absent from Interior Alaska fro the past two years is recovering from a population crash that followed a population peak in 1971. For newcomers to the Interior this may be the first chance to see this creature; others will be reminded of the late 1960’s and early 1970’s when hares abounded in willow and spruce thickets of the Interior.
Snowshoe hares, also known as varying hares, have a buffy brown coat in summer and molt to a sleet camouflage white in the winter. A distinguishing feature is their large hind feet, from which their name derives. Found throughout the boreal forests of Alaska, Canada, and the northern United States, snowshoe hares feed on grasses, herbs and leaves from trees and shrubs during summer, and spruced, willow, alder, birch and other woody plants in winter. In turn, the snowshoe hare is a favorite food for lynx, foxes, and several hawks and owls.
Characteristically, snowshoe hares fluctuate in numbers on a 10-year cycle. Populations may be as dense as 2,000 to 3,000 hares per square mile during a peak year,and as low as 1 to 50 per sq. mile a few years later. Such population cycles have intrigued wildlife biologists for years, and despite numerous studies they have remained largely unexplained. Recent studies give some evidence that may help to explain how the cycleworks.
A member of the Museum of Vertebrate Zoology at the University of California, Berkeley, and during the summer, worked for the Institute of Northern Forestry, U.S. Forest Service, at Fairbanks as a wildlife biologist. He studied snowshoe hares from 1972-2000 to develop an understanding of the factors that influence their populations. An important prat of this study has been to examine the impact of hares on plant regeneration, and to determine their preferred foods and habitats. Results combined with what is already known, have helped develop a better understanding of their 10-year cycle.
As population density increases, hares deplete their winter food supply, which leads to malnutrition and eventual starvation. During periods of high population, hares clip twigs up to half an inch in diameter and girdle or debark birch, aspen and willow up to two inches in diameter. Excessive browsing effects were evident following the winters of 1971 and 1972 in Interior Alaska. Such browsing is an indication of food stress that will eventually and drastically stop the population explosion.
Over utilization of trees and woody shrubs has a detrimental effect on these plants. It reduces growth rates and in some cases, kills the plant. This reduces the amount of food and cover available to hares in succeeding years.
Predators such as lynx, goshawks, and great horned owls specialize by feeding on snowshoe hares and have a great impact on high hare populations. These predators are the major mortality factor for the next three to four years and bring the hare population to a low in the cycle. With a low hare population, the number of predators also decreases.
Lynx were abundant in Alaska from 1962-65, and from 1972-75. During these years trappers averaged about 18 lynx each year; from 1967-70 trappers averaged fewer than three lynx each winter. This cycle of lynx abundance was directly related to the abundance of hares.
Goshawks and great horned owls showed a similar response to the hare cycle. Then hares were abundant, there were more birds nesting, and survival of the young was much greater than when hares were scarce. For the past three years, in Interior Alaska, most hawks and owls, which rely on snowshoe hares for food, have not even attempted to nest.
This automatic response of predators to low hare numbers reduces predation pressure, allowing the hares to increase again.
During the increase phase of the hares cycle, a female snowshoe hare may have as many as 15 young per year, whereas during the population crash, the birth rate may decrease to seven of eight young per female per year. During the increase, female hares normally have three litters during a summer, with an average of five per litter. During the crash, two litters a year can be expected, with about four in each litter. The reduced reproduction of the adults following a winter of food shortage. With smaller populations as plentiful food, adults are in better condition in the spring and they are capable of having more and larger litters.
In Interior Alaska, snowshoe hares peaked at about 1,600 per square mile in the fall of 1971. They were still abundant in 1972, but they crashed to a low of 30 hares per square mile in the winter of 1974 -75. The survival of hares from 1975 through 1976 has been good and hare density last fall (1976) was about 175 hares per square mile. Interior residents can expect to see high numbers of hares in the next few years, with peak densities around 1980 and 1981.
Although snowshoe hares are coming back, the population is still fairly low when compared with the 1971 and the projected 1981 populations. The animals are still confined to their preferred habitats of black spruce or willow-alder thickets. In the next few years, as they increase, hares will start to appear in towns and in open areas. Lynx and other predators will also make a gradual comeback, until eventually the cycle will be completed once again.
Questions: Please answer the following questions on a separate sheet of paper.
1. What two names are used for this rabbit and why?
2. On a 10-year cycle, what is the maximum and minimum of rabbits per square mile?
MAXIMUM: ______MINIMUM: ______
3. If one were walking through the woods and saw twigs up to a half and inch and small trees girdled, what could one guess about the density of the rabbit population?
4. What natural factors did the article say forced an overcrowded rabbit population back down?
5. What happens to the litter size or reproductive rate of both predator and prey when the rabbit population crashed?
6. Study the graph from part A. When can one expect a peak in the rabbit population again?
Part “C” Kaibab Deer Populations
Procedure: 1. Use the following graph of the results of the removal of the predators from the Kaibab forestand the introduction to answer the Part “C” study questions.
*There was a program of no hunting and predator elimination instigated between 1905-1910 *
Questions: Please answer the following questions on a separate sheet of paper. Use a word processor document to type out the question and your answers to the questions given below.
1. Define the term range
2. Between 1910 and 1940, what happened to the carrying capacity of the Kaibab range? Why? (hint: refer to the introduction)
3. What would have happened to the mountain lions of the deer had been killed off instead? How would the carrying capacity of the range been affected?
4. There are relatively few mountain lions or wolves left anywhere today. What takes their place as a population control for deer?
5. If the carrying capacity of a deer range is sharply reduced, how do you think this might affect othernon predators in the area? (e.g. - Birds, Squirrels, etc...)
Part “D” Summary
1. Describe the relationship of a cycling prey population to the predator population.
2. Define: Carrying capacity
3. Explain how predation may be an advantage to both the predator and the prey species.
4. Define density-dependent and density-independent factors to population growth. List and explain the density-dependent and some potential density-independent factors to population fluctuations in the hare and lynx and the Kaibab Deer populations..