Although other species of the genus Microtus, especially M. pennsylvanicus, have been studied intensively in regard to habitat preference (Blair, 1940:149; 1948:404-405; Bole, 1939:69; Eadie, 1953; Gunderson, 1950:32-37; Hamilton, 1940:425-426; Hatt, 1930:521-526; Townsend, 1935:96-101) little has been reported concerning the habitat preferences of M. ochrogaster. Black (1937:200) reported that, in Kansas, Microtus (mostly M. ochrogaster) preferred damp situations. M. ochrogaster was studied in western Kansas by Brown (1946:453) and Wooster (1935:352; 1936:396) and found to be almost restricted to the little-bluestem association of the mixed prairie (Albertson, 1937:522). Brumwell (1951:213), in a survey of the Fort Leavenworth Military Reservation, found that M. ochrogaster preferred sedge and bluegrass meadows but occurred also in a sedge-willow association. Dice (1922:46) concluded that the presence of green herbage, roots or tubers for use as a water source throughout the year was a necessity for M. ochrogaster. Goodpastor and Hoffmeister (1952:370) found M. ochrogaster to be abundant in a damp meadow of a lake margin in Tennessee. In a study made on and near the campus of the University of Kansas, within a few miles of the area concerned in the present report, Jameson (1947:132) found that voles used grassy areas in spring and summer, but that in the autumn, when the grass began to dry, they moved to clumps of Japanese honeysuckle (Lonicera japonica) and stayed among the shrubbery throughout the winter. Johnson (1926:267, 270) found M. ochrogaster only in uncultivated areas where long grass furnished adequate cover. He stated that the entire biotic association, rather than any single factor, was the key to the distribution of the voles. None of these reports described an intensive study of the habitat of voles, but the data presented indicate that voles are characteristic of grassland and that M. ochrogaster can occupy drier areas than those used by M. pennsylvanicus. Otherwise, the preferred habitats of the two species seem to be much the same.

In the investigation described here I attempted to evaluate various types of habitats on the basis of their carrying capacity at different stages of the annual cycle and in different years. The habitats were studied and described in terms of yield, cover and species composition. The areas upon which live-trapping was done were studied most intensively.

These two areas, herein designated as House Field and Quarry Field, were both occupied by voles throughout the period of study. Population density varied considerably, however (Fig. 5). Both of these areas were dominated by Bromus inermis, and, in clipped samples taken in June, 1951, this grass constituted 67 per cent of the vegetation on House Field and 54 per cent of the vegetation on Quarry Field. Estimates made at other times in 1950, 1951 and 1952 always confirmed the dominance of smooth brome and approximated the above percentages. Parts of House Field had nearly pure stands of this grass. Those traps set in spots where there was little vegetation other than the dominant grass caught fewer voles than traps set in spots with a more varied cover. Poa pratensis formed an understory over most of the area studied, especially on House Field, and attained local dominance in shaded spots on both fields. The higher basal cover provided by the Poa understory seemed to support a vole population larger than those that occurred in areas lacking the bluegrass. Disturbed situations, such as roadsides, were characterized by the dominance of Bromus japonicus. This grass occurred also in low densities over much of the study area among B. inermis. Other grasses present included Triodia flava, common in House Field, but with only spotty distribution in Quarry Field; Elymus canadensis, distributed over both areas in spotty fashion and almost always showing evidence of use by voles and other small mammals; Aristida oligantha and Bouteloua curtipendula, both more common on the higher and drier Quarry Field; Panicum virgatum, Setaria spp., especially on disturbed areas; and three bluestems, Andropogon gerardi, A. virginicus and A. scoparius. The bluestems increased noticeably during the study period (even though grasses in general were being replaced by woody plants) and they furnished a preferred habitat for voles because of their high yield of edible foliage and relatively heavy debris which provided shelter.

