Energy Dynamics

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.