While open access to common-pool resources has been equated with a tragedy of the commons, we have found that mobile pastoralists in the Logone Floodplain in Cameroon are sustain ably managing open access to common-pool grazing resources. We have described this pastoral system as a self-organizing complex adaptive system (CAS) in which mobile pastoralists distribute themselves over common-pool grazing resources without central or collective decision-making. We have found evidence of management of open access in the form of an ideal free distribution (IFD). Here we discuss the results of an agent-based model (ABM) simulation and show how pastoralists are able to achieve an IFD with relatively simple movement rules. We describe this system as an Emergent Commons (EC).