Lab – Population Estimate

Mark/Recapture Population Estimate


Laboratory Objectives:

1. Become familiar with population size estimation using a capture, mark, release, recapture technique.

2. Identify factors that influence accuracy of population estimation.



Knowing the size of a wild population is important for many reasons. Management of a game population requires knowledge of the population size. Monitoring and controlling population size is important for the prevention of disease and parasite epidemics. Populations may need to be manipulated to maximize production of large animals, such as in wild game animals.


Population size of wild animals is often estimated by sampling the population. Captured individuals are marked and released into the wild population. The population is allowed to mix then is re-sampled and the proportion of marked individuals in the second sample can be used to calculate an estimate of the number of individuals in the entire population.


Materials Needed:

· Cookie sheet or similar sized flat container (approximately 11 inches by 17 inches).

· Dry Navy Beans- ONE CUP

· Dry Black Beans one-half cup (at least)

· Two bowls large enough to each hold one bag of beans and one small bowl

· Blank paper and tape to cover the bottom of the cookie sheet.

· Ruler and marker

· Six sided die (Usually dice come in a pair, but use only one for this exercise – called a die)



[Photographs of yourself conducting this work must be submitted with the completed lab]

1. Cover the bottom of the cookie sheet with paper and tape it securely. Use the ruler and a marker to divide the bottom of the cookie sheet into 36 equal squares (see figure below). Number the columns across the top (1-6) and the rows down the side (1-6).

1 2 3 4 5 6



2. Using the 6-sided die, randomly choose 10 sections. Roll the die once to get a column number and once to get a row number. The section you will sample is where the column and row intersect. Record the rolls of the die in Table 1. If you randomly select the same section twice, re-roll to select another section.

3. Pour the Navy Beans (1 Cup) into the cookie sheet and spread them out to a reasonably uniform distribution. Shaking the cookie sheet back and forth might help set the distribution.

4. Collect and count all of the beans from each of the sections chosen in Step 2 and place them in the small bowl. Record the number of beans collected from each section in Table 1. You should collect the beans on the line.

5. Replace the navy beans removed in step 4 with the same number of black beans. This is the MARK part of the method. Place the navy and black bean population that is on the cookie sheet into an empty container and mix well. After mixing release the beans into the cookie sheet and distribute them as you did in step 3. For proper mixing, it is important to remove all of the beans from the cookie sheet.

6. Use the 6-sided die to randomly choose 10 different sections (just like step 2). Record the sections chosen in table 2.

7. Collect and count the beans in each of the ten sections, recording the number of marked (black beans) and unmarked (navy beans) individuals collected from each. Record this information in table 2.

8. Use the following equation to calculate an estimate of the number of beans in your population. You do not know the total population estimate so set that as X. Then cross-multiply and solve the equation for the total population estimate based on the data you have collected so far. (If you are unsure how to do this, google ‘how to cross-multiply)

Total population estimate (X) = Total in second sample

Number initially marked Number of marked individuals in second sample


Table 1. Number of animals collected and marked in original sample.

Colony Section Number Collected
Top Side  


Total number of marked individuals (number initially marked)




Table 2. Number of animals collected in second sample


Colony Section Number Collected (Marked) Number Collected (Unmarked)
Top Side    




Total number of unmarked individuals in second sample


Total number of marked individuals in second sample


Total number of beans collected in second sample



Hypothesis: This method accurately estimate the size of the population.






1. What is your calculated estimate of total number of organisms? (from step 8 in the methods)




2. Now go through the entire colony and count all individuals (that you started with) and record below.


Total number of individuals in colony

3. Calculate your percent error by dividing your estimate (Results #1) by the actual number (Results #2) in the colony and multiplying that by 100. Numbers over 100% indicate overestimating the population size and numbers under 100% indicate underestimating the population size.




4. Calculate your margin of error. If your % error is 100% your estimate of population size perfectly matched the real population size. To calculate the margin of error, calculate how far away from 100% your error was. For example, if your error was 72%, the margin of error is 100-72=28%; if your error was 112%, the margin of error is 112-100=12%.




5. Submit your information for Table 3 and consult the class data table when it is available to form your conclusions.



Table 3. Class Results for Mark/Recapture Population Estimation Exercise.


Student Number of Sections Sampled % Error Margin of Error





1. Based on the class results, do mark/recapture techniques estimate population sizes accurately?




2. Was your hypothesis Correct?




3. What factors can or did influence the error rate of this experiment?




4. Should the number of sections sampled affect accuracy of population estimates? If so, in what way?




5. What modifications might you make to your sampling design to get a better estimate?