The results of soil ciliate frequency studies of 47 field studies, from 12 ecosystem types, were combined with recent concepts and observations to assess the importance of soil ciliate biodiversity in ecosystem functioning. A few species typically furnish most of the individuals; increases in biodiversity, produced by less common species, alters soil ciliate communities. Soil ciliates were grouped according to position on the r/K continuum, with polyhymenophoran species as K, and colpodids as r organisms; and according to degree of soil affinity. Grasslands and hardwood forests were characterized by large numbers of K and intermediate species, whereas pine forests and more stressed ecosystems (e.g., arable lands, deserts) had more colpodid and fewer total species. Within these systems, certain species may become prominent, or be absent, suggesting that many soil ciliates exhibit widely overlapping niches, and that while a soil might function with a few species, high diversity allows systems to respond to changing seasons and climate.