The second variable is used to provide more variation across the islands. "Potential" is a representation of the islands ability to use the resources available. Where one island might have a lot of resources but no machinery to use it, another may have more machinery but less resources. So a simple supply and demand enviroment is created, a space which would benefit from a network.
Rule 3: Islands can store "Potential"
There is a natural increase in potential that is a representation of the chaos of events that happen all around us. The more resources there are and the longer the period of time since the last event the greater the potential for a new event will be.
Rule 4: Over time an islands "Potential" is increased