Tasmanian Devil Model The Tasmanian Devil is the world's largest surviving carnivorous marsupial, and is found only in Tasmania, Australia. In recent years the Devil has been struck by a facial tumor disease that is spread by contact. Tasmanian devils with the disease usually die within three to eight months of the lesions first appearing. The disease first appeared in 1996 and is now threatening to wipe out the entire population. You can read more about Devils and the disease here: http://www.parks.tas.gov.au/wildlife/mammals/devil.html In this tutorial we are going to use Starlogo TNG to create a simple model of the spreading of the disease. Our model will follow these rules: Devils move around randomly within their own territory. The disease is spread by one devil contacting another. The disease causes devils to weaken and eventually die. The tutorial will show you how to use Starlogo TNG to create a model for the spread of the Devil Facial Tumour Disease. The following links will take you to each section of the tutorial: Starlogo TNG Model Populate the World First we need to populate our world with Tasmanian Devils.
Go to Edit Breeds. Choose a suitable breed without a skin and rename it Devils. It is important to choose a model without a skin because we later want to use colours to show infected or healthy animals. Move to the Everyone workspace and drag out a Slider control. Attach a shared number variable to it. This will store the number of Devils in the world. Rename it population size. Switch to SpaceLand and change the upper limit for Population size to 300 Switch to SpaceLand to view your world. Move to the Setup workspace. Drag a Setup block onto it. Add a Clear Everyone block. Goto My Blocks>Everyone and drag on a set population size block. Attach a number block and set it to be 200. (Hint: You can quickly create a number block by just typing the number onto a blank workspace area) Goto My Blocks>Devils and drag a Create Devils Do block out. The number should be changed to population size. Under Traits find a set color block, and set it to be black. Then drag a scatter Devils block from the My Blocks>Devils section. Make the Devils Move The next step is to make the Devils move around randomly in the world.
Drag a Procedure block onto the Devils workspace. Rename it Move. Attach Left and Right Movement blocks to it. They are set to 90, but we want the Devils to move randomly, so change these to random 90. Add a forward 1 step block. Change to the Runtime area. From the Setup tab, drag a Forever block onto the Runtime area. From My Blocks > Devils, drag out a Devils Move block and join it on. Change to the SpaceLand window. Click Setup to make your Devils appear, then click Forever to make them move. Setup the Infection The initial rate of infection will be able to range from 0 to 100%. It will be stored in a variable and set by using a slider control. To determine if an individual Devil is infected, we will generate a random number between 1 and 100. If that number is less than or equal to the infection rate, then the Devil will be infected. Move to the Everyone workspace and drag out a Slider control. Attach a shared number variable to it. Name it Infection Rate. Switch to SpaceLand and change the upper limit for Infection Rate to 50. Go to the Setup workspace. We no longer want all Devils to be black. Infected ones will be red. Alter the Setup so it appears like this:
Switch to SpaceLand. Set an infection rate and test it. Spread the Infection The infection is spread by Devils contacting each other, so we will set it up so that if a red and a black Devil collide, the black one will change color to red. Go to the Collisions workspace. Look in My Blocks>Devils for the Collision block and drag it out. Set up the collision as below. Hint: You can use Ctrl-C and Ctrl-V to copy and paste blocks. Both sections of the Collisions block are set up to be identical, as it does not matter which Devil in the collision is red, they both should become red afterwards.
Go to SpaceLand and test to make sure the collisions work as expected. Monitor the Infection We will set up monitors to keep count of the healthy and infected Devils. Goto the Runtime area and drag in a Monitor block (from Setup and Run) Now add a count everyone with block. Then add Color = black Copy and paste the block. Change it to be Color = red. Rename one monitor Healthy and the other Infected. Devils Weaken and Die Sick Devils get weaker over time. We will use a variable to store the energy level for each Devil. For sick Devils, we will repeatedly reduce this value by a small random amount. When it reaches zero, the Devil will die. In the Devils workspace, drag in an agent number block, and name it energy. This is our energy variable. Switch to the Setup workspace, and under My Blocks>Devils, drag the set energy block into the Create Devils action. Set the level to be 200.
Go back to the Devils workspace, and we will create a Procedure for Devils getting weaker, as below. Finally switch back to the Runtime workspace, and from My Blocks, drag a Get Weaker block into the Forever loop. Create Territories We will create territories by drawing different colored regions in SpaceLand. Devils generally move around and stay within their own territory. Switch to SpaceLand, and use the zoom button to zoom out so that you can see the whole land. Click the Edit Terrain button. Use the mouse to select a region, and then click the color palette to make it a different color. Make 8 or 9 patches Stay In Territory Devils should generally stay in their own territory. To do this we will check the patch color ahead. If it is a different color than the patch they are on we will turn them around 180 degrees
and take 3 steps. This should return it to its original territory. We will alter the move procedure to do this. Now your model is built, you can set about making some variations to it. Using your model, try out some of the following: Try altering the starting population. (You could add in another slider for initial population if you like.) Are the devils more likely to become extinct if the initial population is larger or smaller? Try adjusting the speed the devils move around (Make the forward steps 0.5 instead of 1). What difference does this make to the spread of the disease? Try altering the territory size. Does the disease spread faster if the territories are smaller or larger? Under what conditions can some devils survive? Alter the rate at which the Devils die after infection. What difference does it make? Think of a question of your own that you could use your model to answer. In what ways is your model different from the situation of Devils in the real world? What factors are missing that might be important? Extension
Add in a Devil breeding program, so that healthy Devils can reproduce. You can get some ideas about how to set up breeding from the Fish and Plankton example that comes packaged with Starlogo TNG.