A Population Study of the Prairie Vole (Microtus ochrogaster) in Northeastern Part 4

========================================================================= No. of scats or No. containing Predator pellets examined remains of voles Percentage ------------------------------------------------------------------------- Copperhead 25 7 28 Red-tailed hawk 25 3 12 Long-eared owl 25 18 72 Great horned owl 32 6 19 Crow 25 4 16 Coyote 25 3 12 -------------------------------------------------------------------------

The red-tailed hawk (_Buteo jamaicensis_), the long-eared owl (_Asio otus_), the great horned owl (_Bubo virginia.n.u.s_) and the crow (_Corvus brachyrhynchos_) fed on _Microtus_. All four birds were fairly common permanent residents on the Reservation (Fitch, 1952:25). The low density and the strict territoriality of the red-tailed hawk (Fitch, _et al_, 1946:207) prevented it from exerting any important influence on the population of voles, even though individual red-tailed hawks ate many voles. Predation by the long-eared owl was especially heavy; remains of voles were identified in 72 per cent of its pellets examined. Korschgen (1952:39) found remains of voles in 70 per cent of 704 pellets of the long-eared owl. The reason for the heavy diet of _Microtus_ seems to be that both the owl and the vole are especially active at dusk. A group of long-eared owls, living near the edge of Quarry Field, probably exerted an influence on the density of the local population of voles because of the high ratio of predator to prey animals. The crows ate some, and perhaps most, of their voles after the animals had died from other causes. Other birds, mostly raptors, occurring in northeastern Kansas and reported to prey on voles include the sharp-s.h.i.+nned hawk (_Accipiter striatus_), Cooper's hawk (_A. cooperi_), red-shouldered hawk (_Buteo lineatus_), broad-winged hawk (_B. platypterus_), American rough-legged hawk (_B. lagopus_), ferruginous rough-legged hawk (_B. regalis_), marsh hawk (_Circus cyaneus_), barn owl (_Tyto alba_), screech owl (_Otus asio_), barred owl (_Strix varia_) and shrike (_Lanius excubitor_) (Korschgen, 1952:26; 28; 34; 35; 37; McAtee, 1935:9-27; Wooster, 1936:396).

Coyotes, house cats and racc.o.o.ns were identified as predators on voles in the study areas. Remains of voles were present in 12 per cent of the scats of the coyote (_Canis latrans_) examined. In Missouri, Korschgen (1952:40-43) reported remains of voles in slightly more than 20 per cent of the coyote stomachs that he examined. Fitch (1948:74), Hatt (1930:559) and others have reported other species of _Microtus_ as eaten by the coyote. Although coyotes were rarely seen on the Reservation, coyote sign was abundant (Fitch, 1952:29) and coyotes probably ate large numbers of voles. House cats (_Felis domesticus_), seemingly feral, were observed to tour the trap lines on several occasions and were noted by Fitch (_loc. cit._) as important predators on small vertebrates. Four cats were killed in the course of the study and remains of voles were found in the stomachs of all of them. On several occasions, racc.o.o.n tracks were noted following the trap line when the traps had been overturned and broken open, suggesting that racc.o.o.ns are not averse to eating voles although no further evidence of predation on voles by racc.o.o.ns was obtained. Fitch (_loc. cit._) reported racc.o.o.ns (_Procyon lotor_) to be moderately common on the Reservation. Reports of predation by racc.o.o.ns on voles are numerous (Hatt, 1930:554; Lantz, 1907:41). The opossum (_Didelphis marsupialis_), common on the Reservation, occasionally eats voles (Sandidge, 1953:99-101). Other mammals which are probably important predators on voles on the Reservation, though no specific information is available, are the striped skunk (_Mephitis mephitis_), spotted skunk (_Spilogale putorius_), weasel (_Mustela frenata_) and the red fox (_Vulpes fulva_). Eadie (1944; 1948; 1952), Shapiro (1950:360) and others have reported that the short-tailed shrew (_Blarina brevicauda_) was an important predator on _Microtus_. Shrews were present on the Reservation but were not trapped often enough to permit study.

