Grade Level:
Middle School: Sixth Grade through Eighth Grade
State Standards:
Standard 6.4.4 Construct an argument supported by evidence that the stability of populations is affected by changes to an ecosystem. Emphasize how changes to living and nonliving components in an ecosystem affect populations in that ecosystem.

Students first play a board game to learn about resources bighorn sheep need and the threats they face. Outside, students investigate why bighorn sheep are no longer seen along a local trail. Students collect data on other animals in the area by examining tracks. They learn about plants bighorns eat and collect data on food availability. Students also play a bighorn trivia game. Back in the classroom, students put clues together to make a claim about the cause of a population decline in a herd of sheep. 

Essential question: What living and nonliving components to the desert ecosystem affect populations of Moab’s bighorn sheep?

Utah State Science Core Curriculum Topic:

Standard 6.4.4 Construct an argument supported by evidence that the stability of populations is affected by changes to an ecosystem. Emphasize how changes to living and nonliving components in an ecosystem affect populations in that ecosystem.


Populations are groups of organisms that live in the same area. Population size can increase, decrease, or stay the same over time. The stability of populations is affected by many factors, including living and nonliving components to the ecosystem (6th grade textbook). Living components include things like food and predators. Nonliving components are things like weather and geographic factors. In order for populations to be healthy, they must have food, water, and adequate space. When these factors are limited, population size can decrease. Declines in population can happen when more deaths of individuals occur, fewer offspring are born, or when individuals migrate away to other areas with more favorable conditions (6th grade textbook).

Desert bighorn sheep live in the Canyonlands of Utah and the deserts of Arizona, New Mexico, parts of California, and northern Mexico. They are smaller than Rocky Mountain bighorn sheep. Adaptations and life history of Bighorn sheep. (from Hauke, 1998; U.S. Forest Service, Rocky Mountain Region 1995)

Bighorns are generally a medium gray-brown with white on the rump, backs of legs, and muzzle. This coloring allows them to blend in with the rocky landscape surrounding their habitat. Desert bighorn depend primarily on their sense of sight to detect danger. They also have good hearing. Their sense of smell is used to distinguish between foods, detect enemies, and identify their young.

Males are normally larger than females. Adult males, called rams, have a thick, blocky appearance. They have thick necks and large curled horns. Adult female bighorns, called ewes, have small horns. Rams recognize their elders by the size of their horns; dominant sheep having the biggest horns. Average life expectancy is 10-15 years.

The four main needs of desert bighorns include food, water, escape terrain, and space. Their first food preference is Indian ricegrass. They also browse brush and plants with woody stems, especially blackbrush, and sometimes ephedra. They also feed on a few forbs (green leafy plants other than grasses). Water is the primary limiting factor for desert bighorn. Rams can go several days without drinking, getting all the water they need from their food. But ewes, especially during lactation, require a regular source of water. Sources of water include streams, potholes, dew, springs, and water found in food. Desert bighorn prefer open space around their drinking holes so they can see approaching threats. They won’t venture more than a few hundred meters from their escape terrain to get water.

Bighorns use escape terrain to get away from predators, since few animals are able to move as quickly as bighorns through such rugged terrain. Bighorns do not run long distances, but escape their enemies by climbing and hiding in this rugged terrain. Predators of bighorns are coyotes, eagles (feed on lambs), gray foxes, bobcats, and mountain lions. To bighorn sheep, domestic dogs look and smell like predators. Predation is not a primary cause of death for bighorns due to their escape terrain and the variety of wildlife these predators prey upon.

Desert bighorns have a “nursery system.” Two ewes remain with all the lambs on the edge of escape terrain. The other ewes in the band move into open areas to feed on succulent spring plants. Lambs are obedient to the ewes in charge. The feeding ewes return to the lambs on occasion to nurse and exchange places with the nursery ewes. As lambs become older and begin to eat solid foods, they begin to travel with their mothers.

Threats to Bighorn sheep:
Desert bighorns are very sensitive to changes in the environment and these changes can affect the stability of bighorn sheep populations. Threats to bighorn sheep can lead to death or cause changes in populations by impacting reproduction or lamb survival and health. Causes of death include diseases, parasites, tumors, and mineral and dietary deficiencies, as well as accidents, poisonous plants, and extreme climate conditions. Often, several factors interact to cause death. Domestic sheep are the biggest threat to bighorn. They eat similar foods and carry parasites and diseases detrimental to bighorn. Bighorns are known for precipitous herd die-offs. Investigations have attributed the causes to parasites and diseases. Recent management efforts to separate domestic sheep from bighorn range have resulted in decreased die-off. Ear mites cause the most common disease among the bighorn of southeastern Utah. These mites spread down from the head, cause skin problems, and weaken the sheep, making them more likely to succumb to other stresses.

