To successfully preserve or manage a species, we must understand how the community of which it is a part functions. We must know how environmental alterations affect the fitness not only of the species of interest, but also that of its competitors, predators, and prey; and we must know the form and intensity of the pertinent inter-species interactions. Interaction Assessment (INTASS) is a non-manipulative approach to constructing quantitative expressions for fitness, written as functions of measured environmental variables including local population densities of conspecifics and other interacting species. We applied this approach to, and evaluated its consistency from, data on the European land snail (Cepaea nemoralis), American eel (Anguilla rostrata), longnosed and blacknosed dace (Rhinichthys cataractae and R. atratulus), and small-mouth bass (Micropterus dolomieu) until data sets can be compiled on more typically studied vertebrates. INTASS analyses on key species of a community can provide the tools for predicting community-wide ramifications of environmental disturbances. They also can permit the planned manipulation of relative species abundances through habitat alteration.