We experimentally investigated the determinants (consumer vs environmental productivity) and functional consequences of bacterial diversity using a simple laboratory bacterial assemblage and the bacterivorous ciliated protist Colpidium striatum. A two-way factorial design was used, with two productivity levels crossed with the presence/absence of Colpidium striatum. The effects of productivity and predation on bacterial diversity were similar to those previously reported for plant diversity in systems characterized by high productivities, with increased productivity reducing diversity and predation increasing diversity. When Colpidium was present, changes in trophic level biomass and individual bacterial species abundance in response to increased productivity were largely consistent with the keystone predation model. Importantly, Colpidium predation also resulted in an increase in particulate organic matter decomposition, largely due to an increase in the abundance of one single bacterial species that appeared resistant to predation. These results suggest that changes in community structure as a result of trophic interactions and other factors may have profound consequences for important ecosystem functions. We suggest that future research should take an integrative approach and study causes and consequences of biodiversity simultaneously.