I want to create a city filled with virtual creatures.

Say like Sim City, where each creature walks around, doing it's own tasks.

I'd prefer the city to not 'explode' or do weird things -- like the population dies off, or the population leaves, or any other unexpected crap.

Is there a set of basic rules I can encode each agent with so that the city will be 'stable'? (Much like how for physics simulations, we have some basic rules that govern everything; is there a set of rules that governs how a simulation of a virtual city will be stable?)

I'm new to this area and have no idea what algorithms/books to look into. Insights deeply appreciated.

Thanks!