Just how do scientists know how many deer live in an ecosystem? How did they figure out the number of giraffe live in the African savanna? Where do ecologists get these numbers from?

These are some of the questions that my class discussed while studying changes in population and population density. Students guessed how this was accomplished: using satellites, sending people out to look around a lot, catching some of the animals and then bringing them back to the wild, looking at a small area, then estimating.

Although some responses seemed more on track than others, I wanted students to simulate one way scientists do this! The capture-mark-and-release method is the one we chose to try. And because I already had a bag of dried beans from a different lesson, that is what we used as our population!

Materials 

  • Dried beans (these represent the “animal”. You can also use dried pasta, packaging peanuts, or another low cost, small item that can easily be marked)
  • A bag or other container
  • Marker

Set up:

Before beginning the activity, I had divided up the beans into cups for each group of 2 to 3 students. I did not count the number, but used a few handfuls.

I created a chart for students to use like the one below

Estimating Populations Chart

Procedure:

  1. Students first grab a handful of beans from their cup and mark each one that they removed with a marker. Count how many beans were marked and record this on the chart.
  2. Return the marked beans into the container and shake the beans.
  3. Grab another handful of beans from the container.
  4. Count how many total beans are in the new sample, and then how many marked beans were in the sample. Record both numbers on the chart.
  5. Using the formula below, students calculate how many beans are estimated to be in their container.
  6. Repeat steps 3-5 again, recording the numbers on the chart.
  7. Once students have recorded the data, the group can make one final estimation of how many total beans they believe to be their container.
  8. Finally, students count the number of beans they have and compare this number to their estimations!

How does this simulate what scientists do?

Those that study population densities of different organisms will often use this method of capture-count-and-release. A group of organisms is captured and marked with a device or tag. Later in the year, or the following season, scientists will capture another group of that organism and see how many have been previously tagged. If there are only a small number of tagged organisms, scientists assume there is a larger population. If many of the organisms already contain tags, then the total population may be quite small. By using the bean simulation, students can begin to understand this process!

Questions to discuss:

  1. Compare the bean simulation to real life. What is alike and what is different? You might bring up that beans do not die or give birth, so the population will remain static (as long as no beans get lost under the table!). Live organisms always have changing populations, so this would affect the number of organisms in an ecosystem.
  2. How precise is this method? When students compared their estimations to the actual number of beans, they found that some groups were pretty close in their estimation, while others were not. We noticed a pattern of groups that had marked a higher sample initially also had more accurate estimations!
  3. Can this be used for all organisms? When I asked this question, students in my class determined that this would be very difficult to use for types of insects or other very small creatures. Larger animals would work better!

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