Food Web Activity
In a simple food chain with three trophic levels, there are primary producers, grazers, and predators. The population size of each level depends on the abundance of nutrients or prey, and growth, feeding, and reproduction rates. In this activity, the trophic levels are from an upwelling ecosystem such as the Gulf of the Farallones with phytoplankton (diatoms), zooplankton (krill), and whales. For simplicity, the production rate is the combination of the growth, feeding, and reproduction rates.
Students should play at least two rounds of the game with different production rates. The first round is a steady state system where all three trophic levels continue to grow. In the other rounds, one trophic level will go extinct (crash) due to lack of food and then the other trophic levels will slowly crash. Fishery management depends on population dynamics models. In this very simple model, there is only one production rate. In fishery models, growth, feeding, and reproduction rates are included separately. As a fish grows, it feeds on different sized food and is prey to different predators. Also, the rates are different at different life stages and are important in determining population size. Population dynamics models are very complex when they account for the changes in a population at different life stages. When one trophic level decreases, it will affect the abundance of organisms at other trophic levels.
Part A: Energy Relationships
- Assign each person in your group to a trophic level, and one person should be in charge of the stopwatch & nutrients.
- Nutrients (NO3-1: Nitrate, PO4-3: Phosphate): Required for photosynthesis by diatoms
- Primary Producers: Diatoms (Green)
- Primary Consumers/Grazers: Krill (Yellow)
- Secondary Consumers/Predators: Whales (Purple)
- The page with the nutrients is the game board. The other pages should be cut into squares. Each card represents an abundance of organisms that consumes the abundance of one card of their prey (the lower trophic level). For example, one card of krill represents enough krill to feed one whale.
- Each group should play all four rounds.
- Each round lasts exactly 5 minutes.
- To begin, lay down the 6 nutrient sheets on a desk.
- Use the start time and production rate for each trophic level from the chart below.
- The start time is the time at which a trophic level begins growing (start laying down cards).
- The production rate is the combination of the feeding, growing, and reproduction rates. It is the time interval between laying cards down.
- For example in Round 1, primary producers lay down one card at the beginning (t=0) and lay down 1 card every 5 seconds for the entire 5 minutes. Krill (grazers) start after 10 seconds (t=10), and lay down 1 card every 10 seconds. When adding more cards to the board, the population is reproducing and growing.
- Place cards only over their prey (lower trophic level). Do not overlap cards of the same color. Diatoms (green) lay down their cards on the nutrient sheet. Krill (yellow) may only lay down their card (feed) on top of diatoms (green), and whales may only lay down their card (feed) on top of krill (yellow).
- At the end of 5 minutes, record the number of cards remaining uncovered (still alive and feeding) for each trophic level. If the trophic level crashed (unable to add more cards) record this in Table 1.
- When a card is showing, those organisms are alive, feeding and reproducing. Once a card has been covered up, the organisms have been eaten and are dead/removed.
- If all the cards of a trophic level are covered up (they have been eaten), they cannot reproduce or feed. The trophic level has gone extinct or crashed.
- If a trophic level cannot feed (lay down any more cards), because there are no more cards of their prey (lower trophic level) uncovered (there is no more food left), then that trophic level has crashed.
- When a trophic level has crashed, record the time and number of cards uncovered for the crashed trophic level.
Table 1. Number of trophic level survivors and/or crash time per round
- Which levels are growing at the end of the game for all four rounds?
- Round 1 is a steady state system. Compare the production rates of the trophic levels. Predict what would happen to the diatoms and whales if this game were to run another 5 minutes?
- Assuming the krill crashed in Round 2A, hypothesize why this occurred.
- Name two ways a steady state could be restored for Round 2A?
- Compare the abundance of diatoms alive at the end of Rounds 1, 2A, and 2B. What controls the abundance of diatoms in each round?
- What limits the growth of the diatoms in Round 2C? Predict what would happen to the diatoms and whales if this game were to run another 5 minutes?
Part B: Creating Food Webs
Food chains shows the linear flow of energy from one organism to another. Food webs show all of the feeding relationships (what eats what) within an ecosystem and are essentially multiple combined food chains. Part A of this activity examined a single food chain in which energy is transferred in the following manner: Diatoms --> Krill --> Whale. While this simple food chain does exist in marine ecosystems, most marine ecosystems are far more complex.
- Create three food chain examples that contain at least three linkages (four organisms) that show the correct flow of energy and label each individual as either a producer or consumer (include consumer level).
- Construct a food web from the following list of organisms. The food web must include arrows to show the transfer of energy from one organism to another as well as the identification of each organism’s trophic level (may be more than one).
Organisms: Grasses, marmot, chipmunk, red tailed hawk, grouse, grizzly bear, deer, berries and flowers, seeds, fungi, rabbit.