Students will look at single population of organisms and investigate how population growth is influenced by limiting factors such as resources. Using this NetLogo model they will be able to visualize some of the factors that lead to the formation of a carrying capacity for organism within a defined area. Students will then simulate how carrying capacity is affected by the introduction of a predator to the system and will also be able to see the dynamic interaction between these two organisms over the course of a large time scale.

The Wolf-Sheep Predation NetLogo model simulates the interactions between predator and prey within an ecosystem. These systems are looked at as being stable if these populations are able to maintain a relatively steady population over time, whereas an unstable system will result in the extinction in one or more of the populations.

There are two main variations to this model that you will be working with:

In the first variation, wolves and sheep wander randomly around the landscape, while the wolves look for sheep to prey on. Each step costs the wolves energy, and they must eat sheep in order to renew their energy. If wolves are unable to catch enough sheep, they will die. At each time interval both wolves and sheep have a fixed probability of reproducing, depending on the corresponding sliders.

The second variation includes grass (green) in addition to wolves and sheep. The behavior of the wolves is identical to the first variation, however this time the sheep must also eat grass in order to maintain their energy, if they don’t they will die. Grass that is eaten by sheep will regrow after a fixed amount of time depending on the grass regrowth time slider.

Basics of the model

1. Set the GRASS? switch to ON to include grass in the model, or to OFF to only include wolves (red) and sheep (white).

2. Use the default slider settings.

3. Go ahead and hit the "set up" button.

4. Answer the questions before hitting "go"

5. Press the GO button to begin the simulation.

6. Look at the monitors to see the current population sizes.

7. On the top of the world view you will see the word “ticks” with a number next to it. Each tick represents a unit of time passing by.

8. Look at the POPULATIONS plot to watch the populations fluctuate over time.

The behaviors of these organisms are influenced by changing the values of the green sliders. Go ahead and mess around with the model and sliders to get used to interacting with it.

Make sure the "grass?" option is unchecked. When you are ready to run the simulation, click ‘Go’ to start. NOTE: If at anytime you hit ‘Go’ again, this will pause the simulation.  After pausing, hitting ‘Setup’ will clear your simulation and return your initial settings.

Pay attention the increasing number of ‘ticks’.

Let the simulation run for at least 300 ticks and observe the interactions of the wolf and sheep over that time. If either of the animals die off click "go" to pause the simulation.

Change the "grass?" setting to on. Keep all other settings the same.

Hit "setup" and "run". Then let the sim run for 300 ticks before hitting "go" and answering the questions.

As stated earlier, a stable system will tend to have a relatively steady population over the course of time vs. an unstable population which results in the extinction of one or more of the populations.

Your challenge at this point is to turn the unlimited grass ecosystem into a stable system that allows for continuous generations of both wolves and sheep for at least 300 ticks.

The variable "grass?" must be unchecked.

Find three different ways that you can manipulate the simulation so that both populations die off. When you figure this out, describe the variable that you changed for each situation, and then explain why you think both populations were not able to survive.