Ecological distribution of Microtus montanus (Peale) and Microtus pennsylvanicus (Ord) in an area of geographic sympatry in southwestern Montana by James Russell Hodgson A thesis submitted to the Graduate Faculty in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in Zoology Montana State University © Copyright by James Russell Hodgson (1970) Abstract: Distributional patterns of Microtus montanus and Microtus pennsylvanicus in relation to various plant communities and to characteristics of habitats, were studied in the Bozeman vicinity of southwestern Montana from 1968 to 1969 in an attempt to secure information about ecological distribution and habitat preferences of these two vole species. A description of the physiography and vegetation of nine community types is given. During a total of 17,700 trap-nights 762 M. montanus and 583 M. pennsylvaniaus were snap-trapped from 59 study plots. These two species occurred sympatrically in 31 of the 59 sample areas. Preferred habitat of M. pennsylvaniaus is in moist areas where grasses, especially Poa pratensis, and grass-like species are dominant plants, comprising 50 percent or more of the vegetation by canopy coverage, and total canopy cover of all herbaceous material is at least 85 percent. The preferred habitat of M. montanus is not as well delineated as that of M. pennsylvanicus and this species is only poorly responsive to particular physical and physiognomic characteristics of the habitat. Microtus montanus appeared to have a wider ecological tolerance than M. pennsylvanicus, and demonstrated a direct general correlation between abundance and the dryness of the substrate.  © 1971 JAMES RUSSELL HODGSON ALL RIGHTS RESERVED ECOLOGICAL DISTRIBUTION OF MICROTUS MONTANUS (PEALE) AND MICRO.TUS PENNSILVANICUS (ORD) IN AN AREA OF GEOGRAPHIC SYMPATRY IN SOUTHWESTERN MONTANA by JAMES RUSSELL HODGSON A thesis submitted to the Graduate Faculty in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in Zoology Head, Major Department LajiTman9 Examining Committee Graduate' Dean (7 MONTANA STATE UNIVERSITY Bozeman, Montana August, 1970 iii ACKNOWLEDGMENTS The author wishes to express his most sincere appreciation to Dr. Robert E. Moore, Montana State University, for his advice and constructive criticism during this study and manuscript preparation. A debt of gratitude is also due to Drs. Don C. Quimby, P . David Skaar, Harold Watling, and John Rumely, Montana State University, for their help in manuscript preparation. I offer thanks to Mr. Kenneth Greer, Montana Fish and Game Department, for use of the wildlife laboratory facilities. I am obligated to Mr. Louis Jonas, Mr. Robert D o m , and Mr. Paul Sawyer for aid in plant species identification. Special gratitude is extended to Carol Hodgson for her understanding and support during the writing of this manuscript, and for her assistance in typing the rough draft. iv TAIJLE PF CONTENTS VITA . . ............... ' . ........... r Ii ACKNOWLEDGMENTS . . . . .......... , ........ ill TABLE OF CONTENTS . ........................... iv LIST OF TABLES . .......................... yi Page LIST OF FIGURES . .............................. ............ , viii ABSTRACT ............... x INTRODUCTION ............... . . ............. I METHODS . •......................... .. t . 3 Trapping Procedure 3 Species Identification........ .. ............. .. . . . 4 Vegetational Procedure . . . f r t ■ 4 Soil Moisture Procedure 5 Methods of Community Classification . . . . . r ^... . 6 DESCRIPTION OF THE STUDY AREA 8 Grass-sedge Community Type ^ . 10 Mesic Grassland Community Type ^ . 24 Dry Grassland Community Type .......... . . . . . . . . 25 Grass-forb Community Type : ^ . 26 Forb Community Type . . . . . ’........ 27 Sagebrush Community . . . . r . . . . . . . . . . . . . 27 Coniferous Forest Community Type . . . r . . . . . . . . 28 Aspen Community Type ................. , , .............. 28 Alpine Meadow Community Type . . . . . . ............... 29 RESULTS ............................................................ 31 General Distribution . . . . . . . . . . . . . . . . . . 31 Distribution Within Community Types . . . . . . . . . . 34 Vegetational Relationships ................. 38 Cover and Litter Relationships . . . . . . . . . . . . . . 47 Soil Moisture Relationships . . . . . . . . . . . . . . 50 TABLE OF CONTENTS (Continued) V DISCUSSION Page 56 LITERATURE CITED 63 VX I. II. III. IV. Table V. VI. VII. LIST OF TABLES Community type (Com. typ.) , study area number (St. ar.), dominant plants, biomass ratios, soil moisture, and relative densities of both species of Miovotus and associated small mammals from all study areas .......... Total catch, total trap-nights, and percent success of capture of Miovotus montanus3 Miovotus pennsylvanious3 and other species of small mammals based on monthly takes ............................................. . • - Comparison of the number of captures (No. cap.) and..ex­ pected number of captures (Ex. cap.) of Miovotus montanus and Miovotus ’pennsyl'Oanious on the assumption of random distribution within the nine community types .......... Comparison of the number of captures (No. cap.), average number per 100 trap-nights, and the expected number of captures (Ex. cap.) if distribution of Miovotus montanus and Miovotus ’pennsylvanious were random in relation to the percentage coverage of grasses and grass-like species in the canopy ...................................... .. . . Comparison of the number of captures (No. cap.), average number per 100 trap-nights, and the expected number of captures (Ex. cap.) if distribution of Miovotus montanus and Miovotus pennsylvanious were random in relation to total biomass (in grams per square meter) of grasses, forbs, and litter................ . . ................... A comparison between the actual number of captures (No. cap.), and the expected number of captures (Ex. cap.) if distribution were random of Miovotus montanus and Miovotus pennsylvanious in relation to the three dominant plants from all study areas . . . ............ Comparison of the number of captures (No. cap.), average number per 100 trap-nights, and the expected number of captures (Ex. cap.) if distribution of Miovotus montanus and Miovotus pennsylvanious were random in relation to canopy coverage in the herbaceous and shrub layers . . . Page 11 32 33 39 42 45 49 LIST OF TABLES (Continued)- vii Table VIII. Comparison of the number of captures (No. cap.), average number per 100 trap-nights, and the expected number of captures (Ex. cap.) if distribution of Micvotus montanus and Miorotus pennsyIvanious were random in relation to the amount of litter (expressed in grams per square meter) .-. . ............... . . . . . . . . . . . . . . IX. Comparison of the number of captures (No. cap.), average number per 100 trap-nights., and the expected number of captures (Ex. cap.) if distribution of Miorotus montanus and Miorotus pennsylvanious were random in relation to soil moisture . ............................ ............ Page 51 54 viii LIST OF FIGURES Figure Page 1. Approximate location of the study plots in the Bozeman, Montana a r e a ........ ................................. .. . 9 2. Average standing crops of grasses and grass-like species, forks, and litter from all community types as determined from dried clip quadrats. Community types include: the grass-sedge community type (GSC), the mesic grassland community type (MGC), the dry grassland community type (DGC), the grass-forb community type (GFC), the fork community type (FC), the sagebrush community type (SBC), the coniferous forest community type (CFC), the aspen community type (ASC), and the alpine meadow community type (ALC) . . . ................... . . . . . . . . . . . 21 3. Comparison of average canopy^coverage in percent of forks, grasses and grass-like species , and shrubs on all community types. Community types include: the grass-sedge com­ munity type (GSC), the mesic grassland community type (MGC), the dry grassland community ty&e (DGC), the grass- fork community type (GFC), the fork community type . (FC), the sagebrush community type (SBC) , the coniferous forest community type (CFC), the aspen community type (ASC), and the alpine meadow community type (ALC) .......... . . . . 22 4. Soil moisture, (percentage of water by weight as compared to oven dried weight of soil) from all community types. Community types include: the grass-sedge community type (GSC) , the mesic grassland community type (MGC) , the dry grassland community type (DGC), the grass-fork community type (GFC), the fork community type (FC), the sagebrush community type (SBC), the coniferous forest community type (CFG), the aspen community type (ASC), and the alpine meadow community type (ALC). Horizontal lines indicate the range and the central vertical lines represent the means. Dotted lines indicate those values not included in the means 23 .. 1^'- LIST OF FIGURES (Continued) ix Figure Page 5. Numbers of Miorotus montanus and Miorotus pennsylvanicus expressed in average numbers per 100 trap-nights (see Table IV) plotted against the percentage of grasses and grass-like ■ species in the total canopy coverage. Spearman rank correlation coefficients were 0.273 for M. montanus and 0.786 for M. -pennsylvanicus............ .. 40 6. Numbers of Miorotus montanus and Miorotus pennsyIvanicus expressed in average numbers per 100 trap-nights (see Table V) plotted against standing crops (biomass in grams per square meter). Spearman rank correlation coefficients were -0,119 for M. montanus and 0.976 for M. 'pennsyZ- vanious........ x ...........................................43 7. Numbers of Miorotus montanus and Miorotus' pennsylvanious expressed in average numbers per 100 trap-nights (see Table VII) plotted against total herbaceous and shrub coverage in the canopy. Spearman rank correlation coefficients were 0.515 for M. montanus and 0.998 for M. pennsylvanious . . . 48 8. Numbers of Miorotus montanus and Miorotus.pennsylvanious expressed in average numbers per 100 trap-nights (see Table VIII) plotted against the accumulation of litter (in grams per square meter). Spearman rank correlation coefficients were -0.262 for M. montanus and 0.571 for M. pennsyl­ vanious .................................................. 