An individual-based simulation system, named Gecko, is presented for modeling multiple species at multiple trophic levels, on a spatially explicit, continuous two-dimensional landscape. Biologically motivated rules are specified at an individual level, and resulting behaviors are observed at an ecosystem level. Individuals are represented by circles with free range on a resource-producing plane. These circles grow allometrically with biomass of fixed resources. Resource acquisition behaviors include competition by area overlap for producers, and movement based on perception and intent. Individual-level energetics are explicitly modeled with inefficient assimilation, resource transformation, and allometrically specified metabolic costs. Individual growth and reproduction requires a history of successful resource acquisition. Terrestrial producer, herbivore, and carnivore species classes are included, extensible to further classes. A grassland food chain model of “plants,” “grasshoppers,” and “spiders” is used to demonstrate ecosystem-level results of given individual-level behaviors. Ecosystem-level behaviors include a trophic cascade of indirect carnivore-producer interaction effects; stable persistence of all populations; a near-realistic biomass pyramid; and spatial competition and coexistence of multiple producer species. Initial Gecko results show promise for application in both theoretical and natural ecosystem modeling.