Part 2: Quantifying energy flow and the rule of 10
percent
Three hundred trout are needed to support one man for a
year. The trout, in turn, must consume 90,000 frogs, that
must consume 27 million grasshoppers that live off of
1,000 tons of grass.
-- G. Tyler Miller, Jr., American Chemist (1971)
Only a small fraction of the energy available at any trophic
level is transferred to the next trophic level. That fraction
is estimated to be about 10 percent of the available
energy. The other 90 percent of the energy is needed by
organisms at that trophic level for living, growing, and
reproducing.
This relationship is shown in the energy pyramid above. It suggests that for any food chain, the primary producer
trophic level has the most energy and the top trophic level has the least.
8. Why is a pyramid an effective model for quantifying energy flow?
9. Place the organisms from your original food chain on the pyramid provided.
10. Using the rule of 10 percent in energy transfer, record the species names for each trophic level and the
amount of energy available at that level if your producer level had 3,500,000 kilocalories of energy/area.
11. In one or two sentences, describe how the available energy may affect the population sizes of organisms at
different trophic levels.
Part 3: Creating a food web
Food chains are simple models that show only a single set of energy-transfer relationships, but many organisms
obtain energy from many different sources and in turn may provide energy to several different consumers. A
food web illustrates all these interactions and is a more accurate model of how energy moves through an
ecological community.
12. Starting with your original food chain, add another plant and four more animal cards to construct a food
web that shows how energy flows from producers through primary consumers, secondary consumers,
