Presence and stability of populations of Microtus ochrogaster, M. pennsylvanicus and Blarina brevicauda were studied in tallgrass prairie habitat in central Illinois for 25 yr: 52 yr of autumn data were available for M. ochrogaster. M. ochrogaster displayed erratic, very low amplitude population fluctuations in tallgrass; it was absent from the study site 49% of the time. Multiannual cycles were not obvious. Population fluctuations in adjacent alfalfa and bluegrass sites were higher, more frequent and in synchrony with those in tallgrass. Densities recorded in the autumn from 1944 to 1996 revealed distinct, erratic multiannual population cycles. M. pennsylvanicus populations were continuously present at relatively high densities in tallgrass; neither annual nor multiannual fluctuations were evident. Infrequent annual cycles were recorded in the other two habitats. B. brevicauda was persistent and displayed a synchronous annual cycle in the three habitats.
