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Stage 2: Extending the model to represent a population of trees

Step 1
Draw a submodel box to completely enclose your model diagram.
  • Click on the  submodel tool on the toolbar
  • Move the mouse to the top left of your model diagram
  • Drag the mouse (i.e. depress the mouse button and move the mouse) to the bottom-right of your model diagram
  • Release the mouse button
Step 2
Rename the submodel tree.

This submodel will represent a population of trees. However, rather than calling it “trees”, it is conventional to name the class of objects after one individual, so here we call it “tree”.

Step 3
Open up the submodel Properties dialogue window.
  • Choose the  select tool
  • Double-click anywhere inside the submodel
Step 4
Click the radio button labelled “Using population symbols”.
Step 5
Choose a nice background colour for the submodel.
Step 6
Close the submodel properties dialogue window.

Your model diagram should now look like this:

Note that the submodel boundary now has an extra line on the top-and-left, and on the bottom-and-right. This is a visual signal that Simile now represents this as a “population submodel”.

Step 7
Change the expression for gr from its current value (0.2) to rand_const(0.1,0.3).

Since we will now be simulating the growth of several trees, we are giving each one a different growth rate, randomly selected between 0.1 and 0.3. This could reflect, for example, differences in the soil conditions where each tree grows, or genetic differences between trees.

Step 8
Add a  creation symbol to the model diagram, and give it a value of 5.

This is used to specify the initial number of trees (i.e the number at time zero). If we didn’t put this into our model, then our population would start off empty, i.e. with no trees.

Step 9
Add an  immigration symbol to the model diagram, and give it a value of 2.

We are stating that two new trees are created every year.

Step 10
Add a  loss symbol to the model diagram.
Step 11
Rename the loss symbol death.
Step 12
Draw an influence arrow from the compartment size to this symbol.

Your model diagram should now look like this:

Step 13
Enter the expression size>17 for the symbol labelled death.

This states that a tree will “die” (be eliminated from the model) when its size exceeds 17.

Step 14
Rebuild the model, and run it again.

You should see multiple lines on the graph. Each line represents the growth of a single tree. Note that each line has a different slope, since each tree has a randomly different growth rate (as determined by the variable gr). Note also that each year two new trees are created, and that trees disappear when their size value exceeds 17.