The variety of vertebrates preying on voles suggests that they occupy a position of importance in many food chains. Errington (1935:199) and McAtee (1935:4) refer to voles as staple items of prey for all cla.s.ses of predatory vertebrates. An attempt to evaluate prey species was made by Wooster (1939). He proposed a formula which involved multiplying the density of a species, its mean individual weight, the fraction of the day it was active and the fraction of the year it was active to give a numerical index of prey value. Although his methods of determining population densities would now be considered questionable, the purpose of his investigation merits further consideration. He reported _M.

ochrogaster_ to be second only to the jack-rabbit (_Lepus californicus_) as a prey species in west-central Kansas.

MAMMALIAN a.s.sOCIATES

In the course of live-trapping operations several species of small mammals other than _Microtus ochrogaster_ were taken in the traps. Also, from time to time, direct observations of certain mammals were made and various types of sign of larger mammals were noted. These records gave a picture of the mammalian community of which the voles were a part. The three a.s.sociated species which were most commonly trapped were _Sigmodon hispidus_, _Reithrodontomys megalotis_ and _Peromyscus leucopus_. These three species have been commonly found a.s.sociated with _Microtus_ in this part of the country (Fisher, 1945:435; Jameson, 1947:137).

The Texas cotton rat, _Sigmodon hispidus_, was the most commonly trapped a.s.sociate of the voles between November, 1950, and February, 1952.

Although a greater number of individuals of the harvest mouse were taken in a few months, the cotton rat had a greater ecological importance because of its larger size (Figs. 17, 18, 19). The cotton rat was an especially noteworthy member of the community for two reasons. It has arrived in northern Kansas only recently and its progressive range extension northward and westward has attracted the attention of many mammalogists (Bailey, 1902:107; c.o.c.krum, 1948; 1952:183-187; Rinker, 1942b). Secondly, _Sigmodon_ has long been considered to be almost the ecological equivalent of _Microtus_ and to replace the vole in the southern United States (Calhoun, 1945:251; Svihla, 1929:353). Since the two species are now found together over large parts of Kansas their relations.h.i.+ps in the state need careful study.

[Ill.u.s.tration: FIG. 17. Variations in density and ma.s.s of three common rodents on House Field. The upper graph shows the sum of the bioma.s.s of the three rodents. In the two lower graphs the solid line represents _Microtus_, the broken line _Sigmodon_, and the dotted line _Reithrodontomys_.]

[Ill.u.s.tration: FIG. 18. Variations in density and bioma.s.s of three common rodents on Quarry Field. For key, see legend of Fig. 17.]

[Ill.u.s.tration: FIG. 19. Changing bioma.s.s ratios of three common rodents on House Field and Quarry Field. In late 1951 and early 1952 the cotton rats attained relatively high levels and seemingly caused compensatory decreases in the numbers of voles. The solid line represents _Microtus_, the broken line _Sigmodon_, and the dotted line _Reithrodontomys_.]

Both this study and the literature (Black, 1937:197; Calhoun, _loc.

cit._; Meyer and Meyer, 1944:108; Phillips, 1936:678; Rinker, 1942a:377; Strecker, 1929:216-218; Svihla, 1929:352-353) showed that, in general, the habitat needs of _Microtus_ and _Sigmodon_ were similar. Studies on the Natural History Reservation, both in connection with my problem and otherwise, suggested, however, that _Sigmodon_ occurred in only the more productive habitat types used by voles, where the vegetation was relatively high and rank. On the Reservation the cotton rat was found mostly in the lower meadows; they were more moist and had a more luxuriant vegetation than the higher fields. Although a few cotton rats were taken in Quarry Field and still fewer in Reithro Field, the population of those hilltop areas did not approach, at any time, the levels reached on House Field, which produced a more luxuriant cover.

Only when the levels of population were exceptionally high did the cotton rats spread into less productive habitats. At all times, there were areas on the Reservation used by _Microtus_ which could not support a population of _Sigmodon_.