On House Field the most common forbs were Vernonia baldwini, Verbena stricta and Solanum carolinense. On Quarry Field, Solidago spp. and Asclepias spp. were also abundant. All of them seemed to be used by the voles for food during the early stages of growth, when they were tender and succulent. The fruits of the horse nettle (Solanum carolinense) were also eaten. The forbs themselves did not provide cover dense enough to constitute good vole habitat. Mixed in a grass dominated association they nevertheless raised the carrying capacity above that of a pure stand of grass. Other forbs noted often enough to be considered common on both House Field and Quarry Field included Carex gravida, observed frequently in House Field and less often in Quarry Field; Amorpha canescens, more common in Quarry Field; Tradescantia bracteata, Capsella bursapastoris, Oxalis violacea, Euphorbia marginata, Convolvulus arvensis, Lithospermum arvense, Teucrium canadense, Physalis longifolia, Phytolacca americana, Plantago major, Ambrosia trifida, A. artemisiifolia, Helianthus annuus, Cirsium altissimum and Taraxacum erythrospermum. Both areas were being invaded from one side by forest-edge vegetation; the woody plants noted included Prunus americana, Rubus argutus, Rosa setigera, Cornus drummondi, Symphoricarpus orbiculatus, Populus deltoides and Gleditsia triacanthos.

In House Field the herbaceous vegetation was much more lush than in Quarry Field and woody plants and weeds were more abundant. A graveled and heavily used road along one edge of House Field, leading to the Reservation Headquarters, was a barrier which voles rarely crossed. A little-used dirt road crossing the trapping plot in Quarry Field constituted a less effective barrier. The disturbed areas bordering the roads were likewise little used and tended to reinforce the effects of the roads as barriers. There were almost pure stands of Bromus japonicus along both roads. No mammal of any kind was taken in traps set where this grass was dominant.

Because seasonal changes in vole density followed the curve for rate of growth of the complex of grasses on the Reservation, and because years in which there was a sparse growth of plants due to dry weather showed a decrease in the density of voles, the relationships between productivity of plants and vole population levels on the two study areas were investigated. In both fields the composition of the plant cover was similar, and the differences were chiefly quantitative. In June, 1951, ten square-meter quadrats were clipped on each of the areas to be studied. The clippings from each were dried in the sun and weighed. From Quarry Field the mean yield amounted to 1513 ± 302 lbs. per acre; while from House Field the yield was 2351 ± 190 lbs. per acre (Table 1). Using experience gained in making these samples, I periodically estimated the relative productivity of the two areas. House Field was from 1.5 to 3 times as productive as Quarry Field during the growing seasons of 1951 and 1952. Although House Field, being more productive, usually supported a larger population of voles than Quarry Field the reverse was true at the time of the clipping (Fig. 5).

Table 1. Relationship Between Yield and Various Population Data

Although no explanation was discovered which accounted fully for the seeming aberration, two sets of observations were made that may bear on the problem. In June, 1951, the population of voles and cotton rats on Quarry Field was increasing rapidly whereas in House Field that trend was reversed. The trends were reflected by the percentages of immature individuals in the two populations and by the ratios of the June, 1951, densities to the March, 1951, densities (Table 1). Perhaps the density curve was determined in part by factors inherent in the population and, to that extent, was fluctuating independently of the environment (Errington, 1946:153).

The flood in 1951 reduced the population of voles and obscured the normal seasonal trends. Although House Field produced a heavier crop of vegetation, Quarry Field produced a larger crop of rodents, chiefly Microtus and Sigmodon. In House Field, however, the ratio of Sigmodon to Microtus was notably higher. Presumably the cotton rats competed with the voles and exerted a depressing effect on their numbers. The intensity of the effect seemed to depend on the abundance of both species. That this depressing effect involved more than direct competition for plant food was suggested by the fact that in House Field, with a heavy crop of vegetation and a seemingly high carrying capacity for both herbivorous rodents, the biomass of voles, and of all rodents combined, were lower than in Quarry Field which had less vegetation and fewer cotton rats. The relationships between voles and cotton rats are discussed further later in this report.