Individual bighorns can become physically run-down or nervous if harassed by other bighorns, large animals, or people, which can contribute to improper diet. Bighorns are sensitive to people on foot, especially in areas where people seldom travel, so they avoid areas with more people (Sproat 2012).

History of local Bighorn sheep populations:
When settlers arrived in the western U.S., there were probably 1.5 to 2 million desert bighorns. In 1975, there were only about 1000 in the state of Utah. The drop can probably be attributed to diseases brought in by European domestic sheep. In the 1970s, grazing leases within Canyonlands and Arches national parks expired. Around this time, the BLM also ended sheep grazing leases on BLM lands near the bighorn range in the parks. Consequently, the populations of bighorn sheep in this area have vastly increased. When Canyonlands was established in 1964, there were 80 bighorn living in the park. Most bighorn were in the Island in the Sky District; a few were in the Needles District. The population of the Needles herd dropped from 125 to 15 in the mid 1980s after domestic sheep were grazed just outside the park. The domestic sheep were removed, and the bighorn population has since rebounded to 50 head. Now, Island in the Sky is home to approximately 350 bighorn. Some Island in the Sky sheep were transplanted to produce the Arches herd, now 125 head, and the Maze herd, now 50-100 head. Recent ongoing studies by biologists indicate that for a herd of bighorns to survive long-term, there must be a minimum of 100 animals.

Bighorns commonly use pathways dictated by the topography of the land. The Colorado and Green rivers create natural barriers that divide herds and prevent the spread of disease. Unfortunately, roadways, fences, and canals built by people tend to cross bighorn travel routes. This can limit movement from feeding grounds to water, causing herds to become isolated. Inbreeding can occur in smaller herds, which weakens the immunity and health of the herds and creates a serious concern for the herd’s longevity. Fortunately, much of the land occupied by bighorn is under the protection of government agencies. These agencies are attempting to implement new regulations on bighorn range that will benefit these animals.

In the 1980s, biologists relocated bighorns from a native population in Canyonlands National Park in order to establish new herds in Arches, Capitol Reef, and Glen Canyon Recreation area. Sheep were captured in nets fired from helicopters, their health and age were assessed, and suitable animals were transported by ground to their new homes. These populations continue to be monitored (Sloan 2011).

During the closure of the White Rim road during the initial lockdowns of the Covid-19 pandemic in 2020, Bill Sloan reported increases in bighorn sheep use of this historically important area where sheep had previously been displaced due to increased traffic. He observed more use of this important area when traffic and visitation was temporarily halted, indicating what a strong effect tourism has on bighorn sheep distribution (RSS report, April 21, 2020).

A Bighorn Adventure (45 min)

Essential Question: What adaptations allow the bighorn sheep to survive? What is the history of the bighorns in Moab? What are threats to bighorn sheep?

Materials: “Fight for Survival” board game with game pieces, cards, and dice; bighorn skulls; bighorn and mule deer track casts; bighorn pelt; laminated pictures of sheep;

Bighorn sheep video link: should be sent to teacher ahead of time

1) Remind students how population sizes can change due to many factors, like predators and resource availability. Discuss the concept of limiting factors in an ecosystem. An abundant amount of some limiting factors (like food) allow a population to grow. A small amount of those same limiting factors causes a population to shrink. An abundance of other limiting factors (like predators) have the opposite effect. Tell students that on their winter field trip, they are going to be learning about population changes of a large animal living in Arches National Park and other places around Moab—the desert bighorn sheep. Ask students if they have seen bighorn sheep before, and if so, ask them to describe where. Likely responses include near the river or in the Rocky Mountains. Follow up by explaining the difference between Rocky Mountain bighorns and desert bighorns and why they are frequently seen near water.

2) Explain that bighorn near Moab used to be abundant, but their population has changed. Ask students to pay attention to the video for the changes to bighorn sheep numbers and what caused those changes. Show students a video of bighorn sheep. Follow up with your own questions about how the change in the population happened and ask what caused the change. Explain that domesticated sheep are farm sheep. Start a list of the threats to bighorn sheep on the board.

