Using a long-term data set (1989–1999), we examined how changes in abundance of 2 ecologically dominant species, prairie voles (Microtus ochrogaster) and hispid cotton rats (Sigmodon hispidus), affected the local diversity of a grassland rodent community at varying temporal scales. Species richness was associated positively with long-term fluctuations in the abundance of prairie voles and cotton rats over the 10-year period. Although increased vole densities consistently had a significantly negative effect on total community evenness, evenness of other community members was unaffected by fluctuating vole densities. Presence of cotton rats negatively affected both the relative abundance of other community members and the species richness during the years of high density (1991–1995), but did not exhibit longer-term influences. Thus, despite their numerical dominance, changes in prairie vole abundance had little ecological effect on the rest of the community, whereas the influence of cotton rats was density- and temporally dependent.