The cotton rats reacted differently to the floods of July, 1951, than did the voles. Although the population of the cotton rat decreased slightly immediately after the wet period, this decrease was insignificant when compared with the drop in population level of other species of small mammals on the same area. During the autumn of 1951 and until March, 1952, the cotton rat became the most important mammal on the House Field study area in terms of grams per acre (Fig. 17), although the number of cotton rats per acre never matched the density of the voles. A similar, though less p.r.o.nounced, trend was observed on the Quarry Field study area (Fig. 18). One factor in the success of the cotton rat at this time seemed to be the greater resistance to wetting shown by very young individuals. Few adults (of any species) marked before the heavy rains of July, 1951, were trapped in September, 1951, when trapping was resumed after a lapse of one month. Several subadults and some juvenal cotton rats did survive, however, and provided a breeding population from which the area was repopulated. Cotton rats are born fully furred and able to move well, and are often weaned at ten days (Meyer and Meyer, 1944:123-124). Voles, on the other hand, are born naked and helpless and are often not weaned for three weeks. It seems, therefore, that extremely wet soil would harm the voles more than it would the cotton rats.

Several instances of cotton rats eating voles, caught in the same live-trap, were noted. There is reason to believe that young voles, unable to leave the nest, are subject to predation by cotton rats. This would accentuate any compet.i.tive advantage gained otherwise by the cotton rats.

The population of _Sigmodon_ retained its high level, relative to _Microtus_, until February, 1952. In March only one individual was captured and after that none was trapped until August, 1952, when a single subadult male was captured. Early in March, 1952, before the trapping period for the month had begun, the area suffered three successive days of unusually low temperature, with snow, which lay more than six inches deep in places. As suggested by c.o.c.krum (1952:185), such conditions proved detrimental to the cotton rats and, at least to the end of the study period in August, 1952, the population of cotton rats had failed to recover. Perhaps the extremely dry weather which followed the heavy winter mortality delayed the recovery of the population.

These limited data seem to indicate compet.i.tion between _Sigmodon_ and _Microtus_ in Kansas. Extremely wet conditions seem to give _Sigmodon_ a compet.i.tive advantage whereas _Microtus_ is better able to survive dry summers and severe winters. However, these relations.h.i.+ps need further clarification by an intensive study of the life history of _Sigmodon_ in Kansas (especially the more arid western part), including its coactions with the communities it has invaded successfully recently.

The harvest mouse (_Reithrodontomys megalotis_) also was a common inhabitant of the study plots, but this small rodent seemed not to be a serious compet.i.tor of the voles, as its food consists almost entirely of seeds (c.o.c.krum, _op. cit._:165) not usually used by voles. In this study, at least, no conflict over s.p.a.ce was apparent. Harvest mice frequently were taken in the runways of voles and even in the same trap with voles. Reithro Field, the part of the Reservation having the heaviest population of the harvest mouse, differed from the habitats that were better for voles in being higher, drier and less densely covered with vegetation. However, during the summer of 1951 when the voles were most abundant, Reithro Field supported a large population of voles. Estimates of population of the harvest mouse were of doubtful validity in summer because it was readily trapped only in winter and early spring. Many individuals marked in late spring were not trapped again until late autumn although presumably they remained on the area.

This seasonal variation in trapping success seemed to be a matter of acceptance and refusal of bait (Fitch, 1954:45).

The presence of the wood mouse (_Peromyscus leucopus_) on the study plots indicated an overlapping of habitats. Both House and Quarry Fields were on the ecotone between forest and meadow and a mixture of mammals from both types of habitat occurred. No sign of the homes of the wood mouse was found on the study plots, and on the larger trap line, operated by Fitch, wood mice were captured only near the edge of the woods.

Only six deer mice (_Peromyscus maniculatus_) were taken on the study plots. This small number probably provided an inaccurate index of the a.s.sociation of the deer mouse and the prairie vole, because samples from snap-traps and the data of other workers on the Reservation showed a more common occurrence of the two species together. The deer mice seemed to prefer a spa.r.s.er vegetation and did not approach so closely to the forest edge as did the voles. It may have been, in part, the presence of _P. leucopus_ in the ecotonal region which made it unsuitable for _P.

maniculatus_.