52 9. Numbers of Miorotus montanus and Miorotue pennsylvanious expressed in average numbers per 100 trap-nights (see Table IX) plotted against the soil moisture expressed as a percent of the oven dry weight of the soil. Spearman rank corre­ lation coefficients were -0.942 for M. montanus and 0.771 for M. pennsylvanious .......... .............. 53 XABSTRACT Distributional patterns of M-Iovotus montanus and Miovotus penn- syIvanious in relation to various plant communities and to character­ istics of habitats, were studied in the Bozeman vicinity of southwestern Montana from 1968 to 1969 in an attempt to secure information about eco­ logical distribution and habitat preferences of these two vole species. A description of the physiography and vegetation of nine community types is given. During a total of 17,700' trap-nights 762 M. montanus and 583 M. ipennsytVaniaus were snap-trapped from 59 study plots. These two species occurred sympatrically in 31 of the 59 sample areas. Preferred habitat of M. pennsytvanious is in moist areas where grasses, especially Poa pvatensiSj and grass-like species are dominant plants, comprising 50 percent or more of the vegetation by canopy coverage, and total canopy cover of all herbaceous material is at least 85 percent. The preferred habitat of M. montanus is not as well delineated as that of M. pennsytvanious and this specie's is only poorly responsive to partic­ ular physical and physiognomic characteristics of the habitat. Miovotus montanus appeared to have a wider ecological tolerance than M. pennsyt- Vanious3 and demonstrated a direct general correlation between abundance and the dryness of the substrate. INTRODUCTION This study was an attempt to secure information about ecological distribution of Miarotus montanus and Miorotus 'pennsy.lvanicus in an area of geographic sympatry in southwestern Montana. Although there has been much work done on the genus Microtus (see especially bibliography by Golley 1963), proportionally little quantitative work has been done on the vegetational structure of their preferred habitats. The results of various authors (Blair 1940, Eadie 1953, Getz 1961, Zimmerman 1965, and others) have indicated that Miarotus shows a high selective acceptance of dense areas formed of graminoids, primarily in low lying mesic areas. Cameron (1964) and Morris (1969) discussed the significance of insular occurrence.of geographically sympatric species of microtine rodents and suggested competitive exclusion between similar genera. Findley (1951) showed ecological sympatry between Miorotus montanus and M. pennsylvani- aus in a small percentage of the habitat types in Jackson Hole, Wyoming, with M. montanus having the larger ecological amplitude. Koplin and Hoffman (1968), in a northwestern Montana study, accepted the hypothesis of competitive exclusion between sympatric populations of M. montanus and M. pennsyIranians. In southwestern Montana in the areas around Bozeman both M. montanus and M. pennsylvaniaus are sympatric in a large proportion of the studied community types. The objectives of the study are twofold: (I) to study relative numbers of Miorotus montanus and M. pennsylvaniaus in various grassland —2— and forest communities, and (2) by relating numbers of M-iarotus to recognizable vegetational variants within the community types, to at­ tempt to draw conclusions regarding the make-up of preferred habitats and the factors influencing the ecological distribution of these two species. To investigate the importance of these factors under natural conditions, a series of snap-trap collections was made in the various habitat types. The period of study was from June 14 through September 14, 1968, and June 10 through August 22, 1969. METHODS Trapping Procedure Study plots were selected within relatively uniform units of several community types. Snap traps ("museum specials") were used exclusively, and the data were compiled according to the trap-night method. One trap exposed for one night was one trap-night, The traps, baited with a mixture .of rolled oats and peanut butter, were anchored with a 30 inch, small diameter, wire rod on which a numbered colored plastic streamer was tied to facilitate trap location. One hundred traps, one per station and placed equidistant to each other, were usually arranged in a rectangular or square grid pattern at 30-foot paced intervals. In irregularly shaped areas, the boundary of the grid followed the contours of the area. • Traps were left in the field for three nights (300 trap nights) and were checked twice daily, 0800 to 1100 and 1600 to 1800. During the summer of 1968 only one plot was trapped at a time, but in 1969 two areas were sampled simultaneously. Each of the 59 study plots was trapped only once. ■ Sex, age (adult or subadult), capture locations, and standard measurements were recorded for each captured Miovptus. All specimens were taken to the laboratory for identification. All other small mammals trapped were also recorded. —4— Species Identification Field identification was difficult due to the great similarity of M-Lavotus montanus and M. pennsylvaniaus. Species verification was de­ termined by skull characteristics including maxillary tooth features (Hall and Kelson 1959; and Hoffman and Pattie 1968). During 1968, both skulls and skins were collected, but in 1969 pnly skulls were collected. The "casing" method (Anderson 1948) was employed to preserve skins. Vegetational Procedure A method similar to that of Daubenmire (19.59) was utilized to determine canopy coverage and relative frequency of herbaceous species. Two transects were run on each study plot, in which twenty 2 x 5 deci­ meter frames' (40 frames per study plot) were randomly placed at various trapping stations covering the entire grid. Within these quadrats, the percent coverage of each plant species yas visually determined and recorded as one of six coverage classes. The coverage classes used were: Class I = 0-5 percent; Class 2 = 5-25 percent; Class' 3 = 25-50 percent; Class 4 = 50-75 percent; Class 5 =• 75-95 percent; and Class 6 = 95-100 percent. The midpoints of these classes were used in the analysis of data. Plant nomenclature follows Booth (1950) and Booth and Wright (1966). Times of trapping were chosen in such a manner that -5- each area was trapped at an equivalent stage of seasonal vegetational development (for example, the higher the elevation of the study area the later in the season it was trapped). Hve clip quadrats were employed to determine the amount of grasses, forks, and litter present. The quadrats were established at randomly scattered points in those regions of the study plots which showed the greatest Miovotus activity. A 2 x 5 decimeter metal frame was employed. All above ground parts of litter and vegetation lying within the frame were clipped at ground level. The samples obtained were separated, • • bagged, air-dried at room temperature for six months, and weighed. Dry weight biomass ratios (grass to forks to litter) were obtained from the clippings from each area. A single series of such samples (5 frames per area) was taken from each plot. During 1968 clippings were collected in late summer (August and' September), but in 1969 they were obtained during the trapping periods in the areas. Soil Moisture Procedure Substrate moisture was determined on all study plots. Ten soil cores, sampled with an Oakfield sampler, were taken from the upper six inches of the profile. These samples were then canned (5 samples per can) and taken to the laboratory for analysis. Moisture content was determined from a comparison of wet weight and dry weight of the soil after 24 hours of oven drying at H O C., and moisture content for each —6— study plot was expressed'as percent (based on an average of both canned samples) of the oven dry weight of the soil. All areas were sampled between 26 and 28 August, 1969, after a rainless period of several weeks. Methods of Community Classification Various approaches have been employed for the rational classifi­ cation of plant communities, and since these units themselves are highly complex and variable, the classifications are to some extent arbitrary. Several characteristics have been employed as the criteria of classificationamong the widely used ones are species composition, physiognomy, and life form (Hanson and Churchill 1961). The classification of plant community types in this study.was based on plant dominance (expressed as percentage of canopy coverage and frequency of a taxa), on forage class dominance (biomass ratios of grasses and grass-like species to forbs); and to a lesser extent on soil moisture. For meadows, the array of three most conspicuous or prevalent plants of the herbaceous layer (as based on percent coverage in the total canopy) was used as the chief criterion of dominance in community classification. Forested habitats were classified on the basis of tree dominance (aspen or conifers.) and not on the basis of understory composition. The alpine community type was classified as such because of its elevational location. All study —7“ plots were classified into nine community types according to their physiognomy and composition. Those data that lend themselves to statistical treatment have been analyzed by use of the Chi-square test and the Spearman rank correlation coefficient (Tate and Clelland 1957). Also, correlation coefficients were given with statements of correlation or no correlation. DESCRIPTION OF THE STUDY AREA All fifty-nine study plots were located in Gallatin County, Montana within a radius of about twenty miles around Bozeman (Fig. I). Bozeman lies in the Gallatin Valley at about 4,800 feet elevation. The Bridger Mountains rise to about 9,500 feet to the northeast, and the Gallatin Range rises to over 10,000 feet to the south. The valley floor is composed mainly of Tertiary sediments, and the mountainous areas contain Precambrian, Paleozoic, and Mesozoic sedimentary rocks (Perry 1962). The Gallatin Range is also partially composed of Tertiary volcanic rock. Data from the U. S. Weather Bureau (station 104402, Montana State