Mark-Recapture Lab
Determining the Size of an Animal Population
Background:
Scientists can determine the number of
animals in a large population by using a
sampling technique. This is a laboratory that
simulates a population census technique
commonly used by wildlife biologists in the
field. Here is how it works. The first step is to
trap a random sample of animals of the desired
species. These animals are then ear-tagged or
marked in some other manner and released. The
next step is to trap once again. Some of the
animals captured may have been marked from
the first sample. Scientists then calculate the
total population for their specified area based on
the ratio of marked animals to unmarked animals.
In this activity, a bag represents the area of land where the population study is being
conducted. Elbow Macaroni will represent animals. All the Elbow Macaroni in the bag
represent the total animal population being studied.
Purpose/Objective: To estimate the population by using a sampling technique
Materials:
Plastic bag
Elbow Macaroni (including 20 marked Elbow Macaroni)
Calculator
Procedure:
1) Shake your bag of Elbow Macaroni (the marked Elbow Macaroni represent previously
caught and marked animals). Without looking into the bag, reach in and remove 30 Elbow
Macaroni.
2) Record the number of marked Elbow Macaroni (recaught and marked) and the number of
unmarked Elbow Macaroni (caught and unmarked) in your data table as trial 1.
3) Return all the Elbow Macaroni to the bag.
4) Repeat steps 1 to 3 four more times (for trials 2 to 5).
5) Calculate averages for each of the columns.
6) Using the average values, calculate the original size of the bean population in the bag by
using the following formula:
M = number initially marked
CwM = average number caught during the trials with marks
Cw/oM = average number caught during the trials without marks
Calculated Population Size
=
M x (CwM + Cw/oM)
CwM
7) Record the calculated population size in the data table.
8) Verify the actual population size, count the total number of Elbow Macaroni in the bag and
record this value in the data table.
Data/Results:
Bag # _______
Trial
Total
caught
Number caught with
marks (CwM)
Number caught without
marks (Cw/oM)
1
2
3
4
5
Averages
Calculated population size = _________
Actual population size = ________
Discussion:
1) Compare the calculated to the actual population size. Explain why they may not
agree exactly.
2) What changes to the procedure would improve the accuracy of the activity?
3) Explain why this technique is used more often with animals than with plants when
calculating population size.
4) What species of animal would this technique work well for? What species of animal
would this technique not work for? Why?
5) Assume you were doing this experiment with living animals. What would you be
doing before the first step? In the first step?