We compared the effects of 2 common grazers, southern redbelly dace (Phoxinus erythrogaster) and crayfish (Orconectes spp.), on ecosystem structure and function in experimental streams with pool and riffle habitats. Our goal was to identify potentially overlapping roles of these grazers in these systems. Measures of ecosystem structure included algal filament length, particulate organic matter (POM), densities of invertebrate taxa, and algal biomass. Ecosystem function was measured as gross primary productivity (GPP). Biomass-dependent effects of crayfish and dace on ecosystem properties were compared in autumn 2005 when mean water temperature was 12.9°C (range 7.6–27.9°C). Increasing crayfish biomass did not influence ecosystem properties, but increasing dace biomass negatively affected algal filament length and chironomid abundance and positively affected chydorid abundance. Effects of moderately high biomasses of dace and crayfish were compared in spring 2006 when mean water temperature was 21.4°C (range 17.5–29.9°C). Algal filament lengths were generally low relative to values obtained in autumn 2005 in both dace and crayfish treatments. In addition, algal filament length was shorter and chironomid density was lower in crayfish than in dace streams. The contrasting effects of dace and crayfish across sampling days, seasons, and habitats led us to hypothesize that physiological and behavioral traits of these species might limit the redundancy of their effects on ecosystems across broad spatial and temporal scales.