Gecko provides wide flexibility in specifying resources and arbitrarily shaped landscapes. The landscape is a real-valued plane. Scattered on this plane are rectangular sites. These sites arbitrarily overlap, abut, or have empty space between them. Each site produces one or more resources. The simulator makes no provision for wraparound boundary conditions---an edge is an edge. Sites are rectangular rather than circular simply for computational convenience.
In the models discussed here, there are four resources, or substances in the world: sunwater, carbos, proteins, and cproteins. Only sunwater is produced by landscape sites. The other resources are created by agents assimilating their food, and are thus energetically derived from sunwater. Plants turn sunwater into carbos, which comprises their biomass. Herbivores eat plants, assimilating their carbos into proteins. Carnivores eat herbivores, assimilating proteins into cproteins. An agent's volume (biomass) is a quantity of one or more resources. Its taxes are paid on its volume, in the resources that comprise that volume. Resources, and the assimilation steps that convert one resource to another, are the coin of all energetics in Gecko.
As Gecko was originally prototyped on the Echo platform [15,1], it may be helpful to contrast Echo and Gecko here. In Echo, resources are atomic units---agents collect and trade and steal integral quantities of resources, and integral quantities of resources are produced by each site. This site resource production is not spatially distributed within the site, but gifted equably to all agents desiring the resource. The Echo-based Gecko prototype extended this by giving agents the ability to perform integral transformations of resources, such as transforming ten of resource a to one of resource b. Inspired in small part by Gecko's exploration, Echo's conceptual model has since been extended to encompass this resource transformation [5].
Gecko's current resources are real-valued quantities, converted one to another by real-valued conversion rates. Resources are not gifted to agents, but are rather a density function over the landscape, doled out based on agent footprint area. Footprint on the landscape is established via competition. All resources in Gecko must be assimilated to become agent biomass. Thus though the details differ considerably, there remains a fundamental similarity between Echo and Gecko resources, in that growth and reproductive success are predicated on an agent's acquiring and maintaining embodied resources. In other respects, Echo and Gecko are markedly divergent. Gecko does not use resources for genetic behavior specification, and no evolution is modeled within a Gecko run. All members of a species are playing with the same fixed behavior in the course of a Gecko run.
Specifying a site is straightforward. A site is a rectangle with a production function, giving new production of each resource, per unit area, per time step. There is also a site reservoir limit, the maximum density function. At each step, the site adds its production of each resource, times the site area, to a reservoir, then truncates the reservoir according to the specified limit, and determines the density feed function---how much of each resource per unit area is currently available. As each agent claims resources, the reservoir is depleted. Any leftover resources in the reservoir carry over from one round to the next, but diffused evenly across the site, and subject to an upper limit. In the examples used in this paper, the resource limit is equal to current production, so no resources carry over between rounds.
There is great opportunity to expand the role of sites in Gecko. In this paper, when an agent dies or pays taxes, valuable resources are paid back to the site, where they lie unused. The entire decomposer side of ecosystems, the pathway through which the majority of net primary (plant) production passes [2], is not yet incorporated in these models, and every agent is a single organism. In the future, Gecko sites will have a ``metabolism'' that breaks down and recycles resources. In this guise, a site becomes a meta-agent, as a convenient shorthand for the activity of a multitude of soil microbes. Sites could stand in for any composite ecological role that could be abstracted as a metabolism and density function. Examples of this might include a continuous plankton supply, or a uniform distribution of millipedes and other detritivores, or a rain of seeds or spores or larvae, or a more-or-less uniform herbal ground cover punctuated by individually modeled trees.