The purpose of this assignment is to create your own representations of systems that we've discussed in class.

1. Create a descriptive model for a community that has six components:two consumers, two plants, soil nutrients, detritus pool.

a. Draw a picture (not using STELLA icons) that shows the internal flow of nutrients through this community (closed system) and the flow of energy through the community (open system).

b&c. Draw two linked models (using STELLA icons) that show the nutrient cycle and the flow of energy (using a co-flow type arrangement).

d. Use this model to discuss the relative limitations on the flow of nutrients vs. energy. Point out the important controls, such as the input of solar energy and the total amount of nutrient in the system.

e. Discuss the implications for energy and material flow for prey switching by one of the predators.

2. Develop a narrative for a system that is on the brink of a threshold response. Don't use one we've already discussed. Examples might be: a forest being stressed by drought but where the forest stores water itself or, a fish population that imports nutrients to its breeding grounds, but if the population decreases the nutrients for their young will also decrease. The characteristic is that as the stock is decreased there is a mechanism that leads to further subsequent loss.

a. Describe this scenario in several sentences and a drawing using arrows (not STELLA icons).

b. Draw graphs that represent system state vs. time and system state vs. stress level.

c. Constrain your description to using STELLA icons.

d. What information are you loosing from your narrative that isn't represented in the STELLA diagram? What might be the importance of loosing that information or texture?

e. Describe, in detail, how you would mathematically model the mechanism that causes the threshold shift. How does this equation actually work?