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| Yellowstone Park Ranger with ParKids near Mammoth Hot Springs |
Subject
Science
Objectives
The student will
Display understanding that fluctuations in the numbers of an elk population are due to numerous complex factors.
Build a conceptual model depicting stressors and their influences upon an elk population.
Background
Located in the northern part of Yellowstone National Park, the Northern Range sustains one of the world's largest and most diverse populations of free-roaming large animals. For more than 80 years, a debate has centered on the issue of how many elk should be there.
Beginning in the 1920s, managers expressed concern about overgrazing of the range and other effects the elk were thought to be having on their habitat. Control of predators further complicated the relationship of elk with their environment. Early park rangers hunted all predators in the park, including coyotes, mountain lions, and wolves. Between 1914 and 1926, 136 wolves were trapped, shot, or poisoned, effectively eliminating the elks' chief predator.
From 1935-1968, park managers began reducing the number of elk and other ungulates from the Northern Range by trapping or shooting. These removals lowered the elk population from approximately 12,000 to 4,000 animals. However, when direct controls inside Yellowstone National Park were discontinued, elk numbers on the Northern Range increased to approximately 19,000 animals by 1988.
Recently, the restoration of wolves has led to a debate that there may soon be too few elk because of wolf predation. This debate increased in 2002 when numbers of the northern Yellowstone elk herd were the lowest recorded during the last thirty years (approximately 11-14,000 animals).
Most scientists believe fluctuations in wildlife populations are, in fact, due to a number of factors and are extremely complex. Variables affecting an elk population include winter severity, drought, predation (principally by wolves and grizzly bears), and human hunting.
- Severe winters in Yellowstone (indicated by ice, low temperatures and snowfall) typically occur every 5-10 years. This stressor has the greatest effect on calves, 1-2 year old elk, and older elk (greater than 12 years).
- Drought occurs at variable intervals and has the greatest effect on calves (both newborn and those still unborn).
- Predation by wolves, occurring year-round, annually, has the greatest effect on calves, 1-2 year olds, and older elk.
- Predation by grizzly bears occurs annually, though primarily only in the spring. The greatest effect is on calves.
- Human hunting occurs annually, though only in the fall/winter. The greatest effect is upon the prime age elk (3-12 years).
Download an Example of Conceptual Model of Factors Affecting an Elk Population
Get Adobe Acrobat
Procedure
The instructor will
1. Ask students to consider stressors that might influence numbers in an elk population. Write responses on blackboard.
2. Lead a discussion with students in which they consider the frequency of occurrence of each stressor. Write responses on blackboard.
3. Ask students which segment of the elk population [(calf, 1-2 year olds, prime age adults (3-12 years), or older animals (more than 12 years)] would be most affected by each factor.
4. Instruct students to create a conceptual model showing all stressors (with their frequency of occurrence), drawing lines from each stressor to the segment of the population they most strongly affect. (See Example of a Conceptual Model of Factors Affecting an Elk Population for an example.)
5. Test student understanding of population dynamics by asking the following questions:
- Which segment of a population is primarily responsible for the population to persist? (adult prime, needed for procreation)
- Which segment of a population is primarily responsible for population growth? (calf)
6. Ask students to use their models to answer the following questions:
- Which segment of the population has the highest mortality rate? (calves, affected by four separate stressors)
- Which segment of the population has the highest survival rate? (prime age adult, affected by only one stressor)
- Under what circumstances will a population grow? (decrease of winter severity, decrease of drought, decrease of calf predation, decrease of human hunting)
- Which stressor can be controlled outside of Yellowstone National Park (human hunting)
Assessment
Student conceptual model
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National Science Standards for Grades 5-8
NS.5-8.3
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| Funding for this trip was provided by Canon U.S.A., Inc., through The Yellowstone Park Foundation. |
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| Web server services are funded through generous grants to The Yellowstone Park Foundation. |
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