Other mammals noted on the study areas were the following: _Didelphis marsupialis_, _Blarina brevicauda_, _Scalopus aquaticus_, _Canis familiaris_, _Canis latrans_, _Procyon lotor_, _Felis domesticus_, _Sylvilagus florida.n.u.s_, _Microtus pinetorum_, _Mus musculus_ and _Zapus hudsonius_.

SUMMARY AND CONCLUSIONS

In the 23-month period from October, 1950, to August, 1952, the ecology of the prairie vole, _Microtus ochrogaster_, was investigated on the Natural History Reservation of the University of Kansas. In all, 817 voles were captured 2941 times in 13,880 "live-trap days." For some aspects of this study, Dr. Henry S. Fitch, resident investigator on the Reservation, permitted the use of his trapping records. He had captured 1416 voles 5098 times. The total number of live voles used in the study was thus 2233, and they were captured 8039 times. In addition to the voles, I caught 96 cotton rats, 108 harvest mice, 29 wood mice, 2 pine voles and 6 deer mice in live traps. When Fitch's records were used, the live-trapping data covered a thirty-month period and general field data were available from July, 1949, to August, 1952.

Hall and c.o.c.krum (1953:406) stated that probably all microtine rodents fluctuate markedly in numbers. Certainly the populations I studied did so, but the fluctuations were not regularly recurring for _M.

ochrogaster_ as they seem to be for some species of the genus in more northern life zones. The changes in the density of populations described in this paper can be explained without recourse to cycles of long time-span and literature dealing specifically with _M. ochrogaster_ makes no references to such cycles. There is, however, an annual cycle of abundance: greatest density of population occurs in autumn, and the least density in January.

This annual pattern is often, perhaps usually, obscured because of the extreme sensitivity of voles to a variety of changes in their environment. These changes are reflected as variations in reproductive success. In this study, some of these changes were accentuated by the great range in annual precipitation. Annual rainfall was approximately average in 1950 (36.32 inches, 0.92 inches above normal), notably high in 1951 (50.68 inches, 15.28 inches above normal) and notably low in 1952 (23.80 inches, 11.60 inches below normal).

Among the types of environmental modification to which the populations of voles reacted were plant succession, an increase in compet.i.tion with _Sigmodon_, abnormal rainfall and concentration of predators. In the overgrazed disclimax existing in 1948 when the study areas were reserved, no voles were found because cover was insufficient. After the area was protected a succession of good growing years hastened the recovery of the gra.s.ses and the populations of voles reached high levels. In areas where the vegetation approached the climax community, the densities of voles decreased from the levels supported by the immediately preceding seral stages. The higher carrying capacity of these earlier seral stages was probably due to the greater variety of herbaceous vegetation which tended to maintain a more constant supply of young and growing parts of plants which were the preferred food of voles. Later in the period of study the succession from gra.s.ses to woody plants on parts of the study areas also affected the population of voles. Not only did the voles withdraw from the advancing edge of the forest, but their density decreased in the meadows as the number of shrubs and other woody plants increased. These influences of the succession of plants on the population density of voles were exerted through changes in cover and in the quality, as well as the quant.i.ty, of the food supply.

Whenever voles were in compet.i.tion with cotton rats, there was a depression in the population levels of voles. Primarily, the compet.i.tion between the two species is the result of an extensive coincidence of food habits, but compet.i.tion for s.p.a.ce, cover and nesting material is also present. There was one direct coaction between these two species observed. Cotton rats, at least occasionally, ate voles, especially young individuals. In extremely wet weather, as in the summer of 1951, the high survival rate of newborn cotton rats resulted in an increase in their detrimental effect on the population of voles. However, cotton rats proved to be less well adapted to severe cold or drought than were voles.

