Yes predator prey or Lotka Volterra are the same in this case. There is/was a visual formalism called DEVS - Discrete Event System Specification , which has developed a host of visual tools
for modeling and maybe code/stub gen (if i remember correctly. These are other types of [Open source packages on wikipedia](http://en.wikipedia.org/wiki/List_of_discrete_event_simulation_software)
* CD++  is an open source based on the DEVS and Cell-DEVS formalisms (a discrete-event specification of cellular automata models). Hundreds of model samples are available. Varied 2D and 3D visualization engines can be used to improve the analysis of the simulation results.
* PowerDEVS is an integrated tool for hybrid systems modeling and simulation based on the DEVS formalism.
* SimPy is an open source process-oriented discrete event simulation package implemented in Python. It is based on Simula concepts, but goes significantly beyond Simula in its synchronization constructs.
* Tortuga is an open source software framework for discrete-event simulation in Java.
* Facsimile is a free, open-source discrete-event simulation/emulation library.
* Galatea - Galatea is a Agent-based simulation platform.
* MASON is a fast discrete-event multiagent simulation library core in Java, designed to be the foundation for large custom-purpose Java simulations.
The only one I tried is Mason but its Java-based but was good for agent/network sim's. I liked it but it might not be suitable for your purposes