Environmental Science:
How the World Works and Your Place in It


Issues & Investigations Features

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Issues & Investigations

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  • 64 activities, 96 pages, perfect bound, non-consumable
  • Similar to a lab manual for environmental science
  • Each Investigation or Environmental Issue correlates to a section from the Student Edition and includes Objectives, Background Information, a Materials list, Safety Precautions (if applicable), Procedure steps and usually Questions for Discussion or Analysis questions.
  • Worksheets for this manual are provided in the Worksheets for I&I document
  • Included on the Teacher Component disc as Word files

Below is an excerpt from Investigation 1.2 of the Issues & Investigations Manual.

Investigation 1.2

A Sample Census — Wildlife on the Move


• Estimate the size of a population of mobile animals using the mark–return–recapture method. 
• Compare true census and sample census methods of determining population size.

Background Information:
A population refers to the total number of individuals of a single species that live in a designated region at a given time. For example, the population of bison within Yellowstone National Park is about 3,000. The population of humans on planet Earth is approximately 6 billion.

Population density refers to the number of individuals of a species that live in each unit area of habitat at a given time. The habitat is divided into appropriate units such as acres or hectares, square miles or square kilometers. For example, the population density of white-tailed deer in Forest County is 29 deer per square mile. 

A census is a count of a population. The method chosen depends upon the mobility and size of the population and the accuracy required. A true census is an actual count of all of the individuals of a species in a given area. Every ten years, census takers record the number of people living in the United States. Although it provides the most accurate data, it is often impossible to take a true census. This method can only be used if the individuals are large and immobile or slow moving.

A sample census is an estimate of the population. A sample census may be used to determine the population density of almost any plant or animal when an actual count is not possible. The estimated population is calculated by multiplying the average population density by the total area of the habitat. For example—if the area of Forest County is 675 square miles and the census estimate is 29 white-tailed deer per square mile, the estimated population of white-tailed deer in Forest County is:

Estimated Population Density × Area of Habitat = Estimated Population

29 white-tailed deer/square mile × 675 square miles = 19,575 white-tailed deer

Animals are usually harder to count than plants because they move and are more difficult to see. Wildlife biologists or ecologists often use the mark–return–recapture method to estimate the population density of animals. Animals are trapped and marked in some way, and then they are released. The traps are set again and the animals that are caught are checked for the identifying marks.

The estimated population is determined by using the following formula.

P = TFTL /M P = estimated population
TF= total animals captured in first trapping
TL= total animals captured in a later trapping
M= recaptured animals that are marked

The advantage of the mark–return–recapture method is that it allows an estimate of a population to be made when a direct count is impossible. But the method is not always reliable. Marked animals may be more easily spotted by a predator. Some animals may avoid the trap after being trapped the first time. In other cases, the animals may be attracted to the trap by the presence of food.

Charlie and Libby Schwartz were wildlife biologists, who worked for the Missouri Department of Conservation for many years. For more than 20 years, they used the mark-return-recapture method to estimate the population of the three-toed box turtle. They marked the turtles by filing identifying notches in the edge of the top shell, and they trained their labrador retrievers to find and retrieve the turtles. 

Questions for Study and Discussion:
Before beginning this investigation answer the following questions in your laboratory notebook.

  1. Define the following terms:
    sample census
    true census
    population density

  2. As a wildlife manager, you are hired to take a census of the following organisms. Indicate whether you would use a true census or a sample census and justify your choice.
    Bison in the Badlands National Park
    White oak trees in the city park
    Codling moths in an orchard
    Field mice on a farm

  3. After taking the census of bison in the Badlands National Park, the wildlife managers know that there are 500 bison in the park. What additional information do the scientists need in order to determine the population density?

  4. Wildife biologists use the mark-return-recapture method to estimate the population of rabbits. Twenty rabbits are captured and marked. Two weeks later, 12 rabbits are recaptured. Of these twelve, only 5 are marked. What is the estimated population? Be sure to show your work.

  5. What is the advantage of the mark–return–recapture method? Why isn’t this method always reliable?

  6. How did Charlie and Libby Schwartz “trap” the three-toed box turtles in their study?

  7. Make a copy of the chart below.
    Trapping Animals Trapped Marked Animals M Est. Population P
    1st TF = 20    
    2nd TL = 16 _____________________ _____________________
    3rd TL = 19 _____________________ _____________________
    4th TL = 17 _____________________ _____________________
    5th TL = 20 _____________________ _____________________
    6th TL = 14 _____________________ _____________________
    7th TL = 18 _____________________ _____________________
    8th TL = 19 _____________________ _____________________
    9th TL = 20 _____________________ _____________________
    10th TL = 15 _____________________ _____________________
    11th TL = 17 _____________________ _____________________
    12th TL = 12 _____________________ _____________________