Lesson Plan

Hoofin' It! - Population Calculation

distant sheep on a mountain, river valley and other mountains in background
Sheep can be difficult to find in their mountain homes
NPS Photo


The lesson plans in our 'Hoofin' It!' unit help students learn the basics of animal classification and what characteristics are common to mammals, mainly through studying Dall sheep.

Lesson twelve provides students with wild sheep population numbers collected in the field and challenges them to graph and analyze the data. 


Explore the factors influencing the size of Dall sheep populations


The "Hoofin' It!" unit explores the natural resource management of Dall sheep in the national parks of northwest Alaska. Students will learn about Dall sheep, where they live, how they have adapted to their environment, and how wildlife biologists study them to understand how to protect their populations within national parklands. Links to other lessons in the unit can be found at page bottom.

Dall sheep are a wild sheep that lives on steep mountain slopes across the Alaska. The sheep are an integral part of the natural ecosystem, and they are prized by subsistence and recreational hunters. In the early 1990s, the Dall sheep population in the Baird Mountains of Noatak National Preserve declined dramatically, losing half its population in two years. Wildlife managers closed the sheep hunting season for seven years to allow the population to grow again.

Why did the population drop so suddenly? What are the natural and human factors that affect the Dall sheep population? In the spring of 2000, Brad Shults, a wildlife biologist for the National Park Service, began a research project to learn more about Dall sheep population dynamics. Shults hopes to better understand sheep by studying the number of lambs that are born, how long sheep live, what are the most common causes of death, where do they go from season to season, and just how many sheep are there?


Before You Begin 

Provide each student with a copy of the population calculation data. Print out a set of example graphs to check the student work. Review the Dall sheep population size reading with the students. 


  1. Hand out a copy of the population calculation data sheet to each student, and briefly review the type of data that it includes. 
  2. Discuss with the students what types of scientific questions they can ask about population sizes. Discuss that researchers use different types of investigations to answer different types of questions. Which of the questions the students think of can be answered by a population survey? 
  3. Referring to the data sheet, have the class develop together a list of questions that can be answered with the data. Augment the student questions with the discussion questions (below). Ask the students which, if any, of these questions they can guess a reasonable answer, and which require data to investigate? 
  4. Students create a series of graph (x-y or bar chart style), where the year from 1986-2002 is along the x-axis.
    a. Total population (adults + lambs + unknown)
    b. Adult population (rams + ewes)
    c. Number of ewes
    d. Number of rams
    e. Number of lambs
    f.  Percent change in total population (population this year - population last year) / population last year *100 
  5. Students provide answers to each of the questions posed by the class. As a class, review the answers and discuss explanations or further research questions that follow from the questions and answers. How do scientists combine data/observations and knowledge they already have (such as from Dall Sheep Population Size) to develop new understandings? 
  6. Why would scientists want to make the information they learn about Dall sheep populations, public? [other scientists can do further research, managers can protect populations, people (like the students themselves) can learn about Dall sheep, etc.] 
Discussion Questions

  • Which year had the highest total population of sheep? Which year the lowest? 
  • Did the number of rams, ewes and lambs have their highest and lowest population in the same years? 
  • What was the maximum total population from 1986-2002? The minimum, the mean, the range? What about for all adults, for ewes, rams, and lambs? 
  • During which years did the total population rise, and during which years did it fall? 
  • Are there more rams, ewes or lambs in the population? Why might that be? 
  • Which population curve, ewes, lambs or rams, does the total population curve look the most like? Why is that? 
  • Using the graph of Percent Change in Population, which years were the hardest and which were the best for the Dall sheep? Are these the same years when the total population was the highest or lowest, or different years? Why? 
  • What environmental factors could cause the changes in Dall sheep population? 
  • If you wanted to know how snow, predation, and hunting affect the Dall sheep population, what would you measure? For each one, how would you expect the measurement to vary across the years (1986-2002) if it were an important factor? If it were not an important factor? 

Break students into 6 groups, and assign one graph to each group. Have students in each group do their graphing independently.

Have students calculate for themselves: All rams, Adults, Total, and Percent change (of total).

Computer lab: download the population calculation data in text format or Excel format. Have students use spreadsheet software to create the graphs. Students can also use the spreadsheet functions to calculate.

To concentrate on reading graphs rather than creating graphs, review the discussion questions using already-generated population calculation graphs.

Additional Resources

The "Hoofin' It!" unit explores the natural resource management of Dall sheep in the national parks of northwest Alaska. Students will learn about Dall sheep, where they live, how they have adapted to their environment, and how wildlife biologists study them to understand how to protect their populations within national parklands.

This unit is designed for grades K-12. Many of the lesson plans are appropriate for younger grades, although the later part of the unit are geared towards middle and high school. A class needn't do every lesson in the unit to gain insights into wildlife management - each can be approached as a stand-alone lesson on a particular biology-related topic.

Lesson 1
Hoofin' It! - What Do You Know?

(Understanding taxonomy; k - 12th grade)
Lesson 2
Hoofin' It! - Vertebrate Grab Game

(Exploring types of vertebrates; 3rd - 6th grade) 
Lesson 3
Hoofin' It! Vertebrate Mysteries

(A vertebrate matching game; 8th - 12th grade)
Lesson 4
Hoofin' It! Special Parts

(Animal adaptations; k - 12th grade)
Lesson 5
Hoofin' It! Hard to See?

(Camoflague; k - 8th grade)
Lesson 6
Hoofin' It! - Sheep Maneuvers

(A predator-prey game; k - 12th grade)
Lesson 7
Hoofin' It - Year of the Sheep

(Life cycle of a Dall sheep; 3rd - 12th grade)
Lesson 8
Hoofin' It! - Who's Got My Habitat?

(Habitat and wildlife populations; 3rd - 12th grade)
Lesson 9
Hoofin' It! - Habitat Grid

(Exploring wildlife habitat; k - 3rd grade)
Lesson 10
Hoofin' It! - Through the Seasons

(A game looking at seasonal impacts on wildlife; 2nd - 11th grade)
Lesson 11
Hoofin' It! - Population Art

(Intro to counting wildlife populations; k - 2nd grade
Lesson 12
Hoofin' It! - Population Calculation

(Graphing and analyzing sheep population data; 6th - 10th grade)
Lesson 13
Hoofin' It! - Scavenger Hunt

(A game connecting students to wildlife; k - 6th grade)
Lesson 14
Hoofin' It! - Field Sampling

(How scientists count wildlife populations; k - 12th grade)
Lesson 15
Hoofin' It! The Bean Counters: Mark-Recapture

(Learning to use the mark-recapture method for population surveys; 5th - 12th grade)