We began the day by revisiting the grass/cow/hamburger slide from yesterday, Students were challenged to apply their understanding of the food web by figuring out how much grass a cow has to eat to produce a hamburger. Understanding that grass is at the bottom of the food chain (like halobacteria in the Great Salt Lake), and recognizing that cows eat grass (like brine shrimp eat halobacteria), students reasoned that it would take 10 times as much mass in grass to produce one hamburger. If a hamburger has a mass of 100 grams, then a cow would have to eat 1000 grams of grass to produce the hamburger. Several students wondered how much land is required to produce 1000 grams of grass, and I encourage all of my students to research that question for extra credit!
We continued our discussion of the food web game by analyzing group results, assembling a table of class results, and calculating the average number of organisms at each level of the food chain. Students observed the 10-fold decrease in organism mass as we move up the food chain, and we connected the concept of biomass with energy. The results for each class period are reported at the end of the slide deck. We also discussed the variables involved in the food web game activity, with students identifying distance from the seed-trading space, number of students per group, and the efficiency with which groups worked as key variables contributing to how many higher-level organisms a group obtained.
After the slide deck and discussion, students worked in groups to complete the back side of the worksheet from yesterday. Students learned that while the amount of matter in a system stays constant, the amount of energy in a given level decreases as we move up the food chain. This is not always obvious, especially because organisms at the bottom of the food chain tend to be very small (yet great in number).
Today we modeled energy transfer through a food chain. We worked through a brief slide deck and added the words food chain, matter, and energy to our vocabulary list and began thinking about what happens to matter (grass) eaten by an organism (a cow) that does not become food for the consumer of the cow (us, when we eat a hamburger). We then modeled energy transfer through a selection of Great Salt Lake organisms present in a food chain. Using 4 different types of beans, we played the Food Web Game and learned how only 10% of the energy in one level of a food chain is passed along to the next level. The game took us to the end of the period, so tomorrow we will share group data, calculate class averages, and complete page 2 of the Food Web Game worksheet.
Today we turned our attention to the Great Salt Lake. We learned about the organisms that live in the lake, with students taking notes on a worksheet while we worked through a Power Point slide deck. Students learned that in a food web, organisms are drawn as nodes and the edges represent energy. The direction of the edge indicates the predator/prey relationship, with the arrow pointing away from the prey and toward the predator. Students then used the information from the slide deck to construct a food web on the initial model worksheet. Students had a lot of questions about using the initial model, and it’s important to keep in mind that the model of our understanding of the Great Salt Lake ecosystem will develop over time. Scientists often develop models with incomplete data sets and then use the gaps in their understanding to devise and test hypotheses. As scientists in training, we will also learn to work with incomplete data and we will learn to keep track of our questions and devise strategies for filling in the gaps in our knowledge.
Updated 9/30/14: We completed the initial model today, with students using scientific reasoning to assign units of energy moving through their food webs. Students turned in both the Power Point worksheet as well as the initial model worksheet at the end of class.