NAME:______________________PERIOD:____DATE:_________
Lab: Population Size
Question:
How can you model a measuring technique to determine the size of an animal
population?
Research / Background: Field scientists have been conducting population “counts” for
many decades. Because many animals are continually moving around and seeking
shelter and protection in hard to see areas, it is difficult to count them. One commonly
used method which attempts to deal with determining population size for an organism in
the field is called “Mark and Recapture”, or what might be called “Capture-MarkRecapture”. As the name implies, a group of animals is captured, tagged, and then
released. You then capture another group of the same kind of animals and see how
many of them were previously captured and tagged. By knowing how many you caught
the first and second time, and how many were caught twice, you can estimate how
many animals there actually are in the population.
Objectives:
 Model the procedure used to measure an animal population
 Collect data on a modeled animal population
 Calculate the size of a modeled animal population
Materials:
 Paper bag containing fish
 Marker
 Calculator
Procedure:
1. The “brown” fish represent your caught and marked animals.
2. Shake the bag. Without looking into the bag, reach in and remove 30 fish.
3. Record the number of marked “brown” fish (recaught and marked) and the number
of unmarked “orange” fish (caught and unmarked) in your data table as trial 1.
4. Return all the fish to the bag.
5. Repeat steps 2-4 nine more times.
6. Calculate the averages for each of the columns.
7. Using average values, calculate the original size of the fish 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
Pop. Size
= M X (CwM + Cw/oM)
CwM
1
8. Record the calculated population size in the data table.
9. To verify the actual population size, count the total number of fish in the bag and
record this value in the data table.
10. Now collect the class date and figure out the class average calculated population
size.
Data:
Individual Data
Trial
# Caught With
Marks
Class Data
# Caught
Without Marks
# Caught With
Marks
1
2
3
4
5
6
7
8
9
10
Total
Averages
Individual Data Calculated Population Size = __________
Individual Data Actual Population Size = __________
Class Data Calculated Population Size = __________
Class Data Actual Population Size = __________
2
# Caught
Without Marks
Questions:
1. This experiment is a simulation. Explain why this activity was best done as a
simulation.
2. Compare the calculated to the actual population size. Explain why they may not
agree exactly. 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. Based on what you observed in this exercise, do you think that the mark-recapture
method is a good way to estimate population? Explain.
5. If you captured, marked, and released 5 turtles from a pond, and caught 10
unmarked turtles the next day, would you have enough information to estimate the
population using the mark-recapture method? Explain.
6. If you captured and marked, one turtle from a pond and captured the same turtle the
next day, can you conclude that only one turtle lives in the pond? Explain
7. Imagine that you are studying birds that are flying south for the winter. How might
their migration affect the results of a mark-recapture study? Can you accurately
estimate the migrating bird population using the mark-recapture method. Explain.
3
Graph:
Using the graph paper below and the individual data make a line graph showing the
number caught with marks and the number caught without marks. You will have 2
different lines on the graph.
4