University, Bozeman) indicate an annual mean temperature of 42 F. There are relatively short cool summers and long cold winters with variable snow cover on the valley floor and usually constant snow cover in the higher elevations. January temperatures in Bozeman aver­ age between 15 and 20 F., and July temperatures average around 65 F., with extremes that range from 100 to below -50 F. The pattern of annual precipitation is seasonal and averages 15 to 20 inches. Snow fall is around 55 inches. Habitats sampled included both natural and agricultural types on the valley floor as well as in the mountains of the Bridger and Gallatin Ranges. The soils on the valley floor are of an outwash type, and those in the mountains are generally a lithosol type. Agricultural -9- R A N G E LEGEND R iv e rs C ree k s F o re s t & M o u n ta in B o u n d a ry S tu d y P lo ts Figure I. Approximate location of the study plots in the Bozeman, Montana area. -10- types included: alfalfa fields (Medtoago eativa), cultivated grass fields, fallow areas, and pasture lands. Areas were classified as natural if they were only infrequently disturbed by agricultural practices. The study plots were classified into nipe general, com­ munity types, with subunits in some, according to previously mentioned criteria. Quantitative data for each study plot are shown in Table I, and average standing crop values, canopy coverage, and soil moisture are represented in Figs. 2, 3, 4, respectively. Grass-sedge Community Type The grass-sedge communities were located along small streams and ponds, or in poorly drained areas in which the soil was very moist and rich in humus content. Measured soil moistures ranged from 24.80 to 153.69 percent and averaged 55.3 percent. Sedges, grasses, and ground litter were plentiful, and standing crops averaged 570g/34g/286g per square meter for grasses, forbs, and litter, respectively. There was little bare ground. Grasses and grass-like species were dominant as expressed by the canopy coverage. Cavex nekvaskensi-s and other .sedges were frequently found, with Poa 'pvatens'is, Phalavis avundinaoea, Phlenm Ipvatense3 Bvomus inevmis3 or B. mavginatus being found on better drained locations. Grasses and grass-like species averaged 68 percent in the total canopy coverage. Selaginella3 Salix spp. , and various species of forbs were occasionally found. Six (10.2%) of the 59 study plots Table I. Community types (Com. typ.) , study area number (St. ar.) , dominant plants, biomass ratios, soil moisture, and relative densities of both species of Miovotus and associated small mammals from all study areas. Com. typ. ^ s c ar. 2/ 3/ ~ Dominant plants- 4/ Biomas s— ratio Grass/forb- ^ ratio Soil-/ mo s . No.M. mont. - 1Ao .M. penn. a - . ^8/— Associated- mammals GSC 14 Bvomus inevmis 692/40/76 55.5/35.1 35.62 0.67 4.00 Sovex sp. 0.33 Fhteum pvatense Zapus pvinoeps 0.33 Tavaxaoum GSC 17 Cavex Pevomysous nebvaskensis 252/70/144 22.8/47.2 52.79 0.00 0.33 manioutatus 1.00 Gevanium Sovex spp. 3.00 viohavdsonii Z. pvinoeps 0.33 Setaginetta sp. GSC 25 Foa pvatensis 792/56/378 78.7/23.8 153.69 0.00 11.33 P. manioutatus 0.33 Cavex nebvaskensis Sovex spp. 5.67 Agvostis atba 'SoveX patustvis 0.67 Z. pvinoeps 0.33 GSC 38 Bvomus spp. 452/22/814 85.6/5.8 24.80 0.33 16.00 ■ P.' manioutatus 7.67 Poa pvatensis ■ Sovex spp. 6.00 Cavex atvosquama : Z. pvinoeps 1.00 GSC 42 Civsiim sp. 840/8/284 83.4/5.1 33.99 0.33 7.00 P. manioutatus 1.00 Cavex nebvaskensis Fhaiavis avundinaeea GSC 46 Poa pvatensis 392/10/20 81.1/5.5 30.92 0.00 2.00 None Cavex nebvaskensis Agvopyvon smithii Table I. (Continued). Com. St. Dominant plants Biomass Grass/forb Soil No.AL No.AL Associated typ. ar. ratio ratio mos. mont. "perm. mammals MGC 10 Poa pratensis 516/26/58 Dactylis glomerata Taraxacum sp. 38.3/75.6 24.98 2.33 2.33 Mus museulus 0.67 MGC 11 Poa pratensis 366/206/226 57.2/25.6 10.67 0.00 2.00 P. manieulatus 2.00 Bronrus marginatus (9.1)9/ Sorex spp. 1.75 Sonehus sp. Z. prineeps Thomomys 0.25 talpoides Clethrionomys 0.25 gapperi 0.25 MGC 13 Poa pratensis 1086/116/188 52.4/45.5 16.80 5.33 1.00 None Trifolium pratense Daetylis glomerata MGC 20 Bromus inermis 896/68/126 58.0/29.9 . 18.80 2.00 13.33 P. manieulatus 0.67 Poa pratensis Sorex spp. 1.67 Medieago sativa ■ Z. prineeps 0.67 MGC 22 Phleum pratense 552/72/50 47.2/41.7 23.28 7.33 0.33 P. manieulatus 0.33 Poa pratensis Sorex sp. 0.33 Taraxacum sp. Z. prineeps 4.00 - T. -talpoides 1.00 Table I. (Continued). Com. St. Dominant plants Biomass Grass/forb Soil ' No.M. No .Af. Associated typ. ar. ratio ratio mo s. mont. penn. mammals MGC 27 Poa pratensi-s 1076/30/54 77.8/30.7 18.72 0.00 18.33 Af. muscuius 0.33 Bromus -Cnermis P. mahiculatus 1.67 Baotylis glomerata Sorex spp. 4.33 Z. princeps 4.00 T. talpoides 0.33 Eutamias sp. 0.33 MGC 28 Poa pratensis 303/102/140 66.8/26.2 10.00 2.33 0.00 P. maniculatus 1.00 Phleum pratense Sorex spp. 6.67 Taraxacum sp. Z. princeps 3.33 T. talpoides 0.33 Eutamias sp. Microtus 1.00 longicaudus 0.67 MGC 29 Phleum pratense 500/66/25 69.2/28.8 34.80 9.67 1.00 P. maniculatus 5.00 Poa pratensis Sorex spp. 2.67 Bromus inermis Z. princeps 3.33 MGC 31 Poa pratensis 254/116/84 52.8/36.8 11.05 12.67 2.33 P. maniculatus 0.67 Phleum pratense ' Sorex spp. 2.67 Cynoglossum officinale Z. princeps 1.67 MGC 33 Phleum pratense 530/184/76 33.3/52.9 " 20.56 0.33 0.00 P. maniculatus 4.67 Bromus marginatus Achillea millefolium - (3.5)2/ T. talpoides 0.67 Table I. (Continued) Com, St. Dominant plants Biomass Grass/forb Soil N o.M. No.M. Associated typ. ar. ratio ratio mos. front, penn. mammals MGC 34 Phleum pvatense Cirsium sp. Carex sp. 204/150/104 33.0/37.8 13.51 0.00 0.00 P. manieulatus 4.67 Sorex spp. 1.67 Eutamias sp. 0.33 M. Iongieaudus 1.67 C. gapperi 0.33 MGC 36 Poa pratensis Bromus inermis Lychnis alba 346/98/322 69.5/17.7 13.27 13.67 6.00 P. manieulatus16.00 Z. prinaeps 0.33 MGC 37 Bromus spp. 582/150/76' Dactylis gtomerata Medicago sativa 62.8/27.3 10.31 11.33 16.33 P. manieulatus 1.00 £ Sorex sp. 0.33 1 MGC 39 Poa pratensis Agropyron smithii Bromus spp. 264/32/200 65.1/7.4 9.93 7.33 1.00 Sorex sp. ' 0.33 MGC 40 Poa pratensis Brpmus 'marginatus Conium_ maeulatum 786/24/456 77.4/7.9 (3.5)-^ 11.89 3.67 . 7.00 M. -museulus 0.33 P. manieulatus10.33 Sorex spp. 2.67 MGG 41 Poa pratensis 562/12/292 .Juneus baltieus Desahampsia -elongata 72.5/16.3 43.51 0.33 15.33 ■Sorex spp. 1.00 Table I. (Continued). Com. St. Dominant plants Biomass Grass/forb Soil Np.AL No.AL Associated typ. ar. ratio ratio mos. mont. penn. mammals MGC 47 Poa pratensis Agropyron smith'd Bromus -inermis 258/18/188 74.7/14.7 13.60 12.67 4.00 Sorex spp. 0.67 MGC 48 Phleum pratense 494/42/108 78.7/15.2 32.92 3.67 7.00 AL musculus 0.33 Agropyron repens Poa pratensis P. manioulatus 2.67 MGC 49 Poa pratensis 488/26/25 71.9/10.0 22.14 4.33 13.33 P. manioulatus 7.33 Bromus marginatus Agropryon repens Sorex spp. 1.33 MGC 50 Poa pratensis Bromus marginatus 282/28/162 38.9/26.6 10.00 16.33 0.33 AL musoulus 0.33 Baotylis glomerata ■ MGC 53 Poa pratensis 308/18/62 68.8/10.3 19.56 2.00 4.33 P. manioulatus 0.67 Agropyron repens Agropyron smithii Sorex spp. 1.33 MGC ■ 60 Phleum pratense 5-74/26/72 77.5/15.9 8.93 16.33 0.67 P. manioulatus 0.33 ■ Poa pratensis Sorex spp. 2.00 Geranium viseosissimum Z. princeps 5.33 T. talpoides 0.33 DGC 30 Brqmus teotorum 498/112/76 68.5/25.8, 12.05 4.33 0.00 P. maniculatusll.QQ Agropyron sp-ioation Sorex spp. 2.33 Festuoa idahoensis Z. princeps 0.33 Table I. (Continued) .. Com. St. Dominant plants Biomass Grass/forb Soil N o.M. No.M. Associated typ. ar. . ratio ratio mos. mont. perm. mammals DGC 51 Poa pratensis 276/8/38 62.0/12.6 7.08 6.00 1.00 P. maniautatus 1.00 Agropyron smithii Bromus teatorum Sorex spp. 1.67 DGC 52 Poa pratensis 180/102/76 Festuca iddhoensis (5.2)^ J 46.1/16.8 9.63 0.67 0.33 P. maniautatus 1.33 Artenrisia eana DGC 57 Bromus teatorum 240/60/168 44.5/15.0 22.70 2.67 0.00 P. maniautatus 6.00 1 Agropyron spiaatum Sorex spp. 4.67 T Festuea idahoensis T. tatpoides 0.67 DGC 58 Poa pratensis 264/12/196 48.5/15.9 10.15 6.00 0.00 P. maniautatus13.67 Agropyron spiaatum Festuea iddhoensis Sorex spp. 1,00 • GFC . 12 Poa pratensis 230/156/84 Amiaa sororia 28.6/41.7 13.06 0.67 0.00 P. maniautatus 4.67 Lupinus sp. GFC 16 Ranunculus sp. 272/386/166 27.1/51.5 16.90 2.00 0.00 Sorex spp. 0.67 ■ Cirsium arvense Z. prinaeps 6.33 Me Iiea sp. T. tatpoides 0.33 GFC 18 Poa pratensis 464/296/66 28.6/54.4 2.59 0.67 0.00 Sorex spp. 0.67 Phteum pratense Z. prinaeps 3.00 Trifolium repens T. tatpoides 2.33 Table I. (Continued). Com. St. Dominant- plants -Biomass Grass/forb Soil No .AL No.M. Associated typ. ar. ratio ratio mos. mont. penn. mammals GFC 19 BalsamoThiza 42/178/64415/8.3/50.0 sagittata (20.4)5/ Monarda fistulosa Symphoriearpos albus 18.52 0.00 0.00 P. manieulatus Sorex sp. 5.00 0.33 GFC 23 Phleum pratense Trifolium repens Carex hoodii 412/116/156 43.1/68.1 27.18 0.00 2.00 P. manieulatus Z. prineeps 3.33 4.00 GFC 24 Poa pratensis Taraxacum sp. • CirSium arvense 364/206/10 37.5/51.7 16.01 5.33 1.67 Sorex spp. Z. prineeps 1.00 9.00 r GFC 26 Balsamorhiza sagittata 328/98/250 .Cerastium arvense Agropyron spicatum 42.1/58.4 9.13 6.33 0.00 P. manieulatusI A.00 Sorex spp. 1.33 T. talpoides 1.00 - GFC 35 Taracacum sp. Bromus inermis Medieago sativa 108/314/36 23.8/62.9 3.18 15.00 1.00 Af. museulus 0.67 P. maniculatuslk.