Processes structuring riparian corridors can be viewed as a hierarchy, in which primary factors (such as matter, energy, and water) create a spatially extensive and temporally variable physical environment, which becomes habitat for plants and animals. The habitat is further modified by the activities of large animals as they selectively eat vegetation, burrow and wallow in soils, and build dams on streams, among other activities. As a result, the variety of habitats, or "patches," is increased. The vegetation and microorganisms living on the increased variety of habitat patches largely determine the eventual distribution and cycling rates of elements (e.g., nitrogen and phosphorus) as basic population and community processes are carried out. In general, ecologists understand how interactions among water, energy, and matter shape the physical characteristics and habitat patches of river corridors, and how vegetation and microbes cycle elements, grow, reproduce, compete, and otherwise function. However, there has been little recognition of the equal importance of large animals in shaping the character of riverine corridors. This omission is especially surprising, considering that their influence on the habitat mosaic of rivers is second only to the primary factors of matter, energy, and water. By eating plants, moving soil, and dispersing seeds, larger animals (such as mammals, reptiles, and birds) alter vegetative structure, modify channel morphology, and assist in developing microtopography. The ecosystem-level consequences of these physical and trophic activities go far beyond supplying individuals with food and habitat.