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
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).
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.