To test whether population and community attributes, such as self-thinning and competitive displacement of one species by another, are the direct consequence of the physical characteristics of individual plants or the emergent properties of populations and communities, a simple computer model, called Vida, was created and tested. Vida is a spatially explicit, individual-based model that allows one to simulate the growth of individual trees, populations of trees, or mixed communities composed of different species. Despite using geometrically simple individuals to compete for light and space, Vida is able to accurately simulate the growth of populations of real species, and it is able to reproduce a number of community dynamics and relationships observed by field ecologists. Vida, therefore, provides a powerful heuristic tool with which to explore a large number of ecological principles and theories.