Energy Dynamics

Concepts Instructor Prep Student Protocol

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The first law of thermodynamics, energy cannot be created or destroyed, only transformed, can be demonstrated within a classic food web. Here light energy from the sun is first harnessed as radiant energy by plants and is then converted into chemical energy stored as complex carbohydrates. Some vegetation is eventually consumed by animals. In their process of breaking down the sugars, energy is either released as heat, stored in macromolecules as chemical energy reserves to be used later, or passed along to a predator. Each of these stages in the food chain is referred to as the trophic level. Plants are the producers. The squirrel would be the primary consumer, and the predatory fox would be the secondary consumer. The organic matter that is getting transferred from trophic level to the next is called the biomass - usually measured in energy units, calories, or kilocalories.

However, the transfer of biomass is not linear. After primary producers receive energy from the sun and make food, due to cellular respiration a small amount is transformed into unusable heat energy which is released along with carbon dioxide into the environment. The total light energy harnessed is called the gross primary productivity or GPP. If you subtract the energy lost to respiration from GPP the result is net primary productivity or NPP, which is the energy rate at which biomass is stored. Similarly, while primary consumers are harvesting chemical energy from plants, they also release a small amount of heat energy along with carbon dioxide during metabolism. And only a part of the consumed biomass is restored into their tissues. Finally, at the predator level, only a fraction of the original energy harnessed from the sun is available to use. Therefore, the biomass transferred from the producer to the primary consumer is not equal to the biomass transferred from the producer to the secondary consumer. These changes in biomass at each level of a food chain tell us about the productivity of a particular ecosystem as a whole.

In this laboratory you will investigate these principles of energy dynamics and productivity by measuring the transfer of biomass and energy from a producer, the cabbage, to a primary consumer, the cabbage worm.

Print Procedure Steps

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Caterpillar Energy Dynamics
- NOTE: In this lab you will grow caterpillars in conditions with and without food to examine the transfer of biomass in a typical trophic system.
- Weigh and record the wet mass of enough leaves of cabbage to line the bottom of a plastic tub.
- Then, collect two batches of five caterpillars from their cabbage habitat, and weigh and record the wet mass of each batch separately in individual weigh boats. Make sure to tare the balance before each measurement. HYPOTHESES: The experimental hypothesis is that only a fraction of energy will be transferred between the trophic levels of the cabbage and the caterpillars and a significant amount will be lost as waste and heat energy. The null hypothesis is that there will be no significant difference between the energy input from the cabbage, and the obtained energy stored as biomass in the caterpillars.
- Place the weighed cabbage and five caterpillars in a plastic tub lined with damp paper towels, leaving the lid slightly cracked open for ventilation, but not wide enough for the animals to escape.
- Place the other five caterpillars in a second clean container prepared the same way, but with no cabbage.
- After three days, transfer the caterpillars to new weigh boats, and weigh and record the final wet mass of both groups of caterpillars and the cabbage.
- Then place the caterpillar waste, or frass, and cabbage into two separate dishes, and put both dishes into a 37 °C incubator to dry for 48 hours.
- Euthanize the weighed caterpillars by putting them into a minus 20 °C freezer for 24 hours.
- Then, transfer the dishes to a 37 °C incubator for 48 hours.
- After drying, weigh and record the dry mass of the uneaten cabbage, the two groups of caterpillars, and the frass, and use these data to calculate the percent biomass of the caterpillars.
- Finally, discard all of the dried organisms and plates into the hazardous waste bin, and wipe down your workspace with 70% ethanol.
Results
- Take a look at the data you collected. Ensure the initial wet mass, the final wet mass, and the final dry mass columns are accurately filled out in your table. Click Here to download Table 1
- NOTE: Remember an object's biomass is equal to its dry mass. Therefore, percent biomass for each group is the final dry mass divided by the final wet mass times 100. For example, obtain the percent biomass of the cabbage by dividing the final dry mass by the final wet mass and multiplying by 100. This means that approximately 15% of the cabbage's total weight is biomass.
- After this, estimate the initial biomass of the cabbage and caterpillars by multiplying the percent biomass by the initial wet mass, and put the values in this new table. NOTE: Remember, biomass equals dry mass. Therefore, the final dry mass is equal to the final biomass of each group.
- To calculate the approximate initial and final energy in the form of kilocalories within the cabbage, caterpillars, and frass, multiply the biomass values recorded in your table by the conversions, and record the calculated values in the table. Click Here to download Table 2
- After this, subtract the initial energy from the final energy to obtain the change in energy for each group.
- NOTE: The negative sign denotes loss in energy, and positive indicates gain.
- Using your data table, calculate the energy in kilocalories that the fed caterpillars lost to respiration. The formula is the energy gained from cabbage, which is the positive of the energy lost by cabbage, minus the biomass gained from cabbage, minus the energy lost via frass. Click Here to download Table 3
- Then, repeat the same calculation for the unfed group of caterpillars. Make another column in the table and note this down.
- Finally, add up all of the energy gained via biomass and cabbage consumption, and the energy lost via frass and respiration for each group of caterpillars. NOTE: For both groups of caterpillars the net energy is zero, meaning that this experiment obeyed the first law of thermodynamics.