£>1. GFC 43 Poa pratensis. Juneus balticus Sonehus sp. 426/26/240 60.5/18.8 34.42 2.33 6.67 None GFC 44 Bronrus inermis 148/38/64 21.5/9.7 7.74 4.67 0.00 P. manieulatus 5.00 Stipa oomata Melitotus officinalis Table I. (Continued). Com. typ- GFC GFC GFC GFC GFC GFC GFC St. Dominant plants Biomass Grass/forb Soil No.AL No.M. Associated ar. ratio ratio mos. mont. pgMM. mammals 45 Bromus teotorum 210/44/88 30.3/5.0 8.51 1.00 0.00 P. manioulatus 3.00 Elymus oinereus Medioago sativa 54 Trifolium reyens 222/122/16 32.5/52.0 22.81 2.00 0.33 None Daotylis glomerata Feotuoa elatior 55 Poapratensis 208/102/92 58.1/18.8 Festuoa idahoensis Lithospermum ruderale 56 Bromus teotorum 128/240/16 13.4/60.7 Festuoa iclahoensis Melilotus offioinalis 59 Poapratensis 304/256/166 58.3/33.1 Phleum pratense Monarda fistulosa 13.86 13.33 1.00 P. manioulatus 1.33 Sorex spp. 2.67 © Z. prinoeps 0.33 T 7.18 5.33 0.33 P. manioulatus 7.33 Sorex spp. 0.67 20.32' 4.67 0.33 P. manioulatus 0.33 Z. prinoeps 0.33 62 Poapratensis 332/42/44 66.7/17.6 .Bromus marginatus - Medioago■ saiiva 14.33 16.33 0.00 P. manioulatus 2.33 Z. .prinoeps 1.00 63 Poapratensis 526/140/166 50.2/43.3 Trifolium repens Bromus marginatus 38.98 3.33 18.00 Sorex spp. Z. prinoeps 1.00 0.67 Table I . (Continued). Com. St. Dominant plants Biomass Grass/forb Soil No .M. No.A?. Associated typ. ar. ratio ratio mos. mont. penn. mammals FC 15 Taraxacum sp. 54/632/58 Medicago sativa Trifolium pratense tr./96.8^/ 14.00 0.67 4.00 P. manieulatus 1.67 FC 21 Galium boreale 72/446/98 11.3/69.7 24.06 0.00 0.00 B. manieulatus 3.33 Bromus inermis Sorex spp. 3.00 ,Mertensia ciliata Z'. prineeps 9.67 T. talpoides 0.67 SBC 61 Boa pratensis 310/120/116 40.5/15.1 14.36 9.00 0.00 B. manieulatus 3.67 Artemisia tridentata Sorex sp. 0.33 Festuca iddhoensis Z. prineeps 3.67 7. talpoides 1.67 CFC 66 Carex geyeri 14/210/40 • 7.6/4.9 27.43 0.00 0.00 B. manieulatus 1.00 Vaeeinium seoparium (33.2)9/(15.6)12/ Sorex spp. 1.33 Sedum debi Hs Eutamias spp. 2.00 C. gapperi Bhenaeomys 4.33 intermedins 0.33 CFC 67 Carex geyeri,- 72/126/90 24.6/21.0. 20.03 0.00 0.00 Sorex sp. 0.33 Trifolium sp. (5.1)-/ Z. prineeps 0.33 Boa pratensis Eutamias sp. 0.33 C. gapperi ‘ 2.00 P. intermedins 0.33 ASC 64 Lonieera utahensis 146/288/88 11.4/19.6 26.33 0.00 0.67 Sorex spp. 4.00 Bhysoearpus malvaceus Spiraea betulifolia (29.9)9/ Z. prineeps 1.00 Table I. (Continued). Com. St. Dominant plants Biomass .Grass/forb Soil No .M. 'No.M. Associated typ. ar. ratio ratio mo s. mont. penn. mammals ASC 65 Rosa sp. 136/434/24 6.6/22.1 18.00 0.00 0.67 'P. maniouZatus 0.67 - Poa pratensis (29.6)9/ Sorex spp. 2.00 Symphorioarpos aZbus Z. prinoeps 0.33 Eutdmias sp. 0.33 C. gapperi 0.67 M. Zongioaudus 1.33 ALC 32 Lupinus argenteus 202/122/168 31.0/51.2 10.55 6.33 0.00 P. maniouZatus 0.33 Eriogonum umbeZZatum Sorex spp. 1.00 Festuoa idahoensis T. taZpoides 0.33 ALC 68 SaZix woZfii 32/128/8 24.3/20.3 88.61 0.00 0.00 p, maniouZatus 0.33 ^ Erigeron peregrinus SeZagineZZa dens a (10.7)9/ i -I/ Habitat types include: the grass-sedge community (CSC), the mesic grassland community (MGC), the dry grassland community (DGC), the -grass-forb community (GFC), the forb community (FC), the sagebrush community (SBC), the coniferous forest community (CFC), the aspen community (ASC), and the alpine meadow community (ALC). , . 2J See Figure I for location of numbered study areas. 3/ The three most dominant plants as indicated by the percent coverage in the canopy. kj Biomass ratios in grams per square meter: grass/forbs/litter. 5/ Grass-forb ratio in the total coverage of each class in the canopy coverage: grass/forbs. 6/ Soil moisture expressed as a percent of the oven dry weight of the soil. ,Tj Number of Miovotus montanus and M. pennsyivanious expressed in numbers per 100 trap nights. 8/ Additional small mammals captured expressed in the number per 100 trap nights. . 9/ Shrub coverage. 10/ Litter weight contains a high percent of woody stems. 11/ tr. means a trace amount. 12/ 15.6 equals ground cover by SelaginelZa. 650 600- 550- 500- CM 5 450- &_ 400- 60 BI50 ^ $ 4 0 g s 0 30 Figure 3. 0 forbs W\ shrubs I ho K> I GSC MGC DGC GFC FC SBC CFC ASC ALC community types Comparison of average canopy coverage in percent of forbs, grasses and grass-like species, and shrubs on all community types. Community types include: the grass- sedge community type (CSC), the mesic grassland community type (MGC), the dry grass­ land community type (DGC), the grass-forb community type (GFC), the forb community type (FC), the sagebrush community type (SBC), the coniferous forest community type (CFC), the aspen community type (ASC), and the alpine meadow community type (ALC). co m m un ity ty pe s per cent soil moisture Figure 4. Soil moisture (percentage of water by weight as compared to oven dried weight of soil) from all community types. Community types include: the grass-sedge community type (GSC), the mesic grassland community type (MGC), the dry grassland community type (DGC), the grass-forb community type (GFC), the forb community type (FC), the sagebrush com­ munity type (SBC), the coniferous forest community type (CFC), the aspen community type (ASC), and the alpine meadow community type (ALC). Horizontal lines indicate the range and the central vertical lines represent the means. Dotted lines indicate those values not included in the means. —24— included the grass-sedge community type. Mesic Grassland Community Type The mesic grassland community was the most frequently sampled community type and included both natural and agricultural lands. This community type was characterized by a predominance of grasses and rela­ tively few forbs. The grasses averaged 61 percent of the total canopy coverage, and forbs averaged 27 percent. Shrubs played a minor role in the canopy. The grasses also predominated in the standing crops, aver­ aging 519 grams per square meter, and forbs and litter averaged 73 and 141 grams per square meter, respectively. ' Average soil moisture was 19.49 percent with a range of 9.93 to 43.51 percent. Twenty-two (37.3%) of the study plots were of the mesic grassland community type. This community type was recognized as two subunits. The more mesic of the subunits, the blue grass meadow, was. char­ acterized by the grasses Poa. pvatensis3 Daotylis glomerata3 and Bromus marginatus with the forbs Tarajxaoum3 Sonohus3 Trifolium3 Achillea millefolium3 Lychnis alba3 and Medioago sativa found Ipss conspicuousIy^ on various study plots. Frequently more mesic sedges and forbs were found within more poorly drained depressions in a study plot. The soils were moderately drained and the standing crop of graminoids high. Canopy coverage and accumulations of ground litter were variable. -25- The timothy meadow, the more xeric of the subunits, was character­ ized by Phteum pratense as well as Poa pratensiSj Bromus marginatus3 and occasionally Agropyron smithii- and A. repens. The most common forb genera were -Taraxaoim and Cirsium. Similar to the blue grass subunit, standing crops of grasses were high and amounts of litter and canopy coverage were variable. . The soils were moderately well drained. Occa­ sionally there were aggregations of forb and shrub species located within a general study plot. Dry Grassland Community Type The dry grassland communities were located on well drained hill­ sides in which the soil was rocky, sandy and of poor quality. Soil moistures ranged from 7.08 to 22.70 percent and averaged 10.05 percent. Grass standing crops were moderate as was the litter accumulation. Biomass ratios averaged 292g/59g/lllg per square meter for grasses, forbs, and litter, respectively. Grasses made up the majority of the total canopy coverage and averaged 53.9 percent, while forbs made up only 17.2 percent of the total. Areas of bare ground were not uncommon. The vegetational composition was dominated by the grasses Agropyron Spioatum3 Bromus teotorum3 Festuoa idahoensis3 and Poa pratensis. Species density of forbs was variable. Composites were common. There were few mesic depressions with different plant species. The dry grass­ land community type included five (8.5%) of the study plots. —26— Grass-forb Community Type The grass-forb communities were located on both natural and agri­ cultural lands on the valley floor and mountain meadows. This community type was characterized by a variable composition of both grasses and forbs in the canopy coverage. Grasses averaged 37.1 percent of the total coverage and forbs averaged 41.0 percent. Biomass ratios and amounts of ground litter were variable (average values were 278g/159g/ 136g for grasses, forbs and litter, respectively). Soil moisture ranged from 2.59 to 38.98 percent and averaged 15.5 percent. This com­ munity type was also divided into two subunits according to soil moisture and species dominance in the canopy. Seventeen (28.8%) of the sampled areas included the grass-forb community type. The grasses characterizing the more mesic subunit included Poa ppatensisj Phleum pratense^ and Bvomus mavginatus. Various forbs held different dominance positions. These included Civsium spp. , Ranunoulus spp., Tvifolium spp. , Tavaxaoum spp. , Medioago Sativa3 Monavda fistulosa3 and Lithospevmum vudevale. The soils were less well drained than those of the xeric subunit. The more xeric habitats were characterized by the grasses Agvopyvon Spioatum3 Bvomus tectovwn, and B. inevmis. Festuoa idahoensis occupied various dominance positions on the different study plots. Forbs in­ cluded Avnioa spp., Balsamovhiza Sagittata3 Cevastium spp. , Melilotus -27- OffiainaUs3 and Medioago sativa. Soils were moderately well drained. Forb Community Type The two (3.4%) Eorb communities were characterized by a pre­ dominance of Eorb species growing on fertile, mesic soils. Forbs aver­ aged 83.3 percent of the total canopy coverage while grasses made up only an average of 5.6 percent. Similarly, the dominance position of the forbs was expressed in the average biomass ratio (63g/539g/78g for grasses, forbs, and litter, respectively). Soil moisture averaged 19.03 percent with individual means of 14.00 and 24.06 percent. The major forbs included: Medioago Sativa3 Tvifolinm spp. , Taraxacum spp.j Mevtensia OiHata3 and GaHum boveale. Grasses (,Bvomus spp.) played a minor role in the community structure. Standing crops were large and the amount of litter minimal. Sagebrush Community The sagebrush community was distinguished by the grasses Poa pvatensis and Festuoa idahoensis and big sagebrush (Artemisia tvidentata) Species of forbs were few. Grasses made up 40.5 percent of the total canopy coverage, forbs 15.1 percent, and sagebrush 12.9 percent. The soils were sandy and moderately well drained, and soil moisture averaged 14.36 percent. This area contained more mesic gullies of denser vege­ tation. Standing crop of grasses and litter was moderate. The biomass -28- ratio was 310g/120g/116g per square meter for grasses, forbs, and lit­ ter, respectively. A single sagebrush area (1.7%) was sampled. Coniferous Forest Community Type Lodgepole pine (P-inus contorta)3 Engelmann spruce (Piaea enget- manni)3 