A Population Study of the Prairie Vole (Microtus ochrogaster) in Northeastern Part 1

A Population Study of the Prairie Vole (Microtus ochrogaster) in Northeastern Kansas.

by Edwin P. Martin.

INTRODUCTION

Perhaps the most important species of mammal in the gra.s.slands of Kansas and neighboring states is the prairie vole, _Microtus ochrogaster_ (Wagner). Because of its abundance this vole exerts a profound influence on the quant.i.ty and composition of the vegetation by feeding, trampling and burrowing; also it is important in food chains which sustain many other mammals, reptiles and birds. Although the closely related meadow vole, _M. pennsylvanicus_, of the eastern United States, has been studied both extensively and intensively, relatively little information concerning _M. ochrogaster_ has been acc.u.mulated heretofore.

I acknowledge my indebtedness to Dr. Henry S. Fitch, resident investigator on the University of Kansas Natural History Reservation. In addition to supplying guidance and encouragement in both the planning and execution of the investigation, Dr. Fitch made available for study the data from his extensive field work. Interest in and understanding of ecology were stimulated by his teaching and his example. Special debts are also acknowledged to Mr. John Poole for the use of his field notes and to Professor E. Raymond Hall, Chairman of the Department of Zoology, for several courtesies. Dr. R. L. McGregor of the Department of Botany at the University of Kansas a.s.sisted with the identification of some of the plants. Drawings of skulls were made by Victor Hogg.

Of the numerous publications concerning _Microtus pennsylvanicus_, those of Bailey (1924), Blair (1940; 1948) and Hamilton (1937a; 1937c; 1940; 1941) were especially useful in supplying background and suggesting methods for the present study. Publications not concerned primarily with voles, that were especially valuable to me in providing methods and interpretations applicable to my study, were those of Blair (1941), Hayne (1949a; 1949b), Mohr (1943; 1947), Stickel (1946; 1948) and Summerhayes (1941). Faunal and ecological reports dealing with _M.

ochrogaster_ and containing useful information on habits and habitat included those of Black (1937:200-202), Brumwell (1951:193-200; 213), Dice (1922:46) and Johnson (1926). Lantz (1907) discussed the economic relations.h.i.+ps of _M. ochrogaster_; the section of his report concerning the effects of voles on vegetation was especially useful to me.

Fisher (1945) studied the voles of central Missouri and obtained information concerning food habits and nesting behavior. Jameson (1947) studied _M. ochrogaster_ on and near the campus of the University of Kansas. His report is especially valuable in its treatment of the ectoparasites of voles. In my investigation I have concentrated on those aspects of the ecology of voles not treated at all by Fisher and Jameson, or mentioned but not adequately explored by them. Also I have attempted to obtain larger samples.

The University of Kansas Natural History Reservation, where almost all of the field work was done, is an area of 590 acres, comprising the northeastern-most part of Douglas County, Kansas. Situated in the broad ecotone between the deciduous forest and gra.s.sland, the reservation provides a variety of habitat types (Fitch, 1952). Before 1948, much of the area had been severely overgrazed and the original gra.s.sland vegetation had been largely replaced by weeds. Since 1948 there has been no grazing or cultivation. The gra.s.ses have partially recovered and, in the summer of 1952, some gra.s.ses of the prairie climax were present even on the parts of the Reservation which had been most heavily overgrazed.

Ill.u.s.trative of the changes on the Reservation were those observed in House Field by Henry S. Fitch (1953: _in litt._). He recalled that in July, 1948, the field supported a closely grazed, gra.s.sy vegetation providing insufficient cover for _Microtus_, with such coa.r.s.e weeds as _Vernonia_, _Verbena_ and _Solanum_ const.i.tuting a large part of the plant cover. By 1950, the same area supported a lush stand of gra.s.s, princ.i.p.ally _Bromus inermis_, and supported many woody plants. Similar changes occurred in the other study areas on the Reservation. Although insufficient time has elapsed to permit a.n.a.lyses of successional changes, it seems that trees and shrubs are gradually encroaching on the gra.s.sland throughout the Reservation.

The vole population has changed radically since the Reservation was established. In September and October of 1948, when Fitch began his field work, he maintained lines of traps totaling more than 1000 trap nights near the future vole study plots without capturing a single vole.

In November and December, 1948, he caught several voles near a small pond on the Reservation and found abundant sign in the same area. Late in 1949 he began to capture voles over the rest of the Reservation, but not until 1950 were voles present in sufficient numbers for convenient study.

I first visited the Reservation and searched there for sign of voles in the summer of 1949. I found hardly any sign. In the area around the pond mentioned above, however, several systems of runways were discovered.