When the centers of activity (Hayne, 1949b) of individual voles were plotted it was seen that there was a shift in the places of high density of voles on the trapping areas. This shift seemed to be related to the advance of the forest edge with such woody plants as Rhus and Symphoricarpos and young trees invading the area. These shifts were clearly shown when the distribution of activity centers on both areas in June, 1951, was compared with the distribution in June, 1952 (Fig. 1). The shift was gradual and the more or less steady progress could be observed by comparing the monthly trapping records. It was perhaps significant that during the summers the centers of activity were less concentrated than during the winter. The shift of voles away from the woods was more nearly evident in winter when the voles were driven into areas of denser ground cover, which provided better shelter.

From 1948 to 1950 and again in 1952 and 1953 I trapped in various habitat types in a mixed prairie near Hays, Kansas. Before the great drought of the thirties, Microtus ochrogaster was the most common species of small mammal in that area. Since 1948, at least, it has been taken only rarely and from a few habitats. No voles have been taken from grazed sites. In a relict area, voles were trapped in a lowland association dominated by big bluestem. Since 1948 only one vole has been trapped in the more extensive hillside association characterized by a mixture of big bluestem, little bluestem and side-oats grama. None was taken in the upland parts of the relict area where buffalo grass and blue grama dominated the association.

In the pastured areas there are nine livestock exclosures established by the Department of Botany of Ft. Hays Kansas State College. These exclosures included many types of habitat found in the mixed prairie. All of these exclosures were trapped and voles were taken in only two of them. An exclosure situated near a pond, on low ground producing a luxuriant growth of big bluestem and western wheat grass, has supported voles in 1948, 1949, 1952 and 1953. An upland exclosure containing only short grasses also supported a few voles in 1953.

An examination of the nature of the various plant associations of the mixed prairie indicates that yield of grasses, amount of debris and basal cover may be critical factors in the distribution of voles. The association to which the voles seemed to belong was the lowland association. Hopkins et al (1952:401; 409) reported the yield of grasses from the lowland to be approximately twice as great as from the hillside and upland in most years. Probably equally important to the voles was the fact that debris accumulation in the lowland was approximately five times as great as in the upland and approximately 2.5 times as great as on the hillside (Hopkins, unpublished data). The unexpected presence of voles in the short grass exclosure was probably due to two factors. In ungrazed short grass, basal cover may reach 90 per cent (Albertson, 1937:545), thus providing excellent cover for voles. Also, the ungrazed exclosure had greater yield and a thicker mat of debris than the grazed short grass surrounding it and was thus a relatively good habitat, although it did not compare favorably with the lowland type.

Samples of the populations of various areas, obtained by snap-trapping, gave further information regarding the types of vegetation preferred by voles. Voles were taken in all ungrazed and unmown grasslands trapped in eastern Kansas, although some of the areas were not used at all seasons of the year nor in years having a low population of Microtus. Reithro Field, similar to Quarry Field in its general aspect, had a heavy population of voles in the spring and summer of 1951, a time when voles were generally abundant. On the same area the population of small mammals was sampled in the summer of 1949 and, though occasional sign of voles was seen, not one vole was trapped. Later trapping, in the spring and summer of 1952, also failed to catch any voles and Fitch (1953, in litt.) caught none in several trapping attempts in 1953. These later times were characterized by a general scarcity of voles. Reithro Field was drier, with less dense vegetation, than the two main study areas and had larger percentages of little bluestem (Andropogon scoparius) and side-oats grama (Bouteloua curtipendula) and smaller percentages of Vernonia, Verbena, Solanum and Solidago.

