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Description of Study Sites

The study sites were located in the University of Illinois Biological Research Area ("Phillips Tract") and Trelease Prairie, both 6 km NE of Urbana, Illinois (40ß 15'N, 88ß 28'W). Populations of Microtus ochrogaster and M. pennsylvanicus were monitored in 3 distinct habitats: restored tallgrass prairie (March 1972-May 1997), bluegrass, Poa pratensis, (January 1972-May 1997) and alfalfa, Medicago sativa (May 1972-May 1997). Tallgrass prairie was the original habitat of both species in Illinois, and bluegrass, an introduced species, represents 1 of the more common habitats in which the 2 species can be found today in Illinois. Alfalfa is an atypical habitat that provides exceptionally high-quality food for both species (Cole and Batzli 1979; Lindroth and Batzli 1984).

A number of manipulative studies were conducted during the course of the 25-year demographic study (Getz, et al. 1987). These examined effects of supplemental feeding and interspecific competition, and some involved removal of 1 species. Data presented in this paper are from unmanipulated sites in which both species were present. M.pennsylvanicus did not occur in the study region prior to 1972 (Getz, et al. 1978); the species first appeared in the study sites in May 1973.

We trapped 2 restored tallgrass prairies, 1 located in Trelease Prairie, the other in Phillips Tract (Fig. 1; Table 1). Trelease Prairie, established in 1944, was bordered by a mown lawn, cultivated fields, forbs and shrubs, and a macadam county road. In 1980, the cultivated field to the south of the study (Fig. 1) site was converted to grazed pasture. Relative abundances of plants (based on dry weight of vegetation clipped at the surface of 26 randomly located 1/4 m2 plots) in Trelease Prairie were as follows: big bluestem, Andropogon gerardii (17%); bush clover, Lespedeza cuneata (16%); ironweed, Veronia spp. (12%); Indian grass, Sorghastrum nutans, (10%); milkweed, Asclepias spp. (9%); goldenrod, Solidago spp. (9%); bluegrass (5%); switch grass, Panicum spp. (5%); blackberry, Rubus spp. (2%); little bluestem, Andropogon scoparius (2%); about 10 other species with a relative abundance of less than 1% (Getz, et al. 1979).

The tallgrass prairie in the Phillips Tract was established in 1968. This site was bordered on 1 side by an abandoned field that underwent succession from forbs and grasses to shrubs and small trees by the time the study ended. Cultivated fields bordered the other 3 sides. When the Phillips Tract site was first trapped in September 1977, prairie vegetation was well-developed. Lindroth and Batzli (1984) recorded the relative abundances of the most prominent plant species in this site: Andropogon gerardii,(38%), Lespedeza cuneata (25%), Beard tongue foxglove, Penstemon digitalis (16%), and Sorgastrum nutans (19%). All other species represented less than 1% relative abundance. Both prairies were burned during the spring at 3-4-year intervals to retard invading shrubs and trees (Figs. 2 and 4). We trapped 1 or the other of the 2 tallgrass prairie study areas, depending upon the requirements of the overall study at the time (Table 1). Vole populations fluctuated in synchrony in the 2 tallgrass areas (L. L. Getz in litt.).

The bluegrass study sites were established within a former bluegrass pasture located in Phillips Tract (Fig. 1; Table 1). The pasture was released from grazing in June 1971; dense vegetation cover existed by autumn 1971. Relative abundances of plants (same methods as in Trelease Prairie) during this period were: bluegrass (70%); dandelion, Taraxacum officinale (14%); wild parsnip, Pastinaca sativa (4%); goatsbeard, Tragopogon sp. (3%); approximately 20 other species with a relative abundance of less than 1% (Getz, et al. 1979).
To reduce successional changes, especially invading forbs, shrubs, and trees, the bluegrass sites were mowed during late summer every 2-3 years. The entire area was mowed at the same time. The rotary mower was set to cut the vegetation about 25 cm above the surface. This height resulted in suppression of growth of the invading forbs and woody vegetation, but left the bluegrass uncut. The vegetation in this site remained relatively unchanged throughout the first 22 years of the study. During the last 3 years of the study, brome grass, Bromus inermis, rapidly became established throughout the study site, comprising nearly 75% of the vegetation. Timothy, Phleum pratense, also became prominent in parts of the study site at this time, and bluegrass made up approximately 20% of the ground cover.

Two sites supporting alfalfa vegetation were trapped during the study (Fig. 1; Table 1). A site was trapped until the alfalfa plants began to be crowded out by invading forbs and grasses. A year before trapping was terminated in 1 site, the other was planted to alfalfa so that the alfalfa plants would be fully developed when trapping commenced in that site. The sites were separated by a 10-m, closely mown strip. This reduced the incidence of animals whose nests were in 1 field having home ranges extending into the other field during the period when alfalfa was present in both sites.

Animals moved between the 2 sites, however, so we presumed we were monitoring a single population. Initially, alfalfa comprised 75% of the vegetation in each site. During the last year of usage, common plants (in addition to alfalfa) included bluegrass, goldenrod, timothy, brome grass, clover (Trifolium repens and T. pratense) and plantain (Plantago spp). A series of 3-m wide strips were mowed 25 cm above the surface periodically each summer to control invading weedy forbs and to promote new growth of alfalfa plants. The first strips were usually mowed in early June. The mowing normally stopped in mid September. The subsequent strips were not mowed until the vegetation in the previously mowed strips was nearly full-grown. The times of mowing were spaced so that at least two-thirds of the field had dense vegetation cover at all times. Even in the mown strips, live vegetation and recently mown litter provided dense surface cover.