and subalpine fir (Abies tasiooavpa) characterized the coni­ ferous forest community type. The three most conspicuous understory plants in one area were Cavex Qeyevi3 Poa Tpvatensis3 and Tvifolium sp., while the other area was characterized by an understory of C. geyevi3 Vaeeinium Seopavium3 and Sedum debilis. Canopy coverage for grasses, forbs, and shrubs averaged 16.1, 12.9, and 19.2 percent, respectively. Standing crops were moderate, and accumulations of ground litter were minimal (averaged 43g/l68g/65g per square meter for grass, forbs, and litter). The lithosol soils were relatively moist with soil moisture averaging 23.73 percent. Two (3.4%) of the 59 study plots were of the coniferous forest community type. Aspen Community Type The aspen community type was distinguished by quaking aspen (Populus tvemuloides). The understory was dominated by shrubs in one area (Physooavpus malvaceus , Loniceva Utahpnsis3 and Spivaea betuli- folia). The other area was characterized by the shrubs Symphovieavpos albus and Rosa sp. and the grass Poa pvatensis. Grass, forb, and shrub canopy coverage averaged 9.0, 20.4, and 29.8 percent, respectively. In —29— the more open areas forbs and grasses were more commonly found. Soils were lithosols that had an average soil moisture of 22.16 percent with individual values of 18.00 and 26.33 percent. Standing crops were large, but litter accumulations were small. Biomass ratio averaged 141, 361, and 56 grams per square meter for grasses, forbs, and litter, re­ spectively. Two (3.4%) of the sampled study plots were of the aspen community type. Alpine Meadow Community Type The alpine meadow communities actually consisted of a subalpine association at 7,800 feet elevation and an alpine association at 8,800 feet. The subalpine habitat was characterized by the forbs Lupinus avgenteus, Eviogonum umbetlatum and the grass Festuca idahoensis3 with a canopy coverage of 31.0 and 51.2 percent for grasses and forbs, re­ spectively. There was a relatively good standing crop of herbaceous material (202g/122g/168g per square meter for the respective classes). Soil moisture was 10.55 percent. The alpine association was located along a small spring-fed stream in a glacial cirque. The three dominant plants in the canopy coverage were Evigevon pevegvinus3 Sdlix Wolfii3 and SelageneVla densa. Various alpine grasses were present in lesser numbers. The respective coverages in the canopy were 24.3, 20.3, and 10.7 for grasses, forbs, and shrubs. Biomass was small and there was extremely little litter accumulation -30- (32/128/8 grams per square meter). Bare ground areas Were not uncommon. The soils were mesic (88.61 percent moisture). One each (1.7%) of the two subunits was sampled. RESULTS General Distribution During the course of the study 762 Miarotus montanus and 583 M. pennsytvanicus were collected from 59 study plots (Table II). Popu­ lations of both species occurred symmetrically on 31 of the sample plots; on 14 of the plots only M. montanus was captured and only on 8 of the plots 'M. pennsyZtaniaus was captured. On six of the study areas neither species of Miarotus was captured. Capture success increased Miarotus was 2.4 percent in 1968 and increased to 4.3 percent in 1969. At least one species of Miarotus was present ip one or more study plots in eight of the nine community types sampled. An indication of relative abundance of these voles in relation to community types is given in Table III. ' Statistical analysis showed that distribution was not random among community types (Table III) , thus indicating a possi­ ble habitat preference for various community types by the two species. The two species were sympatric in 31 of 52 plots in five community types. Miarotus montanus was captured more often than AL pennsylvaniaus in three of the community types (mesic grassland, dry grassland, and grass-forb community types). In the grass-sedge and forb communities M. pennsylvaniaus was captured more often. A comparison of actual captures with expected captures (based on a distribution of Miarotus if seasonally in favorable habitats. Average capture success for any Table II. Total catch, total trap-nights, and percent success of capture of Miorotus montanus3 Miorotus pennsytVanious3 and other species of small mammals based on monthly takes. Trapping periods Areas trapped Trap- nights No. M. mont. Percent success* No. AL penn. Percent success* Associated mammals ** June, 1968 5 1500 27 1.50 28 1.58 36 July, 19.68 9 2700 38 1.58 60 ■ 2.50 170 Aug., 1968 8 2400 122 5.08 104 4.33 259 Sep., 1968 3 900 21 2.33 0 0.00 40 June, 1969 14 4200 229 5.45 267 6.36 225 July, 1969 13 3900 266 6.82 66 1.69 66 Aug., 1969 _7 2100 59 ' 2.81 58 2.76 84 Totals 59 17700 762 4.31*** 583 3.29*** 880 Based on total trap-nights for that period. Species of other small mammals included: Peromysous maniculatus, Sorex oinereus3 S. Vagrans3 S. palustris3 Zapus prinoeps3 Thomomys talpoides3 Eutamias sp., Miorotus Iongioaudus3 Clethrionomys gapperi, Phenacomys intermedius3 and Mus musoulus. Average total capture success percentage.• k - k - k Table III. Comparison of the number of captures (No. cap.) and expected number of captures (Ex. cap.) of Miovotus montanus and Miovotus pennsylvaniaus on the assumption of random distribution within the nine community types. Community type No. of plots No. of - traps M, montanus M. -pennsylvanious Species ratio (mont./ipenn.) No. cap. Ex. cap. X2 value No. cap. Ex. cap. X2 value Grass-sedge 6 600 4 77.4 69.61 122 59.3 66.29 I : 30. 5 Mesic grassland 22 2200 403 283.8 50.07 347 217.4 77.26 I : 0.86 Dry grassland 5 500 59 64.5 0.47 4 49.4 41.72 I : 0.07 Grass-forb 17 1700 248 219.3 3.76 94 168.0 32.60 I : 0.38 Forb 2 200 2 25.8 21.96 12 19.8 3.07 I : 6.00 Sagebrush I 100 27 12.9 15.41 0 9.8 9.80 Coniferous forest 2 200 0 25.8 25.80 0 19.8 .19.80 Aspen 2 200 0 25.8 25.80 4 19.8 12.61 Alpine meadow _2 200 19 25.8 1.79 0 19.8 19.80 Totals 59 5900 762 761.2 214.51* 583 583.1 276.76* * Denotes a significant value. P<0.005 -34- it were random in relation to all community types) indicated a possible preference by M. montanus for the mesic grassland communities, and possi­ bly an avoidance of the grass-sedge and forb community types. The data also indicated a possible preference for the grass-sedge and mesic grass­ land community types by M. ’pennsy'Lvan-icus, Likewise, a possible avoid­ ance was demonstrated by M. ipennsylVanious for the dry grassland and grass-forb community types. A comparison of ratios relative to community types (number of M. montanus to the number of pennsylvani-cus) is given in Table III. Sample sizes were smaller in the remaining community types. The single sagebrush area and the two alpine community types yielded only M. montanus. In the forested areas, only M. penn^ylvanious was captured. Mtorotus montanus3 in addition to being the most widespread species, was the most numerous species of Miovotus captured on 31 of the 59 study plots. Distribution Within Community Types Relative numbers of these two species (expressed in numbers per 100 trap-nights) in relation to individual study plots are shown in Table I. Although both species were captured in fifty percent (3 plpts) of the sampled areas in the grass-sedge community type, this community type appeared to be favorable habitat for M. pennsylvanious and possibly avoided by M. montanus. A preference by M. pennsylvanious or an -35- avoidance by M. montanus of the grass-sedge community type was evidenced by the large proportion of M. pennsylvanicus in the species density ratio (I : 30.5; Table III) , and large statistical values. The mesic grassland community types appeared to be favorable habi­ tat for both species of Miarotus. Although individual study plots demon­ strated unequal distributions of one of the other species, an overall tally of population numbers showed about equal distributions of species on this community type (species density ratio of I : 0.86, M. montanus to M. pennsytvaniaus3 from 22 sample plots of this community type). Likewise, large statistical values (Table III) were also possibly indi­ cative of a favorable habitat for both species of Miorotus. Within the mesic grassland community types both species were captured on seventeen of the sample areas; exclusively M. montanus populations occurred on three areas, and only M. pennsyIvanicus was captured on two study plots. At least one of the two species of Miorotus was present on each plot. Population numbers of M. montanus considerably outnumbered M. pennsyt- vanious on nine plots, and M. pennsytvanious outnumbered M. montanus on five study plots. On the remaining eight study plots species numbers approximated a I : I ratio. But when this community type was divided into mesic and xeric subunits possibly differences in distribution be­ came more meaningful. Eleven of the sample plots were of the dry subunit; on these, M. montanus was the most numerous species on eight, M. pennsyt­ vanious was the most numerous species on only four. These data suggest a wider ecological tolerance and/or habitat preference for M. montanus. -36- It was also evident that substrate moisture alone was not enough to produce a favorable habitat for M. pennsylvanicus. In the dry grassland community type M. montanus greatly outnumbered M. Tpennsylvanicus at a ratio of I : 0.07 (Table III). This avoidance of the dry grassland community type by M. pennsylvanious was indica­ tive of unfavorable habitat for this species. Although M. montanus greatly outnumbered M. pennsylvanious3 the species density ratio in this community type shows that this is not of a preferred habitat for M. montanus. The numbers of M. montanus per trap effort were much lower than they were in the mesic grassland community, type, which was considered favorable habitat. The only inference that can be made is that M. montanus was more tolerant of xeric conditions than M. pennsyl­ vanious . Miorotus montanus outnumbered M. pennsylvanious at a ratio of I : 0.38 (Table III) in the grass-forb community type. The relatively small population numbers of M. pennsylvanious in this community type may be indicative that this is an unfavorable habitat for this species. The population numbers of