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This model explores the stability of predator-prey relationships in aquatic ecosystems. Such a system is called unstable if it tends to result in extinction for one or more species involved. In contrast, a system is stable if it tends to maintain itself over time, despite fluctuations in population sizes.


Crayfish and smallmouth bass wander randomly around the bay, and what they eat is determined by the Food-Web box. Moving costs energy, and they must eat in order to replenish their energy. When a creature runs out of energy it dies. To allow the population to continue, each crayfish or smallmouth bass has a fixed probability of reproducing at each time step. As periphyton is eaten it regrows slowly, statistically, outward from other living periphtyon.


1. Press the SETUP button to populate the bay.
2. Press the Go/Stop button to begin the simulation.
3. Look at the Population Size plot to watch the populations fluctuate over time
4. Press the Go/Stop button to stop the simulation.

Changing the population conditions:
1. Adjust the sliders to change the population starting conditions.
2. Press the SETUP button to populate the bay with new population sizes.
3. Press the Go/Stop button to begin the simulation.

Changing the food web relationships
1. Click Change to edit the Food-Web box. You can change who is the predator and who is the prey. The species in this ecosystem include: periphyton, crayfish, and smallmouth bass.


Population sizes vary over time. How do different species population sizes relate to one another? How is time a factor in this relationship?

When pressing the SETUP button to start a new run of the model, it will not be an exact duplication of how the model ran the previous time before. Each press of the SETUP button randomly starts the model off with new initial parameters.


Try adjusting the parameters under various settings. How sensitive is the stability of the model to the particular parameters?

Notice that under stable settings, the populations tend to fluctuate at a predictable pace. Can you find any parameters that will speed this up or slow it down?

Try changing the food web relationships. Can all species eat one another?


The next model in this series, Aquatic Bioacculumation, adds additional information and graphs. You can observe how toxins move through the ecosystem as different species eat one another.


Note the use of breeds to model two different kinds of "turtles": crayfish and smallmouth bass.


Aquatic Bioacucmulation uses the same food web relationships but adds parameters to be studied.

Aquatic Invasive Species adds more species to the food web to increase complexity of relationships.

Terrestrial ecosystems can be investigated in the Wolf Sheep Predation model.

Look at Rabbits Grass Weeds for another model of interacting populations with different rules.