The Wild Bean Population:
Estimating Population Size Using the “Mark and Recapture” Method
I have always found it difficult to find quality labs to use with my students when teaching my unit on ecology. My school is on a busy city street, we have no access to a pond or woods, and only very limited access to grass in the school yard! Therefore, any labs we do in ecology have to be labs that can be carried out within the classroom or within the laboratory. This year, for the first time, I tried this lab that I called "The Wild Bean Population".
Anyone can do this lab since it uses very simple materials! All you need is dry white navy beans, dry red pinto beans, and a brown lunch sack.
In order to effectively study living organisms, scientists often need to know the size of a given population. A population is a group of organisms of the same species that live in a particular area. It is not reasonable to think that every individual in the population can be counted, and it is often difficult to get an accurate estimation of population size since organisms tend to hide, move around, etc.
Population biologists have developed several methods for sampling a population. In this lab, you will use the sampling technique known as “the mark and recapture method” to estimate the size of a population of wild beans! This method involves capturing a number of individuals from a population, marking or tagging them, and then releasing them back into the wild.
In the photo to the right, the white beans represent the population of wild beans that lives within the brown paper sack. Students grab a handful of beans and remove them from the sack. This represents the initial capture of organisms. The white beans are counted, marked and returned to the sack. The easiest way to do this would be to have the students mark each bean with a Sharpie before returning it to the sack. I teach 5 biology classes, and did not want to have to throw away that many marked beans at the end of each class. My solution was to have the students replace the white beans from the initial capture with red beans. The red beans represent organisms that were initially captured and returned to their environment.
At a later time, a second capturing is conducted. Some of the organisms in the second recapture were previously marked while others in the second recapture will have no mark. If you know the following information: (1) The number of individuals initially marked, (2) the total number of individuals recaptured in the second group, and (3) the number of marked individuals in the second recapture, it is possible to make an estimation as to the total population size.
This lab satisfied three purposes:
1. To learn the “mark and recapture” technique for estimating the size of a population.
2. To calculate the size of a population from given data.
3. To make predictions about the size of a population under various conditions.