Various species of foxtail (Setaria) dominated most roadsides in the vicinity of the Reservation. Voles almost always used these strips of grass but never were abundant in them. Voles were taken near the margin of a weedy field, fallow since 1948, but there was none in the middle of the field. Most individuals were confined to the grassy areas around the field and made only occasional forays away from the edge. The dam of a small pond on the Reservation and low ground near the water were used by Microtus at all times. In the summer of 1949 no voles were taken anywhere on the Reservation but their runways were more abundant around the pond than in the other places examined. Of all the areas studied in the summer of 1949, only the pond area had been protected from grazing in previous years. Polygonum coccineum was the most prominent plant in the pond edge association. A few voles were trapped in large openings in the woods, where a prairie vegetation remained and where voles seemingly lived in nearly isolated groups.

Voles were rarely taken in grazed or mown grassland or in fields of alfalfa, stubble or row crops. The critical factor in these cases seemed to be the absence of debris or other ground cover under which runways and nests could be concealed satisfactorily. Woods, rocky outcroppings and bare ground were not used regularly by voles. Fitch (1953, in litt.) has taken several Microtus in reptile traps set along a rocky ledge in woods but most of these voles were subadult males and seemed to be transients. Fields in the early stages of succession also failed to support a population of voles. Such areas on the Reservation were characterized by giant ragweed, horse weed, thistles and other coarse weeds. Basal cover was low and debris scanty. Not until an understory of grasses was established did a population of voles appear on such areas. The coarse weeds seemed to provide neither food nor cover adequate for the needs of the voles.

An analysis of trapping success at each station in House Field further clarified habitat preferences. The tendency of voles to avoid woody vegetation was again demonstrated. Not only was the population concentrated on that part of the study plot farthest from the forest edge but, as a general rule, voles tended to avoid single trees or clumps of shrubby plants wherever these occurred on the area. As an example, trap number 18 never caught more than one per cent of the monthly catch and in many trapping periods caught nothing. This trap was under a wild plum tree. Adjacent traps often were entered; the general area was the most heavily populated part of the study plot. Only under the plum tree was there a relatively unused portion.

Traps number 29 and 30, in the shade of a large honey locust tree, also caught but few voles. Trap number 30 was only six feet from the base of the tree and caught but one vole throughout the study period. These two traps caught more Peromyscus leucopus than any other pair, however, and both of them also caught pine voles (M. pinetorum). The area shaded by this tree permitted an extension of parts of the forest edge fauna into the grassland.

In spite of the marked general tendency to avoid woody plants, some voles made their runways around the roots of blackberry bushes, sumac and wild plum trees. Some nests were found under larger roots, as if placed there for protection. More vegetation was found under the woody plants which the voles chose to use for shelter than under those which they avoided. It seemed probable that the actual condition avoided by voles was the bareness of the ground (a result of the shade cast by the woody plants) rather than the woody plants themselves.

Running diagonally across the eastern half of the trapping plot in House Field there was a terracelike ridge of soil. On each side of this ridge there was a slight depression. Observations of the study plot in the growing season showed this strip to produce the most luxuriant vegetation of any part of the plot. Clip-quadrat studies confirmed this observation and showed the bluegrass understory to be especially heavy. This strip included the areas trapped by traps numbered 4, 5, 17, 18, 22, 23 and 37. With the exception of trap number 18, discussed above, these traps consistently made more captures than traps set in other parts of the plot. In winter, these traps also caught more harvest mice (Reithrodontomys megalotis) than any other comparable group of traps.

Although the amount of growing tissue of plants probably is at least as important to voles as the total amount of vegetation, some correlation seemed to exist between the density of grassy vegetation and the density of populations of voles. A mixed stand of grasses, with an obvious weedy component, can support a larger population of voles than can either a nearly pure stand of grass or the typical early seral stages dominated by weeds. Probably the more or less continual supply of young plants provided preferred food easily available to voles. A more homogeneous vegetation would tend to pass through the young and tender stage as a unit, thus causing a feast to be followed by a relative famine.