Many field studies have examined how site fertility, soil differences and site history influence the diversity of a plant community. However, only a few studies have examined how the identity of the dominant species influences the diversity in grasslands. Plant species differ widely in phenology, growth form and resource uses; thus, communities dominated by different species are also likely to strongly differ in the environment that they create and in which the subdominant species exist. We examined the correlation between the four most dominant species and community diversity in 2100 plots, located in 21 abandoned agricultural fields in central Minnesota over a 23-year period. The four most common species were two non-native C3 cool season species, Poa pratensis and Agropyron repens, and two native C4 warm season species, Schizachyrium scoparium and Andropogon gerardii. We found that the differences in the dominants explained up to 27% of the community diversity. Thus, the identity of the dominant species can have a strong influence on community diversity and studies examining factors that influence plant community diversity need to incorporate the effect of the dominants. Secondly, we found that the non-native C3 grass dominated communities had lower overall and lower native species richness relative to the native C4 grass dominated communities. Therefore, a shift in dominants from C4 to C3 may lead to a large community diversity decline. We found that Poa pratensis, the most abundant non-native C3 grass increased in abundance over the 23 years; thus, the negative influence of non-natives on the community diversity is not decreasing over time and active management is required to restore native grassland plant communities.