In the flow of energy through the ecosystem, energy is lost at each trophic level. Some of this loss is due to metabolic needs, such as moving, and some is due to the formation of inedible tissue such as shells. Typically, only between 10% and 40% of the energy stored at each level can be used directly by the following trophic level. Therefore, few food chains extend beyond five members (from producer through decomposer) because the energy available at higher trophic levels is not enough to support further consumers. The flow of energy through the ecosystem drives the flow of mass (nutrients) within the ecosystem. Nutrients are chemical elements and compounds that are necessary for life. Unlike energy from the Sun, which originates from outside the ecosystem and is continuously lost, nutrients are cycled within the living (biotic) and non-living (abiotic) components of the ecosystem. The flow of nutrients such as carbon, oxygen, nitrogen, or phosphorus is called biogeochemical cycling. Decomposers play a key role in the biogeochemical cycles by returning nutrients to the soil, water, or air.

NOTE: An interesting related activity is "Red Tide" Links
  • This lively activity illustrates a simple ocean food chain and how the amount of phytoplankton limits the number of animals that can live in that ecosystem

30 phytoplankters (pieces of popcorn, "Phytoplankton" cards or other markers), 1 small paper or plastic bag for each "student Copepod", armband (or "Copepod card") for each "student Copepod," armband of different color (or "Herring card") for each "student Herring," armband of different color (or "Seal card") for each "student Seal"

You may wish to issue "Food web cards" for your students to wear instead of armbands. You can download an Adobe Acrobat Reader version of the cards by clicking here. You'll need to make several copies of the phytoplankton, Copepod and Herring cards.
  1. Review the concepts of food web and energy flow with your class.
  2. Mention that some copepods and other zooplankton migrate towards the ocean's surface at night to feed on phytoplankton. This may help the zooplankton avoid becoming prey instead of predators! Herring eat copepods where they find them. Seals usually hunt for herring during the day.
  3. Move your clss to a field or gymnasium, or rearrange your classroom so there is room for this activity.
  4. Divide the class into three groups in the ratio of 1 SEAL : 4 HERRING : 20 COPEPODS. (If you have greater than 25 students, add more copepods; if you have fewer than 25 students, decrease the herring by one and then decrease the copepods.) Two classes can be combined for this activity. Be sure to increase the amount of phytoplankton (i.e., popcorn or other markers), as well.
  5. Hand each copepod an empty bag. This represents the copepod's "stomach" into which it will place "phytoplankton."
  6. Instruct the class that you will be scattering phytoplankton around the area. Then they should turn their backs to you as you distribute the phytoplankton around the large open space.
  7. Ask the class to face you. Tell the copepods that, on the "count of three," they should migrate to the phytoplankton area by jumping and moving their arms (because copepods swim by pulling themselves through the water using their antennae) to collect phytoplankton and place the food in their stomachs (i.e., the bags). At the end of 20 seconds the copepods should stop collecting phytoplankton.
  8. Next the herring are allowed to hunt for copepods by swimming (by holding two arms together in front in "fin-like fashion" and making swimming movements). Copepods try to jump away from the herring. Depending on the size of the "hunting area," allow the herring to try and catch copepods until each has had time to capture one or more (this may take as little as 10 seconds). Any copepod caught by a herring must give its bag of food to the herring and then leave the ecosystem (i.e., hunting area).
  9. Any "un-captured" copepods may continue to feed on phytoplankton. Herring may still try to catch copepods but they are replaced as "top predator" when the herring-hunting seal appears. Any herring tagged by the seal should surrender its bags (i.e., copepods) and then leave the ecosystem.
  10. Stop the game when one seal tags three herring.
  11. Choose a few of the copepods and herring that left the ecosystem to pick up and count all the "leftover" phytoplankton. Count and record these "surviving" phytoplankton and place them in a separate bag.
  12. Each surviving copepod should count the number of phytoplankton (i.e., popcorn or other markers) in his / her "stomach."
  13. Each surviving herring should count the number of copepods (i.e., bags) in his / her "stomach."
  • Rules for survival:
    • A seal must have at least 3 herring or it will starve
    • A herring must have at least 4 copepods or it will starve
    • A copepod must have at least 30 phytoplankton or it will starve
  • Have the seals, herring and copepods that did not catch enough food stand in three separate areas. (They did not survive.)
  • Have the herring and copepods that were food for their predators stand in two separate areas. (They did not survive.)
  • Record the numbers for use in discussion about the activity:
    • How many seals survived?
    • How many herring survived?
    • How many herring were eaten by the seal?
    • How many herring starved?
    • How many copepods survived?
    • How many copepods were eaten by herring?
    • How many copepods starved?
    • How many phytoplankters survived?
  • What percentage of the original number of phytoplankters survived?
  • Are there enough phytoplankton to feed more copepods the next night when the copepods come to the surface to feed?
    • If not, what will happen to the food web?
  • Discuss what conclusions can be learned from this activity. (Food web has a limited number of steps that can be supported by the amount of food energy in the system. It takes a lot of phytoplankton to support very few top predators.Too many predators may overeat their prey and everyone loses.)
  • Louis Pasteur said, "The role of the infinitely small in nature is infinitely great." Ask students to write a short essay discussing whether they agree or disagree with the quote. They should include supporting evidence for their stand.
Adapted from Sea Soup Teacher's Guide: Discovering the Watery World of Phytoplankton and Zooplankton, Copyright 1999, by Betsy T. Stevens, Tilbury House, Publishers. Inquiry-based activities for use with Sea Soup: Phytoplankton and Sea Soup: Zooplankton, children's picture books by Mary R. Cerullo, photography by Bill Curtsinger, Tilbury House, Publishers