M. montanus within the grass-forb community provided no evidence that this was a favorable habitat type, but may be indicative of a wider ecological tolerance for this habitat type than shown by M. pennsylvanious. As in the dry grassland community, capture numbers of M. mOntanus per trapping effort were smaller than in the more favorable mesic grassland community. Both species of Miorotus occurred -37- sympatrically on eight of seventeen plots in this community type, and M. montanus outnumbered M. pennsylvanieus on six of them. On seven of the plots only M. montanus was captured. Only M. yennsylvanicus was captured on one plot, and no Micvotus were captured on another. When the sites were divided into mesic and xeric subunits, the data revealed that M. IpennsyttarL-Ious outnumbered M. montanus on only three of eight mesic subunits, while M. montanus was most numerous on five of them. Two areas which had large M. pennsylvanious populations (areas 43 and 63) had soil moistures that exceeded thirty-four percent. On the nine xeric subunits M. montanus was the most numerous species on eight, and on one plot no Miovotus were captured. These data suggest an avoidance of the grass-forb community type by M. pennsyttanious3 except for ex­ tremely moist situations. It is also indicative of a wider ecological tolerance by M. montanus. Due to small sample sizes, only conjectures or generalities can be made concerning Miovotus distribution in the remaining six community types. On the two forb community areas sampled no Miovotus were cap­ tured on one study plot (area 21), and on the other (area 15) M. montanus was outnumbered by M. pennsylvanious at a ratio of I : 6 (Table III). On the single sagebrush area sampled only M. montanus was collected (27 individuals), possibly this is indicative of a favorable habitat type. Data revealed that both the coniferous forest and the aspen community types were not favorable areas for either species of -38- Miovotus (Table III) , since only four individuals were captured in a total of 1200 trap-nights on both areas. In the alpine community (area 68) no 'Miovotus were captured, but in the subalpine community (area 32) nineteen M. montanus were taken. Vegetational Relationships The results discussed above indicate that the majority of Miovotus occur in grass-like and grass-forb meadows and avoid areas where woody plants predominate (conifers, willows, and aspens). An analysis of the relative abundance of Miovotus (Table IV) in relation to the abundance of grasses and grass-like species in the canopy cover showed a non- random distribution. Those study plots in which grasses and grass-like species made up at least fifty percent of the canopy (29 plots and ex­ clusive of those ten plots of'60-70 percent coverage) were seemingly preferred habitat types for M. pennsylvanious. likewise, those areas with less than fifty percent graminoid coverage were seemingly unfavor­ able habitat types for M. pennsylvanious and were avoided by this species. The correlation coefficient between the total percentage of grasses and grass-like species in the canopy and the relative abundance of M. yenn- sylvanious was 0.786 (Fig. 5), a roughly linear relationship between the abundance of graminoids in the canopy and the relative density of M. ■pennsylvanicus is suggested. Table IV. Comparison of the number of captures (No. cap.), average number per 100 trap-nights, and the expected number of captures (Ex. cap.) if distribution of Miorotus montanus and Miorotus pennsylvanious were random in relation to the percentage coverage of grasses and grass-like species in the canopy. Abundance, of grass in canopy Number of plots M, montanus M. pennsylvanious No. cap. Av. No./100 trap-nights Ex. cap. value No. cap» Av. No./100 trap-nights Ex. cap. value' 80% or more 3 2 0.33 38.8 34.81 75 8.33 29.6 69.42 70-80% 8 123 5.12 103.4 3.73 231 9.63 79.0 292.15 60-70% 10 226 7.53 129.2 77.09 108 3.63 98.8 0.86 50-60% 8- 126 5.25 103.4 4.60 126 5.25 79.0 27.90 i 40-50% 7 96 4.57 90.4 0.34 8 0.38 69.2 54.09 30-40% 8 102 4.25 103.4 0.02 14 0.58 79.0 53.52 20-30% . 8 69 2.88 103.4 11.42 4 0.17 ■ 79.0 ' 71.24 20% or less _7 18 0.86 90.4 58,02 17 0.81' 69.2 39.34 Totals 59 762 762.3 190.08* 583 582.9 608.52* * Denotes a significant value. E<0.005 av . n o/ 10 0 tr ap -n ig ht s M -fr O) CO O O M. pennsylvanicus • M. montanus dU 40 50 60 70 abundance of graminoids in per cent Figure 5. Numbers of Miarotus montanus and Miorotus pennsylvanicus expressed in average numbers per 100 trap-nights (see Table IV) plotted against the percentage of grasses and grass-like species in the total canopy coverage. Spearman rank correlation coefficients were 0.273 for M. montanus and 0.786 for M. pennsylvanicus. -41- An analysis of M. montanus distribution relative to the ampunt of grasses and grass-like species in the canopy cover suggested that areas with a graminoid cover ranging from about thirty percent to eighty per­ cent were favorable habitat types for M. montanus ■(IahIe IV). Those areas in which grasses and grass-like species made up at least eighty percent of the canopy (3 plots) as well as those that had less than thirty percent graminoid coverage (15 plots) appeared to be unfavorable M. montanus areas and were avoided by this species. M. montanus demonstrated less of a linear correlation with the amount pf grasses and grass-like species in the cancpy than did M. pennsytvanious (rd = 0.273; Fig. 5). Standing creps of herbacecus material (biomass) also were related to the local distribution and the relative densities pf both species of Miovotus (Table V). The data indicated a direct positive correlation between numbers of M. pennsylvanicus trapped and the amount of biomass of a given area (r^ = 0.976; Fig. 6). Those study plots in which herbaceous standing crops were in excess of 700 grams per square meter (19 plots) appeared to be favorable habitat types for M. 'pennsytvanious.. The data revealed little linear correlation between amounts of standing crops and population densities of M. montanus (r^ = -0.119; Fig. 6). Those with a biomass in excess of 800 grams per square meter ap­ peared to be avoided by M. montanus. Likewise, the same response was observed in areas with less than 300 grams herbaceous material per Table V. Comparison of the number of captures (No. cap.)-, average number per 100 trap- nights, and. the expected number of captures (Ex. cap.) if distribution of Miorotus montanus and Miorotus pennsyIvanious were random in relation to total biomass (in grams per square meter) of grasses, forbs, and litter. M. montanus M. pennsylvaniaus Biomass in grams per m2 Number of plots No. cap. Av. No./100 trap-nights Ex. cap. X2 value No. cap. Av. No./100 trap-nights Ex. cap. X2 value 900g. or more 7 35 1.67 90.4 40.39 221 10.52 69.2 333.36 800-900g 6 21 1.17 77.5 41.21 111 6.17 59.3 45.12 700-800g 6 ' 91 5.05 77.5 2.34 85 4.72 59.3 11.16 600-700g 8 120 5.00 103.4 2.68 50 2.08 79.0 10.67 500-600g 9 117 4.33 116.3 0.01 59 2.19 88,9 10.07 400-500g 14 314 7.48 180.9 97.97 37 0.88 138.3 74.22 i 300-400g 5 50 3.33 64.6 3.30 20 0.13 49.4 17.50 M CO O O OQ O less _4 14 1.17 51.7 27.47 0 0.00 39.5 39.52 1 Totals 59 762 762.3 215.37* 583 582.9 541.63* * Denotes a significant value. P<0.005 av . n o. /1 00 tr ap -n ig ht s o M. pennsylvanicus # M montanus 500 600 700 800, total biomass in grams / M: Figure 6. Numbers of Miarotus montanus and Miarotus pennsylvanicus expressed in average numbers per 100 trap-nights (see Table V) plotted against standing crops (bio­ mass in grams per square meter). Spearman rank correlation coefficients were -0.119 for M. montanus and 0.976 for M. pennsylvanicus. -44- square meter. The remaining areas had larger numbers of M. Tnontanus3 but the greatest apparent preference was shown for those areas in which biomass ranged between 400 and 500 grams per square meter. An- analysis of dominant plants (the three most conspicuous plants in each study plot as determined by percent coverage and frequency in the canopy) and the local distribution of Miorotus revealed a possible response by these two species of voles to particular plant species (Table VI). Only those data are discussed in which,the particular plant species was dominant on at least four study plots. Both species demonstrated a positive correlation indicating favorable habitats in areas dominated by the grasses Bromus marginatus3 Daotytis gtomerata3 and Poa pratensis. Miorotus montanus preferred areas dominated by Bromus ineTmis and Festuoa idahoensis3 but M. pennsylvanious did not. Miorotus montanus appeared to avoid areas dominated by Cgvex nebrask- ensis3 and M. pennsylvanious avoided areas characterized by a dominance of Agropyron Spioatum3 Bromus teoiorum, and Feotuog idaho- ensis. When the forb species Medioago sativa was dominant in an area it appeared to be favorable habitat for both species of Miorotus. Cirsium arvense was seemingly preferred by M. montanus3 a.nd those areas characterized by Trifolium repens were apparently avoided. Miorotus pennsylvanious avoided areas in which Taraxacum sp. was a dominant forb. Plots characterized by a subcanopy of shrubs were generally avoided by both species of Miorotus3 except possibly areas -45- Table VI. A comparison between the actual number of captures (No. cap.), and the expected number of captures (Ex. cap.) if distribution were random of Miovotus montanus and Miovotus pennsyIvanious in relation to the three dominant plants from all study areas. M. montanus M. ^pennsytvanious Dominant plants from all areas Number of plots No. cap. Ex. cap. value No. cap. Ex. cap. X2 value GRAMINOIDS Agvopyvon vepens 3 30 38.6 1.91 74 30.0 64.. 69 Agvopyvon smithii 5 84 64.3 6.04 37 50.0 3.36 Agvopyvon spioatum 4 58 51.4 0.84' 0 40.0 39.96 Agvostis alba I 0 12,9 12.8$. 34 . 10.0 57.71 Bvomus inevmis 8 175 • 102.9 50.55 88 79.9 0.82 Bvomus mavginatus 7 127 90.0 15.19 122 69.9 38.77 Bvomus sp. 3 ■57 38.6 . 