Quantifying Ecosystem Energy

 

STUDENTS WILL BE ABLE TO:
- Explain distribution of biomass between producers, herbivores, carnivores, and decomposers.
- Identify the three principles of thermodynamics.
- Apply the principles of thermodynamics to energy flow within an ecosystem.

GUIDING QUESTION: How does the sun's energy flow through the ecosystem?

MATERIALS:
Days 1 and 2:
- Flags (5 for each group)
- Meter sticks (1 per group)
- Scissors (at least one pair for each group)
- Measuring tape (1 per group)
- Bags and vials for collection
- Calculators
- Scales
Day 3:
- Poster board
- Art supplies
Optional Extensions:
- Refer to specific activity (listed at end of Class Activities)

CLASS ACTIVITIES:
Day 1: Explore
1. Before class, go outside and place a numbered flag in the center of various areas - enough for groups of 2-4 students in your class to each have their own spot. Try to get a representation of grassy areas, shrubby areas, flowerbeds, trees, etc.
2. At the start of class, assign each group a number corresponding to the numbers on the flags. (Numbers may be randomly chosen by each group unless you wish to challenge particular groups with more structurally diverse plots of land.)
3. Explain to the students that their task is to find out who has the energy in this ecosystem. Each group is to determine where the energy is in one square meter. The methods that they use to estimate the amount of energy present is up to them. Introduce the term biomass: the first step in figuring out how much energy is present is to track the amount of biomass at each level in the food chain. Remind the students of the four main levels (producer, herbivore, carnivore, and decomposers), and brainstorm (for no more than 5 minutes) ways that they could sample or estimate the biomass of each level.
4. Take students outside and have each group find their designated area. Using the flag as a central point, they should map out 1 square meter around the flag and mark each of the corners with another flag.
5. Once the study area is marked, each group must develop a plan of action - how will they quantify the biomass at each level? Students should consider the following questions:

• What producers are present? How could they estimate their biomass? How could they sample the biomass? Make scissors etc. available in case groups would like to clip vegetative biomass. Groups should focus on green biomass - in other words, if there is a tree or shrub present, their main concern should be quantifying the biomass of the leaves that are within their plot, not that of the trunk and branches.
  
• What herbivores are present? Keep in mind that herbivores are not just large mammals; for instance, they will likely find insects in their plot.
  
• What carnivores are present? How could they estimate their biomass? How could they sample the biomass?
  
• What decomposers are present? How could they estimate their biomass? How could they sample the biomass?
  

6. Students should let you know if there is any additional equipment they need for their sampling.
7. For greatest accuracy, plant samples may be dried overnight in an oven.

Day 2: Identify
1. If necessary, continue sampling plots.
2. Back in the classroom, give students time to weigh samples, calculate mass from measurements, etc.

Day 3: Organize
1. Each group should create a pyramid or other display that visually represents the proportions of each trophic level in their plot.
2. Groups can use any additional time to prepare a brief (under 5 minute) presentation covering their methods, results, further questions, and error analysis.

Day 4: Share
1. Each group should present their findings, incorporating their visual display into this presentation.
2. Discuss:

• Was the same trend (more producers than herbivores, more herbivores than carnivores) visible in each plot? How did results between plots differ and why?
• Is biomass equivalent to energy? Could different types of organisms have more stored energy than others? (For instance, animals vs. plants) What about different parts of organisms? (For instance, seeds vs leaves) What steps could you take to figure this out?

3. Introduce the three principles of thermodynamics.
4. Homework: Students should draw a picture or flow chart showing the flow of energy within the food chain that they mapped out in the "Interactions in the Food Chain" lesson or a in food chain that includes them. (Students who dislike drawing may also use pictures from magazines or the Internet.) Food chains should start with the sun and include a decomposer. Keeping in mind that energy is neither created nor destroyed, the picture should account for all energy. Students can use color-coded arrows and boxes to indicate whenever energy changes forms. Possible forms of energy that students can include are:

• Heat
  
• Light
  
• Potential (energy of position)
  
• Kinetic (energy of movement)
  
• Chemical

Optional Extensions:
1. Do an inquiry lab investigating what organisms and parts of organisms contain the most energy: Set up stations with birdseed, mealworms, and grass or plant leaves. Track the number of visits by birds to each station. What stations do they go to most frequently? Why?
2. Instead of just focusing on biomass as a measure of energy, use a bomb calorimeter the amount of calories in certain types or parts of vegetative material in the student plots. Variation: Compare sugar amounts in different plants.
3. Hold a "sugar relay," in which students must move sugar from one bin to another bin, to a third bin (representing plants to a plant consumer to an animal consumer.) The catch? They need to move the sugar using a cup that is full of holes, and sugar drains away every second. Discuss how energy is "lost" (no longer stored in chemical form) between trophic levels in real life. (Disposable cups or yogurt containers work well.) This activity should be conducted outside, and water may be used if cost precludes using sugar.
4. Hand out gummy worms (or other gummy animal) to each student, along with a clean knife. Explain to your class that scientists estimate that only about 10 % of the energy is passed along at every step in the food chain. Let students give examples of where the other 90% of the energy goes. Everyone should then cut away 1/10 of their gummy worm, and set aside the rest. The worm was just eaten by a fish! But then, a larger fish comes along, and eats the little fish. Students should again discard all but 1/10 of the remaining worm. Students catch the larger fish - they can now eat the last 1/10. Was it a big enough snack? How many worms would the little fish need to eat to get enough energy to survive? How many little fish would the big fish need to eat? How does this relate to the distribution of biomass in their study plots?