Laboratory Notes for BIO 3001

© 20 April 2004, Mary Jean Holland

Mark, Release, Recapture—Estimating Population Size

How many individuals are in a population? Assume that the population is restricted by the availability of resources the species needs, e.g., trees to climb or water to swim in. There are lots of squirrels in Central Park, but you can’t count them all; there are many turtles in Turtle Pond, but you can’t stick your head under water and count them all. The technique is to trap some individuals, put a mark on them that you can identify in the future, and release them back into the larger population. At a later time, trap more individuals; the ratio of marked to unmarked will allow you to estimate the total size of the population. One major assumption of the method is that the marked and released individuals mix freely with the rest of the population; if they didn't, you might trap a lot of them again, if you sample at the place where they were released. Another concern is that the marking does no harm to the individual; if it harms or kills the individual, you will get a skewed result. Some animals are smart and the free meal and bedding in a trap may encourage them to be trapped again. The method is not flawless.

In this exercise you will estimate the number of red beans in a container by a modification of the “mark, release, recapture” method used by field ecologists to estimate the size of an animal population in an ecosystem.

  1. Remove 100 red beans and substitute 100 white beans for them. This process will simulate the “marking” of 100 individuals from the population with white.
  2. Add the white beans to the original population and mix the beans thoroughly to distribute the marked individuals in the population.
  3. Withdraw a sample (a handful) of the bean population to simulate “recapture.” Count the numbers of each type of bean in the sample. Be sure to record your results before returning the sample to the population and mixing again.
  4. Find unknown N, the estimated number of beans in the population, using the following relationship:

  5. Total # marked (white)                       =     # of recaptured marked (white)                
    Unknown N (population size) Recaptured sample size (red & white)

    Unknown N = (Total # marked) × (Recaptured sample size) ÷ (# of recaptured marked)

  6. Return the sample to the population and mix well again. Repeat this sampling process for a total of 5 times.
  7. Look at your five estimates of population size. How well do they agree? Calculate the range and mean (average) for your estimates.
  8. Now count the population (total number of beans, both red and white) to see how close your estimates were to the actual population.

Data Sheet: Mark, Release, Recapture

Sample Trial Number of Red Beans in Sample (SMARKED) Number of White Beans in Sample Total Number of Beans in Sample (STOTAL) Estimated Total Number of Bean in Population
Trial #1        
Trial #2        
Trial #3        
Trial #4        
Trial #5        

Estimates of PTOTAL

Range: ___________________________

Average: ___________________________

Actual Count of PTOTAL: ___________________________

Please sort the red and white beans and replace them in the original bags or containers, so they are ready for the next group of students.

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Last updated 24 March 2017 (MJCH/JHW, & L. Song ed.)