8.63 99 30.0 159.00 Bvomus tectovum 5 58 64.3 0.62 4 50.0 42.27 Cavex atvosquama I I 12.9 10.94 48 10.0 144.62 Cavex geyevi 2 0 25.7 25.72 0 20.0 19.98 Cavex hoodii I 0 12.9 12.86 6 10.0 1.59 Cavex nebvaskensis 4 I 51.4 49.46 62 40.0 12.16 Cavex sp. I 0 12.7 12.86 0 10.0 9.99 Daotylis glomevata 6 112 77.2 15.73 116 59.9 52.44 Desohampsia elongata I I 12.9 10.94 46 10.0 129.80 Blymus oineveus I 3 12.9 7.59 0 10.0 9.99 Festuoa elatiov I 6 12.9 3.69 I 10.0 8.09 Festuca idahoensis 8 143 102.9 15.65 5 79.9 72.12 Junous baltious 2 6 25.7 15.12 68 20.0 115.41 Melioa sp. I 6 12.9 3.69 0 10.0 9.99 Phalavis avundinaoea I I 12,9 10,94 21 10.0 12.13 Phleum pvatense 13 175 167.2 0.37 108 129.9 3.68 Poa pvatensis 36 556 463.0 18.70 ‘ 476 359.5 37.65 Stipa oomata I 14 12.9 10.94 o ■ 10.0 9.99 FORBS Aohillea millefolium I I 12.9 10.9,4 :o 10.0 9.99 Avnioa sovovia I 2 12.9 9.17 0 10.0 9.99 Balsamovhiza sagittata 2 19 25.7 2.95 0 20.0 19.98 -46- Table VI. ' (Continued). Dominant plants from all areas M. montanus M. pennsyIvanicus Number of plots ■ No. cap. Ex. cap. X2 value No. cap. Ex- cap. X2 value Cerastium arvense I 19 12.9 2.93 0 10.0 9.99 Cirsium arvense 4 23 51.4 15.73 26 40.0 4.88 Conium maeulatum I 11 12.9 0.30 21 10.0 12.13 Cynoglossum officinale I 38 12.9 ■ 49.15 7 10.0 0.89 Erigeron peregrinus I 0 12.9 12.86 0 10.0 9.99 Eriogonum umbellatum I 19 12.9 2.93 0 10.0 9.99 Galium boreale I 0 12.9 12.86. 0 10.0 9.99 Geranium richardsonii I 0 12.9 12.86 I 10.0 8.09 Geranium viscossisimum 2 49 . 25.7 21.07 3 20.0 14.46 Lithospermum ruder ale I 40 12.9 57.28 3 10.0 4.89 Lupinus argenteus 2 21 25.7 0.89 0 20.0 19.98 Lychnis alba I 41 12.9 61.58 18 10.0 6.42 Medicago sativa 6 139 77.2 49.56 101 59.9 28.13 Melilotus officinalis 2 30 25.7 0.71 I 20.0 18.03 Mertensia ciliata I 0 12.9 12.86 0 10.0 ■ 9.99 Monarda fistulas a 2 14 25.7 5.30 I 20.0 18.03 Ranunculus sp. I 6 12.9 3,67 . 0 10.0 9.99 Sedum debilis I 0 12.9 12.86 0 10.0 9.99 Sonchus sp. 2 5 25.7 16,69 26 20.0 1.81 Taraxacum sp. 7 100 90.0 1.11 40 69.9 12.81 Trifolium pratense 2 18 25.7 2.3% .15 20.0 1.21 Trifolium repens SHRUBS ■ 5 12 64.3 42.54 . 61 50.0 . 2.44 Artemisia cana I 2 12.9 9.17 i 10.0 8.09 Artemisia tridentata ■ I 27 12.9 15.55 0 10.0 9.99 Loniaera utahensis I 0 12.9 12.86. 2 10.0 6.23 Physoearpus malvaceus I 0 12.9 12.86 2 10.0 6.23 Rosa sp. I 0 12.9 12.86 2 10.0 6.23 Salix wolfii I 0 12.9 12.86 0 10.0 9.99 Spiraea betulifolia I 0 12.9 12.86 . 2 10.0 6.23 Symphoriaarpos albus 2 0 25.7 25.72 2 20.0 16.16 Vaccinium scoparium I 0 12.9 12.86 870.55 0 10.0 9.99 L389.51 P<0.005 P<0.005 -47- of Avtemtsia tvidentatas which were seemingly preferred by M, montanus. The calculated chi-square value (Table VI) was much larger for M. penn- syIvanieus distribution, thus indicating more of a selective response to various plant species than was observed for M. montanus. These statistical correlations demonstrated by either species to particular plant species were probably not responses to that species exclusively, but might be indicative of other factors of the habitats also associated with these plant species. Cover and Litter Relationships Cover is generally a vague term implying various things and has been applied occasionally to that particular physiognomic type of vegetation most suitable for runway construction (Getz 19.61). In this study the term litter is applied to that material used for runway construction, and cover is used to imply total canopy coverage in the herbaceous and shrub layers. The data revealed evidence of a correlation between total canopy coverage of the herbaceous and shrub layers and the relative density of M. pennsylvanicus (r^ = 0.998; Fig. 7). Those areas (31 plots) in which coverage exceeded eighty-five percent appeared to be favorable habitat types for M. pennsyivanicus3 and likewise, areas with less than seventy-five percent coverage were apparently avoided by M. pennsylvanicus (Table VII). av .n a/ 10 0 tra p- ni gh ts M. pennsylvanicus o M. montanus # I 70 75 80 85 per cent total canopy coverage Figure 7= Numbers of Miarotus montanus and Miarotus pennsylvanicus expressed in average numbers per 100 trap-nights (see Table VII) plotted against total herbaceous and shrub coverage in the canopy. Spearman rank correlation coefficients were 0.515 for M. montanus and 0.998 for M. pennsylvanicus. Table VII. Comparison of the number of captures (No. cap.), average number per 100 trap- nights, and the expected number of- captures- (Ex. cap.) if distribution of Miavotus montanus and Miovotus pennsyIvanious were random in relation to canopy . coverage in the herbaceous and shrub layers. .. M. montanus M. -pennsyVoanious Total can. coverage Number of plots No. cap. Av. No./100 trap-nights Ex. cap. value No. cap. Av. No./100 trap-nights Ex. cap. X2 value 95% or more 8 73 3.04 103.4 8.92 120 5.00 79.0 21.23 90-95% 10 141 4.70 129.2 _ 1.08 192 6.40 98.8 87.92 85-90% 13 226 5.79 168.0 20.06 181 4.64 128.4 ■21.51 80-85% 5 83 5.33 64.6 5.24 40 2.67 49.4 1.79 75-80% 5 59 3.93 64.6 0.49 36 2.43 49.4 3.63 , 70-75% 6 59 3.28 77.5 4.42 8 0.44 59.3 45.49 65-70% 2 51 8.50 25.8 24.50 2 0.33 19.8 15.96 I 60-65% 3 18 2.00 38.8 11.19 ' 2 0.22 29.6 25.75 55-60% 4 35 2.92 51.7 5.38 2 0.17 39.5 35.62 55% or less _3 17 1.89 38.8 12.22 0 0.00 29.6 29.64 Totals 59 762 762.3 '93.50* 583 582.9 288.54* * Denotes a significant value. P<0.005 -50- A seemingly small correlation between M. montanus distribution and the total canopy coverage was evidenced by a low correlation coefficient (r^ = 0.515; Fig. 7). There was some evidence that M. montanus may have preferred areas in which coverage ranged between 65 and 100 percent (Table VIT). Amounts of litter seemingly had little linear effect on the local distribution of M-Icvotus (Table VIII), as indicated by the small cor­ relation coefficients (r^ = -0.262 for M. montanus and r^ = 0-571 for M. pennsylvari-lous; Fig. 8). In those areas in which the amounts of litter exceeded 250 grams per square meter (7 plots) and those five areas with 150 to 200 grams litter per square meter may have been favor­ able habitats for M. pennsylvanious. No trends were demonstrated by M. montanus distributions relative to amounts of litter, except possibly that M. montanus prefers areas with moderate amounts of. ground litter. Soil Moisture Relationships There was some evidence of a correlation between high soil moisture and the distribution of M. pennsylvanious (r^ = 0.771; Fig. 9). The data indicated a possible preference by M. pennsylvanious (Table IX) for those areas in which substrate moisture was greater than thirty percent (11 plots). There was a reduction in M. pennsylvanious population numbers in the drier areas (48 plots) . /Tab16 VIII. Comparison of the number of captures (No. cap.), average number per 100 trap- nights, and the expected number of captures (Ex. cap.) if distribution of Miovotus montanus and Miovotus pennsylvanious were random in relation to the amount of litter (expressed in grams per square meter). M. montanus M. pennsylvanious Litter in Number grams/m^ of plots No. ; cap. Av. No./100 Ex. trap-nights cap. X2 value No. cap. Av. No./100 Ex. tr ap-nights c ap. X2 value 350g. or more 4 13 1.08 51.7 2 8 . 9 5 103 8 . 5 8 39.5 101.95 300-350g. I 41 13.67 ' 12.9 60.84 18 6.00 9.9 6 . 6 7 250-300g. 2 2 0.33 25.8 21.99 67 11.16 19.8 114.32 200-250g. 3 36 3.17 3 8 . 8 0.20 26 2 . 8 9 2 9 . 6 0.45 150-200g. 12 207 5.75 155.0 17.41 80 2 . 2 2 118.6 1 2 . 5 4 IOO-ISOg. 5 45 3.00 64.6 5.95 63 4.20 4 9 . 4 3 . 7 5 50 -IOOg. 19 213 3.74 245.5 4.30 161 2 . 8 2 187.7 3 . 8 0 50g. or less 13 215 . 5.51 168.0 13.17 . 65 4.33 128.4 31.33 Totals 59 762 7 6 2 . 3 152.81* 583 5 8 2 . 9 274.72* * Denotes a significant value. P<0.005 o M. pennsylvanicus e M. montanus grams of litter / M Figure 8. Numbers of Miarotus montanus and Miarotus pennsylvanicus expressed in average numbers per 100 trap-nights (see Table VIII) plotted against the accumulation of litter (in grams per square meter). Spearman rank correlation coefficients were -0.262 for M. montanus and 0.571 for M. pennsylvanicus. O M. pennsylvanicus • M. montanus per cent soil moisture Figure 9. Numbers of Miarotus montanus and Miorotus pennsylvanicus expressed in average numbers per 100 trap-nighcs (see Table IX) plotted against the soil moisture expressed as a percent of the oven dry weight of the soil. Spearman rank cor­ relation coefficients were -0.942 for M. montanus and 0.771 for M. pennsylvanicus. Table IX. Comparison of the number of captures (No. cap.), average number per 100 trap- nights , and the expected number of captures (Ex. cap.) if distribution of Miovotus montanus and Miovotus pennsyIvanious were random in relation to soil moisture. Soil moisture Number of plots M. montanus M. pennsyIvanious No. cap. Av. No./100 trap-nights Ex. cap. X2 value No. cap. Av. No./100 trap-nights Ex. cap. X2 value 50% or more 3 0 0.00 38.8 38.78 35 3.89 29.6 2.36 40-50% I I 0.33 12.9 10.99 46 15.33 9.9 132.05 30-40% 7 60 2.86 90.4 10.25 137 6.52 ' 69.2 66.55 20-30% 14 91 2.17 180.9 44.66 106 2.57 138.2 7.69 10-20% 22 362 5.50 284.2 21.82 245 3.73 217.4 3.51 10% or less 12 247 6.86 155.0 54.54 ■ 14 0.39 118.5 92.21 Totals 59 762 762.3 181.02* 583 582.9 304.37 * Denote a significant value. P<0.005 The observed distribution of M. montanus was correlated with the dryness of the substrate (r^ = -0.942; Fig, 9). Those areas in which soil moisture was greater than twenty percent (25 plots) appeared to be unfavorable habitat for M. montanus. Likewise, the thirty-four areas in which soil moisture was less than twenty percent appeared to be favorable habitat type for M. montanus. Those moist situations seemingly preferred by M. pennsylpccnious frequently supported dense stands of grasses and grasSrlike species, and in those areas there may have existed an interaction between the associated factors of moisture and vegetation. Variations in M. montanus numbers similarily may reflect an interaction between the more xeric habitats and their characteristic vegetation. -55- DISCUSSION Optimum habitat of M. ipennsyIvanicus appears to be in areas where grasses and grass-like species , especially Poa -pratensis3 are dominant plants, comprising fifty percent or more of the vegetation, and total canopy cover of all herbaceous material is at least eighty-five percent. This type and amount of vegetation; which is in part a reflection of substrate moisture, appears to be an important factor influencing the local distribution of M. ‘pennsylvan-icus. Amounts of litter