This area had been protected from grazing for several years prior to the reservation of the larger area. In House Field, where my main study plot was to be established, there was no sign of voles. Slightly more than a year later, in October, 1950, I began trapping and found _Microtus_ to be abundant on House Field and present in smaller numbers throughout gra.s.sland areas of the Reservation.

GENERAL METHODS

The present study was based chiefly on live-trapping as a means of sampling a population of voles and tracing individual histories without eliminating the animals. Live-trapping disturbs the biota less than snap-trapping and gives a more reliable picture of the mammalian community (Blair, 1948:396; c.o.c.krum, 1947; Stickel, 1946:158; 1948:161).

The live-traps used were modeled after the trap described by Fitch (1950). Other types of traps were tested from time to time but this model proved superior in being easy to set, in not springing without a catch, in protecting the captured animal and in permitting easy removal of the animal from the trap. A wooden box was placed inside the metal shelter attached to each trap and, in winter, cotton batting or woolen sc.r.a.ps were placed inside the boxes for nesting material. With this insulation against the cold, voles could survive the night unharmed and could even deliver their litters successfully. In summer the nesting material was removed but the wooden box was retained as insulation against heat.

Bait used in live-traps was a mixture of cracked corn, milo and wheat, purchased at a local feed store. The importance of proper baiting, especially in winter, has been emphasized by Howard (1951) and Llewellyn (1950) who found an adequate supply of energy-laden food, such as corn, necessary in winter to enable small rodents to maintain body temperature during the hours of captivity. The rare instances of death of voles in traps in winter were a.s.sociated with wet nesting material, as these animals can survive much lower temperatures when they are dry. Their susceptibility to wet and cold was especially evident in rainy weather in February and March.

Preventing mortality in traps was more difficult in summer than in winter. The traps were set in any available shade of tall gra.s.s or weeds; or when such shade was inadequate, vegetation was pulled and piled over the nest boxes. The traps usually were faced north so that the attached number-ten cans, which served as shelters, cast shadows over the hardware cloth runways during midday. Even these measures were inadequate when the temperature reached 90F. or above. Such high temperatures rarely occurred early in the day, however, so that removal of the animals from traps between eight and ten a. m. almost eliminated mortality. Those individuals captured in the night were not yet harmed, but it was already hot enough to reduce the activity of the voles and prevent further captures until late afternoon. When it was necessary to run trap lines earlier, the traps were closed in the morning and reset in late afternoon.

Reactions of small mammals to live-traps and the effects of prebaiting were described by Chitty and Kempson (1949). In general, the results of my trapping program fit their conclusions. Each of my trapping periods, consisting of seven to ten consecutive days, showed a gradual increase in the number of captures per day for the first three days, with a tendency for the number of captures to level off during the remainder of the period. Leaving the traps baited and locked open for a day or two before a trapping period tended to increase the catch during the first few days of the period without any corresponding increase during the latter part of the period. Initial reluctance of the voles to enter the traps decreased as the traps became familiar parts of their environment.

At the beginning of the study the traps were set in a grid with intervals of 20 feet. The interval was increased to 30 feet after three months because a larger area could thus be covered and no loss in trapping efficiency was apparent. The traps were set within a three foot radius of the numbered stations, and were locked and left in position between trapping periods.

Each individual that was captured was weighed and s.e.xed. The resulting data were recorded in a field notebook together with the location of the capture and other pertinent information. Newly captured voles were marked by toe-clipping as described by Fitch (1952:32). Information was transferred from the field notebook to a file which contained a separate card for each individual trapped.

In the course of the program of live-trapping, many marked voles were recaptured one or more times. Most frequently captured among the females were number 8 (33 captures in seven months) and number 73 (30 captures in eight months). Among the males, number 37 (21 captures in six months) and number 62 (21 captures in eight months) were most frequently taken.

The mean number of captures per individual was 3.6. For females, the mean number of captures per individual was 3.8 and for males it was 3.4.

Females seemingly acquired the habit of entering traps more readily than did males. No correlation between any seasonally variable factor and the number of captures per individual was apparent. To a large degree, the formation of trap habits by voles was an individual peculiarity.

In order to study the extent of utilization of various habitats by _Microtus_, a number of areas were sampled with Museum Special snap-traps. These traps were set in linear series approximately 25 feet apart. The number of traps used varied with the size of the area sampled and ranged from 20 to 75. The lines were maintained for three nights.