3) Tell students that on our field trip, we will consider a mystery about local bighorn sheep. To learn more about them, students will play a board game about the survival of the bighorn sheep. Give instruction, and divide the class into groups. Stress that students must read the cards aloud to the other players because, as a group, they will need the information in the game to solve the mystery of the bighorn sheep.

4) As students play, invite one group at a time to a touch table. Show the skulls and horns to introduce basic sheep biology. Describe identifying features such as brown fur, white butts, split hooves, and curved horns. Explain how to tell the difference between males and females and how to tell the age of the males. Show pictures of the helicopters that moved the sheep, and explain that wildlife managers used helicopters because sheep live in remote, difficult to access places.

5) Once everyone has played for a while, have students do a pair share to list all limiting factors of bighorn sheep. Then lead a class discussion to generate a list of these on the board (and explain each of them). Tell students they will search for clues to what is affecting the bighorn sheep in Moab using this list as a guide during the field trip. (Note: take a photo of each list so each class has a record of their list to refer to on the field trip.)

6) Tell students that the upcoming field trip will involve a moderately strenuous hike, and they need to be prepared. Explain the trail may be snow-packed in places, so all students should wear shoes with good traction. Review appropriate behavior when near steep drop-offs. Emphasize the importance of bringing the right clothes, including layers, hat, mittens, and warm socks. Tell students a thermos with hot soup or a hot beverage for lunch will help to keep them warm.

Essential Questions: Why do we no longer see bighorn sheep on the middle earth trail?

Materials: Three Track Pattern Cards with a description and drawings of one basic track pattern; laminated construction paper “tracks”; big pictures of tracks; pictures of actual sizes of tracks; copies of “Bighorn Sheep Menu” laminated small plant guides; 7 different colors flagging tape or ribbon; dry erase markers (one per student); bighorn quiz game questions and answer sheet; data collection sheets; lists of resources and threats developed by students in pre-trip; three white boards.

1) Have students think back to the pre-trip and remind you of the animal they will be discussing. During the field trip, students will act as wildlife biologists to figure out if this trail is prime bighorn sheep habitat or not. Explain that in the past, sheep have been seen in the area, but not recently. Today, students will attempt to figure out why. Go over four resources sheep need in their habitat, (water, food, wide open spaces, and escape terrain) and tell students we will look for these resources as we hike. Discuss the list of threats students made in the pre-trip, and ask students to look for these as well.

2) Plant Station: (Before the activity, flag several examples of each of the plants on the “Bighorn Café Menu” plant cards.) Gather students. Show students the list of resources. Students should point out where sheep could find water, food, open spaces and escape terrain. For food, tell students bighorn sheep have favorite foods. Ask if anyone had their favorite food for snack. Repeat with foods students kind of like and foods students only ate because it was all they had. Ask what students can do as wildlife biologists to investigate if food limits bighorn sheep.

Tell students you have flagged seven different plants bighorns eat with different colored ribbons. Their task will be to identify the plants. Show the “menu” and tell students they will write the color of the flagging they see on the plant under the description of the plant on the menu. They should then read the information on the cards and find their favorite plant fact. Divide the class into groups of two or three, and pass out a menu and dry erase marker to each group. Identify the first plant together as a class, and give boundaries. Bring the class back together when most of the groups are done, and review the plants. Discuss the bighorns’ favorite foods, and relate them back to the favorite foods of students. Have students share their favorite facts about each category of plants. Discuss factors, such as lack of precipitation or competition from other animals, that might affect the amount of favorite foods and force sheep to eat foods they don’t care for. Ask students if the number of bighorn sheep could be limited by how many and what types of plants are available for them to eat. What might happen to sheep populations if there was not enough food? As scientists, they are going to figure out if this spot is the equivalent of a bighorn sheep’s favorite restaurant.

Give students a copy of their data collection sheet. As they walk, they should tally the plants they see within 2 feet, or an arm’s length, of the trail on either side. At the top of the hill, have students count the number of favorite, OK, and least favorite foods. Students should raise their hands according to the most common group they found. If students found favorite foods the most, lack of food has not driven sheep away.

Ask students if the availability of food is why sheep are not seen in this area. Ask kids to look around as they are hiking the rest of the way to lunch and see if they notice anything else that might affect bighorn’s desire to hang out in this habitat.

3) At the top of the hill: Ask students to listen to the ambient noise for 2 minutes and count instances of human activity, such as cars going past on the roads above and below. Compare their findings with data from observations made in April and September when more visitors were in town.

