The extent to which Echo captures the essence of real systems is still largely undetermined. An Echo world consists of a lattice of sites. Each is populated by some number of agents, and there is a measure of locality within each site. Different types of agents use different types of resources and can store these resources internally.
Sites charge agents a maintenance fee or tax.
This tax can also be thought of as metabolic cost. Agents fight, trade and reproduce. Fighting and trading result in the exchange of resources between agents. This allows the possibility of the evolution of social rules and potentially of mimicry, a phenomenon frequently observed in natural ecosystems. The interaction rules rely only on string matching.