Perform a short demonstration of a food chain. Call on seven student volunteers, and have them stand at the front of the classroom. Designate four of them as “squirrel,” “mountain lion,” “acorn,” and “fox.” Give a sheet of paper with an arrow to each of the other three students. Tell them they will represent energy flow. Have the class arrange the first four students side-by-side, in order of “who eats whom” from left to right. Then, have the class arrange the three students with arrows, so that the arrows point in the direction of energy flow because of the feeding relationships shown. The correct order is:
acorn → squirrel → fox → mountain lion
Have students copy the food chain into their notes. Define trophic levels. Then guide students in labeling the acorn as a producer, the squirrel as a primary consumer, the fox as a secondary consumer, and the mountain lion as a tertiary consumer. Explain the flow of energy between each of the trophic levels. Also, define the terms herbivore, carnivore and omnivore, and have students identify an example of each on the food chain. (The herbivore is the squirrel, the carnivore is the mountain lion, and the omnivore is the fox.) Also explain the role of decomposers, which are mostly bacteria and fungi. These organisms recycle nutrients from decaying organic material. Be sure to explain that decomposers can be placed at any point in a food web.
Ask the class to name more organisms that could belong to the forest ecosystem from the food chain. Begin creating a food web, writing each new organism on the board or overhead projector, and using arrows between the organisms to show the flow of energy.
Individual: After demonstrating how to place a few organisms into the food web, students continue creating the food web in their notes individually.
Tell students ecologists use many different models to study the flow of energy through ecosystems. Food chains and food webs both show the direction of energy flow, but “energy pyramids” can show the direction and the amount of energy flow between trophic levels. Show them an energy pyramid like the one below (S-8-9-1_Marine Energy Pyramid.docx):
Explain the ten percent rule, the concept that only about 10 percent of the energy available in one trophic level is available to organisms in the next trophic level. Explain that some of the energy in each level is used for life processes and some energy is given off as heat. Ask students,
- “If 100 percent of energy is stored in the producers in the food chain, what percent is available to primary consumers?” (10 percent)
- “What percent is available to the secondary consumers?” (1 percent)
- “What percent is available to the tertiary consumers?” (0.1 percent)
Tell students that energy available at each trophic level is measured in units called kilocalories (kcal). Explain that if 7,000 kcal are available in producers, then only 700 kcal can be transferred to primary consumers. Have students calculate the kcal available for secondary consumers and tertiary consumers.
In small groups, have students complete the Energy Pyramid Activity (S-8-9-1_Energy Pyramid Activity-Student Version.doc and S-8-9-1_Energy Pyramid Activity-Teacher Version.doc). Have each student answer the questions to the Energy Pyramid Activity individually.
- Students who might need an opportunity for additional learning can arrange a list of names of other organisms in the food web from a list that you provide (e.g., deer, sparrow, owl, wolf, bear, mouse, skunk, insects, fish). Also, before the lesson begins, provide a list of the vocabulary terms and definitions. Students may reinforce learning by practice creating a food web with the Pond Food Web interactive activity or watching the Energy Pyramid tutorial (see Related Resources).
- Students who may be going beyond the standards can create a food web with specific organisms that live in the local Pennsylvania ecosystem.
- Students who may be going above the standards can write a short essay on the impact of an ecosystem’s food web if one organism becomes extinct or loses its habitat within the ecosystem. The essay should touch on how other organisms in the food web could be impacted by the event.