4) Lunch and Tracking Station: Gather students in a circle. Ask if other animals share their habitat with bighorn sheep? Ask students how animals interact with bighorn sheep. Since we don’t see many animals, invite students to suggest evidence we could find to tell us which animals might interact with bighorns? Encourage students who go hunting with their families to share evidence of animals they have observed.

Tell students three characteristics are used to determine what animal left a track: shape, size, and pattern. Use blown up pictures of tracks as well as plaster tracks to demonstrate differences in size and shape. Have students share similarities and differences they notice. Compare deer vs. bighorn sheep, mountain lion vs. domestic dog, dog vs. coyote. Distance between the tracks can often provide another clue as to what animal left a track. For example, a smaller animal’s tracks are going to be much closer together than a larger animal. Discuss how patterns can also tell a story of what has occurred, such as overlapping mountain lion and bighorn sheep tracks. Use cardboard tracks to replicate several patterns. Explain and demonstrate how the animal moved, and then encourage a few students try to replicate each movement.

As students walk back, challenge students to look for different types of tracks and tally sets of tracks they see on the back side of the data collection cards. Encourage them to use their new knowledge of track, size, shape, distance, and pattern to help figure out who left the tracks. Remind students not to step on tracks or others won’t be able to see them.

At the rocky area, stop and discuss the tracks students observed. Once again, have students raise their hands according to the most common groups of tracks they’ve seen. Discuss why the animals in the area may influence the number of bighorn sheep. Do we see evidence of animals that would compete for food or water? Are there a lot of predator species? Are there animals that might scare bighorns? Bring out the threats list, and discuss if the animals they observed could affect the presence of bighorn sheep. Remind students that to a sheep, a pet dog looks and smells a lot like a predator. Ask how a sheep would react to seeing lots of dogs.

Have students share their theories about why sheep are not visible on this trail anymore. Tell students this particular area has not been studied, but during the Covid-19 lockdown in the spring of 2020, park scientists studied the presence of sheep along the White Rim road in Canyonlands. They found that without people on the road, sheep ventured back into areas they had not been seen for a long time. Discuss whether the presence of people and dogs has increased on the middle earth trail.

5) Bighorn quiz game: Divide the student into two teams. Explain the rules for the bighorn quiz game, and elect an adult to keep score. Questions will be asked of the two students closest to you. The first contestant to say “Buzz” gets to answer the question. Have everyone practice buzzing in to build the excitement. If the first contestant gets the answer wrong, the second contestant gets to try. If the second contestant gets the answer wrong, contestants may discuss with their team and try to answer. After each question, discuss the answer with the group. The contestants go to the end of the line. As questions are read, remind students where they learned the answers, such as the pre-trip or the tracking station. Play as long as time permits. For the last question, teams should work together to write as many threats to bighorn sheep as they can.

Bighorn Quiz Game
1. Name the four needs of bighorn sheep. (wide open fields with forage, escape terrain, water, food)
2. Name the top two favorite foods of bighorn sheep. (Indian Rice Grass, Blackbrush)
3. What is a food bighorn sheep only eat if there is nothing else around? (yucca, cactus)
4. You can tell the age of a bighorn sheep by........ (the dark rings on its horns)
5. What is the difference between the horns of male and female bighorns? (the size and curl of the horns)
6. Bighorn rams fight each other for dominance by.............. (head butting each other)
7. True or false: bighorn sheep are ferocious carnivores. (false)
8. What animals compete for food with bighorn sheep? (domesticated sheep, mule deer, cows)
9. What animal can Bighorn sheep easily catch diseases from?  (domesticated sheep)
10. When faced with a threat, a bighorn sheep will usually............... (run away)
11. Bighorns can walk on narrow ledges and up steep, rocky slopes because......... (they have separated toes that can grip the rock and suction cup-like pads on their feet.)
12. What animal smells and sounds like a predator to bighorn sheep but is also a pet? (dogs)
13. Bighorns will run from …… (people, predators, domestic dogs, loud vehicles)
14. The bighorn's main wild predator is the............... (mountain lion)
15. Wildlife biologists study bighorn sheep by putting these around their necks. (radio collars)
16. True or False: it is easy for bighorn sheep to cross rivers and roads. (false)
17. How have humans started new herds of bighorns where they were once found? (moved by helicopter - remember from the pretrip)
18. True or False: We are likely to see lots of sheep on popular hiking trails. (false)
19. (Group question using white board) List as many threats to bighorn sheep as you can.