appear to be of little importance to favorable habitat. Thus, those areas which have relatively high substrate moistures along with well developed grassy herbaceous canopy layers are preferred by. M. pennsylyanieus. Zimmerman (1965) has shown that a definite correlation exists between numbers of M. ’pennsytvan-icus and grassy areas at Terre Haute, Indiana, and that this species was taken only where grasses made up at least fifty percent or more of the vegetation. It is possible that M. penn- Sytvani-CUS has a high evaporative loss of moisture so that water loss becomes too great in drier situations to maintain a proper water bal­ ance (Lindeborg 1952 and Getz 1963). Also, M. pennsytvanicus distri­ bution is in general correlated with the amount of herbaceous vegetation in the canopy and light penetration (Mossman 1955). Thus, the positive response by M. pennsytvanicus to hydroseric and mesic communities may be related to higher humidity, decreased light penetration, and certain vegetational features. -57- The preferred habitats of M. montanus were not as well delineated as those of M. pennsyIvanieus. Those particular physical and physio­ gnomic characteristics of the habitat strongly influencing M. yennsyl- Vanieus distribution only moderately influenced M. montanus distribution. The observation that M. montanus occupied a greater range of habitats than M. pennsyZvanieus and seemingly was less responsive to various habi­ tat features suggested a wider ecological tolerance by M. montanus. Findley (1951, 1954) reported a wider tolerance for M. montanus in Jackson Hole, Wyoming. Findley (1951) also reported that M. montanus was less tolerant than other species of Mievotus of extremely dry or wet habitats (sage and willow-sedge savannah), but this species was relatively common in the drier habitats in the present study. The distribution of M. montanus generally correlated with the dryness of substrate, but overall population numbers did not indicate the dry grass­ land community type to be a favorable habitat. The habitat preference of M. ipennsyZvanieus3 especially in the eastern United States, is well documented (Blair 1940, 1948; Linduska 1950; Eadie 1953; Getz 1961; Zimmerman 1965; and others). These studies indicate that dense cover formed by grassland vegetation, primarily in ■ low-lying humid areas is preferred habitat. Hoffmann and Pattie (1968) also report that Montana M. pennsytvanieus occupy forested areas with dense herbaceous ground cover. Less information is available concerning M. montanus habitat preference. Findley (1951), Hoffmann and Pattie -58- (1968), Koplin and Hoffmann (1968), and Goertz (1964) indicate that preferred habitats are usually dry grassland or sagebrush-grassland, grassy alpine meadows, and agricultural lands. Wecker (1963) stated that habitat preference of wild populations of Peromysaus maniaulatus bairdi was an expression of an innate pattern of behavior and that this pattern was elicited by certain k$y environ- • mental stimuli. Harris (1952) showed that P, m. QraciZyLs and P. m. bairdi in artificial habitats demonstrated a selection for particular kinds of objects in the environment. Harris (1952) also maintained that these objects probably act as clues by which the mice recognize the environment most suitable to them and that habitat selection by Peromysaus is basically genetic in nature. According to the principle of stimulus summation (Hilden 1965), not all suitable habitats need to possess all the features characteristic of optimal environments. All that is necessary is that the combined effect of the individual stimuli exceeded the threshold of the selection reaction. Hilden (1965) also maintains, at least for birds, that sometimes one key stimulus ("super stimulus") may outweigh others and without its presence other stimuli are never sufficient to induce a selection response, whereas it alone may elicit a selection for a submarginal habitat. This key stimulus may be an ultimate factor (food, the requirements imposed by structural and functional characteristics of the species, and shelter from enemies and adverse weather) that is being substituted for a sequence of -59- proximate factors in the selection of habitat. Such a factor might be operative in Miorotus selection of habitat in certain study plots. The combined effect of positive stimuli may be diminished by neg­ ative stimuli of the habitat, which then would have a repelling effect. The actual factors influencing ecological distribution by the two species are not completely known. Intraspecific competition may also determine the width of habitat range (HiIden 1965). When the population is sparse, only optimal habi­ tats are exploited, but with population increase less favorable habitats will serve, in order of suitability. During 1968-69 Miorotus populations were believed to be at peak densities. Thus, the large densities during the present study may have been responsible for movement of individuals into otherwise submarginal habitat types. Distributional patterns of the two species indicate that they were sympatric on many of the study plots. Also, since both species were captured in adjoining traps or in the same trap on successive days, it was concluded that these two species were not demonstrating competitive segregation on all study areas to the extent reported by Findley (1951, 1954) or Koplin and Hoffmann (1968). There is some evidence that these two species were ecologically separated within the mosaic vegetational patterns of various study plots. Various authors (Morris 1969, Findley 1951, Cameron 1964, and Clough 1964) have reported that one species of microtine rodent may occupy habitats used by another spepies in the -60- absence of that species. Getz (1962) has shown that M. pennsylvaniaus was less aggressive than other species of microtine rodents, but Murie (1963) has contended that M. pennsyIvanrLeus was dominant over M. montanus* Brown (1962) believes that a dominant species might prevent the spreading of another species. Many of the areas of coexistence had a predominance of one or the other species of Mierotus. It is possible that once a species is established in an area, either by preference or accident, its presence may restrict the abundance of the other species. But, in the present study, even if one species had a competitive advantage over the other it was not enough to maintain segregation of species in many of the small homogeneous study plots. In the present study, when species were coexistent, they occurred together in habitats apparently preferred or tolerated by both species, and when species were segregated they were in areas either preferred or tolerated by one species and avoided by the other species. Although Morris (1969) is of the opinion that habitat preference in itself is not an active enough mechanism to segregate closely related species of microtine rodents, it appears from this study that habitat preference is of major importance in species separ­ ation. The niche model proposed by Hutchinson (1957) and reviewed by Miller (1964, 1967) provides a concept for interpreting Mierotus distri­ bution. This model involves an intersection between two fundamental niches and includes a smaller niche (Hg) as a subset or included niche I —61— within the fundamental broader niche (N^) . Hutchinson (19,57) suggests two alternative outcomes of competition under natural conditions: (I) if competition proceeds in favor of one species in all elements of the broader niche (N^) of the second species, then only the first species survives; or (2) competition favors the second species in some elements corresponding to part of the intersection subset (Ng) and both species survive. If we examine Miarotus species distribution^ at the extremes (very mesic or xeric habitats) of their habitat spectra this model seemingly fits. On the other hand, in many of the other plots with a greater degree of sympatry and less evident differences in habitat utilization the fit of the above model becomes less exacting. This sympatric distribution of species with only slightly different ecologi­ cal preferences has also been reported by Hutchinson (1959) for Mioro- tus arvaliSj M. agrestis, and Clethrionomys glareolus in western Europe.. Koplin and Hoffmann (196,8) contend that similarities in habitat utili­ zation are adaptive, and after only several generations of genetic recombination one species could theoretically fill a vacated niche of the other decimated species. They are also of the opinion that compete itive exclusion would result in selection against genotypes of M. montanus adapted to mesic habitats and genotypes of M. pennsylvaniaus adapted to xeric habitats. Trends in habitat preference of M. ■pennsylvaniaus and M. montanus have been demonstrated in the present study, but the mechanisms are not completely understood. No' one factor is probably responsible for maintaining habitat and/or species separation. Those proximate factors involved may include habitat preference (Hilden 1965, Wecker 1963, and Harris 1952), behavioral intolerance as a result of olfactory dis­ crimination (Moore 1965), interspecific competition (Findley 1954, and Getz 1962) , or some combination of these. -62— LITERATURE CITED Anderson, R. M. 1948. Methods of collecting and preserving vertebrate animals. National Museum of Canada. Bull, 69. 162 p.' Blair, W. F . 1940. Home ranges and populations of the meadow vole in southern Michigan. J. Wildlife M|gt.*4: 149-161. ___________ . 1948. Population density, life span, and mortality rates of small mammals in the blue-grass meadow and blue-grass field association of southern Michigan. Amer. Midi. Nat. 40: 395-419. Booth, W. E. 1950. Flora of Montana. Part I. The Research Foundation at Mont. State College. Bozeman. 232 p. ___________ . and J. C. Wright. 1966. Flora of Montana. Part II. Mont. State Univ., Bozeman. 305 p. Brown, L. E. 1962. 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A comparison of habitat and food of two species of MiorotuS' J. Mamm. 46: 605-612. LLLEEGN I TRftARIES 1762 I uuuvv<-~ ■ D378 Hodgson, James Russell H667 Ecological distributic cop.2 of Microtus montanus (Peale) & Microtus penn- sylvanicus (Ord)___ # mmm