Estimating the Number of Individuals in a Population:
Mark-Recapture
This lab is designed to help you understand the theory and practice of mark-recapture as a method
of estimating the number of mobile individuals in a population. This method is used for many
species. You will identify the advantages and limitations of the technique and when it would be
appropriate and inappropriate as a method of population size estimation.
Activity:
1. Get a sealed bag of pasta shells and a marker.
2. Without looking into the bag, take out one scoop full of pasta.
3. Mark each ‘individual’ in your scoop and note the number that you have marked.
4. Return the marked pasta to the bag.
5. Shake the bag, then repeat step 2.
6. How many “marked individuals” did you get in the second sample?
Use these values to solve the mark-recapture equation:
N=[(M+1)(n+1)/(m+1)]-1
Where:
N = population size estimate
M = number of individuals marked
n = total number of individuals (marked and unmarked) “captured” in second scoop
m = number of marked individuals “recaptured” in second scoop
Discussion:
Within your group, answer the following questions:
What is your estimate of the population size?
How confident are you that this represents the true value of the population?
What would increase your confidence in your prediction?
How would you test the accuracy of your prediction?
In what ways does our example differ from the use of the mark-recapture method used to determine
the size of mobile organisms in the wild (butterflies in the state park, catfish in Lake Michigan, etc.)?
(Note: we would call these differences “violations of the assumptions of the model”, because the
model that the equation represents is based on the assumption that the natural population has the
same characteristics as our artificial population.)
How would each of these violations of the assumptions of the model affect the accuracy of your
estimate (be sure to include both the direction and general magnitude of the difference)?
Which populations (species, habitat, population size, etc.) are likely to violate which assumptions?
Activity:
Using your artificial population, test your hypothesis of how violations of the model’s assumptions
will affect your estimate.
Be prepared to briefly report on:
1. The accuracy of your estimate (both a numerical value and a judgment of usefulness).
2. The violations of assumptions that you would have in actual populations.
3. Your hypotheses about the direction and magnitude of the effect of at least one of violations on
the accuracy of the estimate.
4. Your test of your hypothesis and the result.
5. Your evaluation of the mark-recapture estimating technique as a whole, and the effect of violating
the assumption you looked at in your test.
6. What techniques would make this method as effective as possible.
7. What populations would be best suited to estimation by this method.
8. What populations would be least suited to estimation by this method.