Since northern mixed prairies evolved under the influence of bison, grazing may be an important process maintaining diversity in these ecosystems. However, it is unclear whether grazing by cattle has the same ecological consequences as grazing by bison. I surveyed plant communities that were grazed at a range of intensities by bison or cattle, or were mechanically mowed. I used generalized linear mixed models and geostatistical techniques to evaluate the effects of grazing intensity and species of grazer on structural and floristic responses, and analysis of variance to evaluate the effects of mowing. I found that both grazing and mowing increased diversity and reduced evenness of the plant community. Spatial patterns of grazing were similar for bison and cattle, although bison created more discrete patches at the highest intensities of grazing. My results suggest that despite some differences in their selective preferences, the two species may have similar ecological effects.