ABSTRACT
In nature, events are concurrent and the influence of one event upon another propagates through yet other events. Thus, individual-based or individual-like models, which embody the concept of the interaction of model entities, are more true to nature than classical serial models controlled by high level data structures. However, in nature events and processes at one scale constrain or enable events and processes at other scales. The uniform time step and simple local interactions of models which simulate concurrent behavior are special cases of more complex and rich possibilities of organization. A model medium is presented which allows crafting of simulations embodying the principles of concurrent interacting entities which are also hierarchically ordered. The entities of such models advance the virtual time of the simulation. Actions may operate at different scales as the virtual occasions are composed by the model entities. This hierarchical and concurrent individual based model (HICBM) medium is presented in an object oriented idiom. Code for class methods essential to the concept of the medium is given in the programming language Smalltalk. The medium is created by hierarchical restrictions on the order and scope of message-passing among model entities in the HCIBM. The HCIBM scheme should provide a framework for the design of parallel computers to run concurrent ecological models. It is suggested that a better theoretical understanding of concurrency could refresh our understanding of nature. The notion of individual action appears inextricably involved in the concept of concurrency.