Heavy rainfall reduced the densities of populations of voles by killing a large percentage of juveniles. During the summer of 1951 the compet.i.tion of cotton rats further depressed the population level of the voles, but the relative importance of compet.i.tion with cotton rats and superabundant moisture in effecting the observed reduction in population density is difficult to judge. Perhaps most of the decrease in population which followed the heavy rains was due to compet.i.tion rather than to weather. Subnormal rainfall, as in 1952, reduced the population of voles by inhibiting reproduction. Presumably because of an altered food supply, reproduction almost ceased during the drought. Utilization of the habitat was further reduced in the summer of 1952 because the voles did not grow so large as they otherwise did.

Predation, as a general rule, does not significantly affect densities of populations, but large numbers of predators concentrating on small areas may rapidly reduce the numbers of prey animals. In the course of my study, such a situation occurred but once, when a group of long-eared owls roosted in the woods adjacent to Quarry Field. The population of voles in that area was probably reduced somewhat as a result of predation by owls.

Population trends in either direction may be reversed suddenly by changes in the factors discussed above. In the fall of 1951, a downward trend in the density of the voles was evident. At this time, populations of cotton rats were increasing rapidly and compet.i.tion between cotton rats and voles was intensified. In February, 1952, the population of cotton rats was decimated suddenly by a short period of unusually cold weather. The voles were suddenly freed from the stress of compet.i.tion and the population immediately began to rise. The upward trend began prior to the annual spring increase and was subsequently reinforced by it. In the last part of May, 1952, the upward trend of the population was reversed, as the drought became severe, and the density of the population decreased rapidly. This drop was too sudden and too extreme to be only the normal summer slump. The relatively rapid response of voles to a heavy rain after a dry period, first by increased breeding and later by increases in density, is one more example of abrupt changes in population trends caused by altered environmental conditions.

In the population changes that I observed, no evident "die-off" of adults accompanied even the most drastic reductions in population density. The causative factor directly influences the population either by inhibiting reproduction or by increasing infant and prenatal mortality. The net reduction is due to an inadequate replacement of those voles lost by normal attrition.

Most voles, under natural conditions, live less than one year. Those individuals born in the autumn live longer, as a group, than those born at any other time. Since the heaviest mortality is in young voles, adults which become established in an area may live more than 18 months and, if they are females, may produce more than a dozen litters. No decrease in vigor and fertility was found to accompany aging. A relations.h.i.+p between the condylobasilar length of the skull and the age of a vole was discovered and, with further study, may yield a method of aging voles more accurately than has been possible heretofore. Other characteristics, varying with age, were described. The most reliable indicator of age seemed to be the prominence of the temporal ridges.

Runway systems and burrows are used by groups of voles rather than by individuals. Most of the activity of voles is confined to these runways and an exposed individual is seldom seen. A home range may include several runway systems, and the ranges of individuals overlap extensively. Both home ranges and patterns of runway systems change constantly. Runways seem to be primarily feeding trails, and are extended or abandoned as the voles change their feeding habits. Groups of adult voles using a system of runways seem to have no special relations.h.i.+p. Juveniles tend to stay near their mothers, but as they mature, they s.h.i.+ft their ranges and are replaced by other individuals.

Males wander more than females, and s.h.i.+ft their ranges more often. No intolerance of other voles exists and, in laboratory cages, groups of voles lived together peaceably from the time they are placed together.

Crowding does not seem to be harmful directly, therefore, and high densities will develop if food and cover resources permit.

As a prey item, the prairie vole proved to be an important part of the biota of the Reservation. It was eaten frequently by almost all of the larger vertebrate predators on the Reservation and was, seemingly, the most important food item of the long-eared owl. The ability of the prairie vole to maintain high levels of population over relatively broad areas enhances its value as a prey species.

LITERATURE CITED

ALBERTSON, F. W.

1937. Ecology of a mixed prairie in west-central Kansas. Ecol.

Monog., 7:481-547.

BAILEY, V.

1902. Synopsis of the North American species of _Sigmodon_. Proc.

Biol. Soc. Was.h.i.+ngton, 15:101-116.

1924. Breeding, feeding and other life habits of meadow mice. Jour.

Agric. Res., 27:523-536.