Part of Lesson Plan: Population Trends -- Jennifer Terpstra

Activity Overview / Details

Give each lab group (my lab groups consist if 4 people) a box of 100 paperclips. Tell the students that the paperclips represent amoebas, unicellular organisms that reproduce simply by splitting in half. First, to represent exponential growth, have students lay out paperclips in a branching dichotomous "tree," which represents exponential growth through six generations (it should look like a Christmas Tree). The results will be 1 amoeba in the first generation, 2 in the second generation, 4 in the third generation, 8 in the fourth generation, 16 in the fifth generation, and 32 in the sixth generation. Students may need to see an illustration on the board to get them started. They most likely will need to be reminded that the amoebas reproduce by splitting in half, therefore the population doubles with each generation. Then have students model logistic growth by repeating the procedure, but this time removing paperclips to represent deaths of amoebas. On the board at the front of the class, write the number of deaths which will occur in each generation. In generation 3, remove 1 amoeba, in generation 4, remove 2, and in generation 5, remove 3. Remind students that they are starting over with a new set of amoebas, however, they should remove amoebas as they construct each new generation. Have students compare the number of organisms in the last generation of each model; 32 in the exponential model and 10 in the logistic model. Provided below is a diagram that may help you as the teacher visualize the logistic growth model, or use as an overhead which can be shown to the class in case some groups have trouble with the logistic model. Ask students why/how the first model represented exponential growth. Answers should include the idea that nothing stopped the amoebas from multiplying. Ask students why/how the second model represented logistic growth. Answers should include the idea that some amoebas died starting with the third generation, and something must have happened to cause their death, which in turn affects the number of births in the following generation. Note: A diagram for exponential growth is not provided as each generation simply doubles the number in the previous generation- 1,2,4,8,16, and 32. This activity should really only take 5 minutes at the most for a group to complete; two minutes for each model. Allow an additional minute for answering investigative questions asked by the instructor.