Murder Ewe Wrote
(adapted from U.S. Forest Service, Rocky Mountain Region, 189-195)

Essential Question: How can different factors interact to cause a rapid population change in bighorn sheep?            
Materials: copies of the questions for each students (with group numbers at the top); dry erase markers (one per student); posters and poster stands; stickers (one set per student);

1) Explain that since students are now experts on bighorn sheep (BHS) populations, they are being invited to help solve the mystery of a recent die-off of Rocky Mountain BHS that happened in Montana near Yellowstone National Park. Explain the differences between Rocky Mountain BHS and desert bighorns, and provide background about the die-off. (2-3 min)

2) Explain that each student will be assigned a different role as a specialist in their field. Explain the roles of meteorologist, disease specialist, and bighorn sheep scientist. Show where the posters for each scientist type are located, hand out clipboards with questions for each of these roles, and have students work collaboratively with other similar specialists to read the posters and answer their questions. (15-20 min)

3) Once most students have answered the questions, have each group of scientists present their answers to the entire class. Remind students that to solve the mystery, they need to put the information together, so students need to listen for clues while each group presents. Next, have students join a group of three. Students will find their group numbers in the top right corner of their question sheet. (5 min)

4) Give each group of three a whiteboard and dry erase marker. Challenge students to brainstorm all the possible causes of the die-off. Each student should take turns listing a single threat and then passing the white board around the circle, so everyone writes. If a student needs help thinking of another idea, they should talk to other members of their team (but every student must write). After 3 minutes, have students share their lists and add ideas they hear from other groups. (5 min)

5) Demonstrate how to create a concept map using a simple example. Concept maps help scientists organize ideas and show how concepts relate and interact. Write “drought” and “Sheep Decline” on the board and connect them with an arrow. As students brainstorm how these concepts could be connected, write their ideas such as “food loss” or “no water” on the arrow. (2-3 min)

6) After explaining how to construct a concept map, pass out concept stickers and concept map worksheets. Instruct students to create their own concept maps with their teammates by arranging stickers in logical order, drawing arrows, and adding connecting words between concepts. Remind students they don’t need to include all the stickers, and there is no one “right” way to create their concept map. (5 min)

7) Explain how to make a claim. A claim is a statement linking a cause and effect with a reason. For example, for the example used in step 5, the cause is the drought, the effect is sheep decline, and the reason is food loss. Invite students to make a claim by first selecting two or three main causes for sheep population decline. Students will use these concepts to fill in the claim statement “_______, ______ and _____ caused bighorn sheep to die off because ____.” If students are stuck, have each student in the group suggest one of the concepts (the stickers) used on the map made by their scientist group. (5 min)

8) Call on different groups to present their claims. (2-3 min)

Elmore, F. (1976). Shrubs and trees of the southwest uplands. Globe, AZ: Southwest Parks and Monuments.

Fagan, D. (1998). Canyon country wildflowers: A field guide to common wildflowers, shrubs, and trees. Helena and Billings, MT: Falcon Publishing.

Hauke, Craig. (1998). Bighorn Sheep: Monitoring and management of desert bighorn in the national parks of southeast Utah. Lecture, Dan O’Laurie Canyon Country Museum and the Moab Information Center Lecture Series. (Video)

Monello, R.J., Murray, D. L, and E. F. Cassirer. (2001) Ecological correlates of pneumonia epizootics in bighorn sheep herds. Can. J. Zool. 79: 1423-1432.

RSS Biweekly update (21 April 2020). Bill Sloan’s Wildlife Report- How wildlife are responding to lack of visitors.

Stall, C. (1990). Animal tracks of the southwest. Seattle, WA: The Mountaineers.

Sloan, W.B. (2011) Desert Bighorn Sheep Monitoring, 2009-2010. Southeast Utah Group Natural Resource Program Annual Report.

Sproat, K. (2012) Alteration of behavior by desert bighorn sheep from human recreation and desert bighorn sheep survival in Canyonlands National Park. BYU Dissertation.

U.S. Forest Service, Rocky Mountain Region. (1995). Ecosystem matters: Activity and resource guide for environmental educators. U.S. Government Printing Office: 1995-577-064.

Williams, D. (2000). A naturalist’s guide to canyon country. Illus. by G. Brown. Helena, MT: Falcon Publishing.


Last updated: July 26, 2022