Investigation 10: Energy Dynamics

Energy doesn't show up on a scale as a number you can read off directly. You get at it indirectly, through biomass. Dry, organic tissue is built from carbon compounds that store chemical energy in their bonds, so the mass of dried tissue is a stand-in for the energy it holds. That's why this investigation always dries samples before weighing or burning them: water adds bulk but no usable energy, and its amount varies by hour, by rain, by how juicy the leaf is. Drying strips that variable out so the mass left is a fair measure of stored energy.

For producers, you measure dry biomass at the start of a growth period and again at the end. The increase is net primary productivity: the energy a plant has captured through photosynthesis and locked into new tissue, after its own respiration costs are paid. For a consumer like a cabbage butterfly larva, energy arrives as food it eats. Some of that ingested energy is never absorbed and leaves as feces. Of what is absorbed, a large share is burned in respiration to run the larva's cells and is lost as heat. Only the leftover gets built into new larval biomass, growth you can measure as a mass gain.

This is why the math is just a ratio: biomass gained divided by biomass eaten, times 100, gives the percent of ingested energy that ended up stored rather than lost to feces or respiration. That number is always a small percentage, which connects directly to why energy pyramids taper: each trophic level only passes on a small fraction of the energy it takes in, and this lab is where you calculate that fraction yourself instead of just citing the 10% rule.