I assembled bacterial communities to explore the effects of bacterial diversity on multiple ecosystem functions, including bacterial community biovolume, decomposition of particulate organic matter, and biomass transfer to the next trophic level. The experiment used a two-way factorial design with four levels of bacterial diversity (one to four species) and the absence/presence of a bacterivorous ciliated protist Tetrahymena pyriformis as two main factors, and all possible combinations of the four bacterial taxa nested within each diversity level. Bacterial community biovolume increased as bacterial diversity increased, a result due largely to positive selection effects. Decomposition and consumer abundance, however, were unaffected by bacterial diversity, though both varied among bacterial composition treatments. Negative selection effects, the dominance of species that do not contribute significantly to ecosystem functioning, accounted for the lack of diversity effects on decomposition and consumer abundance. The presence of Tetrahymena reduced bacterial community biovolume but increased decomposition, without altering the diversity-functioning relationships. Decomposition was strongly linked with consumer abundance such that communities supporting larger consumer biomass exhibited higher decomposition rates. This study suggests that if the negative selection effect is common, as it might be when examining ecosystem variables other than biomass (due to the presence of keystone species that can contribute disproportionably to ecosystem functioning relative to their abundances), basic bacteria-mediated ecosystem processes, such as decomposition and energy transfer to the next trophic level, may not increase with bacterial diversity.