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Stage 3: Re-casting the model diagram

The present model is based on a number of assumptions that are biologically unsound. For example, it assumes that the number of rabbits eaten per fox is directly proportional to the number of rabbits, so doubling the number of rabbits doubles the number eaten per fox — even if there are millions of rabbits around!

We will tackle the problem of making the model more biologically realistic in two stages. First, we will change the model diagram a little, adding in a couple of variables and changing the influence arrows, but retaining the same mathematical structure: the revised model should produce exactly the same results. These changes provide more appropriate handles in our model for expressing biologically-meaningful relationships. Then, in the following stage, we will replace some of the simplistic assumptions in this model with others that are biologically more realistic.

Step 1
Use the  delete tool to remove the influence arrows from rabbits to predation, and from predation to fox repro.
Step 2
Add in two new  variables, naming them rfox and eaten per fox.
Step 3
Draw  influence arrows:
  • from the rabbits compartment to eaten per fox;
  • from eaten per fox to rfox;
  • from eaten per fox to the predation flow;
  • from rfox to the fox repro flow; and
  • from the foxes compartment to the fox repro flow.

Your model diagram should now look like this:

Step 4
Enter the following expression for each variable or flow:
  • eaten per fox : 0.01*rabbits
  • rfox : 0.01*eaten_per_fox
  • predation : eaten_per_fox*foxes
  • fox repro : rfox*foxes

Satisfy yourself that these changes have not altered the mathematical nature of the model.

Step 5
Re-build and run the model again, confirming that you do indeed get the same behaviour that you had before.