The catch was a.s.sumed to indicate the relative abundance of _Microtus_ and certain other small mammals but no attempt to estimate actual population densities from snap-trapping data was made. In August, 1952, when the live-trapping program was concluded, the study areas were trapped out. The efficiency of the live-trapping procedure was emphasized by the absence of unmarked individuals among the 45 voles caught at that time.

Further details of the methods and procedures used are described in the appropriate sections which follow.

HABITAT

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 a.s.sociation 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 bluegra.s.s meadows but occurred also in a sedge-willow a.s.sociation. 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 gra.s.sy areas in spring and summer, but that in the autumn, when the gra.s.s began to dry, they moved to clumps of j.a.panese honeysuckle (_Lonicera j.a.ponica_) and stayed among the shrubbery throughout the winter. Johnson (1926:267, 270) found _M. ochrogaster_ only in uncultivated areas where long gra.s.s furnished adequate cover. He stated that the entire biotic a.s.sociation, 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 gra.s.sland 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 gra.s.s const.i.tuted 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 gra.s.s. Those traps set in spots where there was little vegetation other than the dominant gra.s.s 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 bluegra.s.s. Disturbed situations, such as roadsides, were characterized by the dominance of _Bromus j.a.ponicus_. This gra.s.s occurred also in low densities over much of the study area among _B.

inermis_. Other gra.s.ses present included _Triodia flava_, common in House Field, but with only spotty distribution in Quarry Field; _Elymus canadensis_, distributed over both areas in spotty fas.h.i.+on 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; _Panic.u.m 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 gra.s.ses 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 const.i.tute good vole habitat. Mixed in a gra.s.s dominated a.s.sociation they nevertheless raised the carrying capacity above that of a pure stand of gra.s.s. 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 _Taraxac.u.m 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...o...b..culatus_, _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 const.i.tuted 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 j.a.ponicus_ along both roads. No mammal of any kind was taken in traps set where this gra.s.s was dominant.

Because seasonal changes in vole density followed the curve for rate of growth of the complex of gra.s.ses on the Reservation, and because years in which there was a spa.r.s.e growth of plants due to dry weather showed a decrease in the density of voles, the relations.h.i.+ps 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 quant.i.tative. 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. RELATIONs.h.i.+P BETWEEN YIELD AND VARIOUS POPULATION DATA

====================================================================== House Field Quarry Field ---------------------------------------------------------------------- Yield in June, 1951, lbs./acre 2351 190 1513 302 _Microtus_, June, 1951, gms./acre 3867 5275 Per cent immature _Microtus_, June, 1951 29.85 38.02 Ratio _Microtus_, June/March 0.73 2.63 _Sigmodon_, June, 1951, gms./acre 1376 746 Per cent immature _Sigmodon_, June, 1951 35.72 44.44 Ratio _Sigmodon_, June/March 1.40 2.25 _Microtus-Sigmodon_, June, 1951, gms./acre 5243 6021 _Microtus_ mean, gms./acre/month 2922 1831 _Sigmodon_ mean, gms./acre/month 802 335 _Sigmodon-Microtus_, gms./acre/month 3728 2166 ----------------------------------------------------------------------

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 compet.i.tion 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 bioma.s.s of voles, and of all rodents combined, were lower than in Quarry Field which had less vegetation and fewer cotton rats. The relations.h.i.+ps 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 s.h.i.+ft in the places of high density of voles on the trapping areas. This s.h.i.+ft 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 s.h.i.+fts 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 s.h.i.+ft 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 s.h.i.+ft 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.

[Ill.u.s.tration: FIG. 1. Progressive encroachment of woody vegetation onto study areas, and the accompanying s.h.i.+ft of the centers of populations of voles. Activity centers of individuals were calculated as described by Hayne (1949b) and are indicated by dots. The cross-hatched areas show places where the vegetation was influenced by the shade of woody plants.]

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 a.s.sociation dominated by big bluestem. Since 1948 only one vole has been trapped in the more extensive hillside a.s.sociation 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 gra.s.s and blue grama dominated the a.s.sociation.

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 gra.s.s, has supported voles in 1948, 1949, 1952 and 1953. An upland exclosure containing only short gra.s.ses also supported a few voles in 1953.

An examination of the nature of the various plant a.s.sociations of the mixed prairie indicates that yield of gra.s.ses, amount of debris and basal cover may be critical factors in the distribution of voles. The a.s.sociation to which the voles seemed to belong was the lowland a.s.sociation. Hopkins _et al_ (1952:401; 409) reported the yield of gra.s.ses 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 acc.u.mulation 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 gra.s.s exclosure was probably due to two factors. In ungrazed short gra.s.s, 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 gra.s.s surrounding it and was thus a relatively good habitat, although it did not compare favorably with the lowland type.