Part 1: The Producers
Go to http://www.learner.org/courses/envsci/interactives/ecology/food_web.php Challenge
Try to get two plants to happily co–exist. In any given ecosystem, most organisms will carve out a niche for themselves where they can obtain all of the necessities to survive. Often, different species within the ecosystem will compete for the resources that a niche provides. However, certain species live well together—symbiotically, parasitically, or by staying out of each other's way. For example, lichen and moss, often the primary colonizers of a new ecosystem, tend to live fairly harmoniously in each other's vicinity. Let's see what happens in this model.

Step 1
Imagine the ecosystem is newly forming—the previous ecosystem has been destroyed by fire or flood—and the first colonizers of the successive ecosystem are, of course, producers. Given the two fictitious species of plants in the simulator, predict what will happen in this young system and record your prediction in the Data Table. Then run the simulator to 100 time steps and record the population numbers for both plants. Answer the following: 1. What assumptions does this model make about co-dominance as well as the general terrain of the ecosystem? 2. Do you find one producer to be dominant? Why might one producer be dominant over another?
Step 2
Now you'll introduce an herbivore into the environment. In theory, an herbivore native to the ecosystem should feed primarily on the dominant species. In this system, the herbivore may consume enough of the dominant species to give the non-dominant species a chance for proliferation and survival. Click on herbivore A (the rabbit) and choose "eats plant A." Predict and record what will happen to the population numbers in the ecosystem. Then, run the simulator and record your results. Answer the following: 1. Does adding the herbivore establish a more equal field? Is one producer still dominant over the other? Why might one producer be dominant over another? 2. If the simulation included decomposers, how would your current results change? 3. How do producer population numbers with the presence of an herbivore compare to the primary colonizer model?
Food Web
Challenge
Now that you have a sense for the interrelationships between the trophic levels, see how big you can make your food web and still have all of the species you add survive through the end of the simulation run. Keeping the ideas of succession and the competitive exclusion principle in mind, think of the many factors that may go into sustaining an ecosystem. Is there any way we can all get along and live side by side?

Step 1
First you'll run a less than "real-life" scenario. Choose only one organism from each trophic level and make sure that the food chain goes in a straight line from one trophic level to the next, i.e., Herbivore A eats Plant A, Omnivore A eats Herbivore A, and the Top Predator eats Omnivore A. Let Plant B survive on its own and see what happens. Predict whether each species will survive, and whether it will increase or decrease in number, as well as whether Plant B will survive to the end. Record your prediction in the Data Table and then run the simulation twice and record your data. Use X for "die out," ↑ for "increase in numbers," and ↓ for "decrease in numbers." Answer the following: 1. Was your prediction correct? How did you arrive at your prediction? What differences were there between your prediction and the simulation? 2. What would happen to this imaginary ecosystem if the producers were to die out? 3. Did any of the species increase in number? What could account for this increase? Which species decreased in number and what might account for this decrease? 4. Which populations would benefit the most from the presence of decomposers?
Step 2
Now try a more "real-life" scenario and experiment with what might happen in an ecosystem that is more like a web. This time click the "all on" button. The model shows who eats whom and the paths by which energy is transferred. Predict which populations will die out, increase in numbers, or decrease in numbers and record your predictions. Run the simulation twice and record the results in your Data Table. Then try to modify who eats whom in order to ensure the survival of all species and record what was changed in your chart. Finally, answer the following:1. 1. Was your prediction correct? How did you arrive at your prediction? What differences were there between your prediction and the simulation? 2. Were you able to modify the parameters so that each species survived? Explain how you decided what changes to make. 3. Which way does energy flow and how does eating an organism result in energy transfer?