North of Long House, Wetherill Mesa
Pinyon-juniper forest with a dominant ground cover ofPoa fendlerianawas described by Erdman (1962) as one of the three distinct types of pinyon-juniper woodland on Wetherill Mesa. Such a woodland occurs adjacent to the Bobcat Canyon drainage, and is continuous across the Mesa from above Long House to the area near Step House. Plants in the ground cover include:
Cryptantha bakeriOpuntia rhodanthaChrysothamnus depressusSolidago petradoriaKoeleria cristataLupinus argenteusYucca baccataPhlox hoodiiEriogonum racemosumEriogonum umbellatumCordylanthus wrightiiPedicularis centrantheraPenstemon linarioidesPenstemon strictus
Two traplines were run from July 9 to 12, 1962, in the area south of the Bobcat Canyon drainage at an elevation of 7,100 feet. No mice were caught in three nights of trapping. Four additional lines were established on July 24, 1962, and were run for three nights, in the area north of the Bobcat Canyon drainage at elevations of 7,100 to 7,150 feet.
P. maniculatusandP. trueiwere caught here (Table 1). This vegetational association may have few rodents because there is a shortage of places where they can hide. AlthoughPoa fendlerianais abundant, the lack of shrubs leaves little protective cover for mammals.
Mug House—Rock Springs
A juniper-pinyon-mountain mahogany association extends from the area of Mug House to Rock Springs, on Wetherill Mesa. On that part of the ridge just above Mug House, the understory is predominantlyCercocarpos montanus(mountain mahogany), but northward toward Rock Springs the understory changes toFendlera rupicola,Amelanchier utahensis,Cercocarpos, andPurshia tridentata. The ground cover is essentially the same as that in the pinyon-juniper-muttongrass association described previously.
Four traplines were run from July 31 to August 2, 1962, and from August 13 to 15, 1963. These lines ran northwest-southeast, starting 1,000 feet southeastof, and ending 3,000 feet northwest of, Mug House. The lines traversed elevations of 7,225 to 7,325 feet. Individuals ofP. maniculatusandP. trueiwere caught here (Table 1).
Deer and rabbits inhabit the trapping area. Bobcats have been seen, by myself and by others, near Rock Springs. Lizards of the generaCnemidophorusandSceloporus, as well as gopher snakes were seen in this area.
Juniper—Pinyon—Bitterbrush
Three pairs of traplines were run from August 7-9, 1962, in a juniper-pinyon-bitterbrush stand on the southern end of Wetherill Mesa, starting 200 yards southwest of Double House (Fig. 1).
The forest on the southern end of the mesas consists of widely-spaced trees, which reflect the low amounts of precipitation at these lower elevations. Juniper trees are more numerous than pinyons, and both species are stunted in comparison to trees farther north on the mesa.Purshia tridentata(bitterbrush) is the understory codominant.Artemisia nova(black sagebrush) is present and grasses are the most abundant plants in the ground cover. Herbaceous species in the sparse ground cover include the following:
Opuntia polyacanthaSolidago petradoriaLathyrus pauciflorusPenstemon linarioidesLupinus caudatusYucca baccataPhlox hoodii
OnlyP. maniculatuswas caught in this stand; all mice were caught in the first night of trapping.
Five areas were selected for trapping in the summers of 1963 or 1964, in order to test hypotheses concerning habitat preferences of each of the species ofPeromyscus. Four of these areas appeared to be ideal habitat for one species, but not for the other. The fifth area was expected to produce both species ofPeromyscus. Each of these areas is discussed below.
One Mile Southeast of Park's Entrance
A small stand ofArtemisia tridentata, occurring one mile southeast of the entrance to the park, is bordered to the north and northeast by a grassy meadow, discussed in the following account. Kangaroo rats have been reported in this general area, and I wanted to determine whetherP. maniculatusandDipodomysoccurred together there. Fifty trap nights in this sagebrush, on June 20, 1963, yielded onlyP. maniculatus(Table 1).
Meadow, One-Quarter Mile Southeast of Park's Entrance
A grassy meadow lies just to the east of the highway into the park, one-quarter of a mile southeast of the park's entrance. On July 30, 1963, one hundred traps were placed in two lines through the meadow, and were run for one night. Only individuals ofP. maniculatuswere caught (Table 1).
M-2 Weather Station, Chapin Mesa
The M-2 weather station of the Wetherill Mesa Archeological Project was on the middle of Chapin Mesa at an elevation of 7,200 feet. This site was in an old C. C. C. area, about one mile north of the park's U. S. Weather Bureau station. The vegetation surrounding the M-2 site was a pinyon-juniper-muttongrass association. It was thought that both species ofPeromyscuswould occur in this habitat.
On May 10, 1964, 25 traps were placed in this area and were run for one night. Only individuals ofP. trueiwere caught (Table 1).
Grassy Meadow, Southern End Moccasin Mesa
This large meadow is located eight miles south of the northern rim of Moccasin Mesa. The meadow lies in a broad, shallow depression that forms the head of a large drainage (Fig. 1). To the south of the meadow the drainage deepens, then reaches bedrock as it approaches the pour-off.
On August 23, 1964, one hundred traps were set in pairs in a line through the middle of the meadow; adjacent pairs were 20 feet from each other. Only individuals ofP. maniculatuswere caught (Table 1).
Grasses are dominant in the ground cover, andSphaeralcea coccinea(globe mallow) is codominant. The abundance of globe mallow is due to the present and past disturbance of this meadow by a colony of pocket gophers. Trees are absent in the meadow. Species of plants include the following:
Opuntia polyacanthaChenopodiumsp.Artemisia ludovicianaChrysothamnus nauseosusKoeleria cristataPoa pratensisLupinus ammophilusCalochortus gunnisoniiErigeron speciosusGutierrezia sarothraeTetradymia canescensTragopogon pratensisBromus tectorumSphaeralcea coccineaEriogonum racemosumPolygonum sawatchenseComandra umbellataPenstemon strictus
Bedrock Outcroppings, Southern End Moccasin Mesa
Two miles south of the preceding site, much of the mesa is a wide expanse of exposed bedrock, which extends approximately 100 feet inward from the edges of the mesa. Pinyon-juniper-mixed shrub woodland adjoins the bedrock.
On August 23, 1964, 25 traps were placed along the bedrock, near the edge of the forest. Only two mice, bothP. truei, were caught. (Table 1).
Home Range
In order to learn how extensively mice of different ages travel within their habitats, whether their home ranges overlap, and how many animals live within an area, it was necessary to determine home ranges for as many mice, of each species, as possible (Hayne, 1949; Mohr and Stumpf, 1966; Sanderson, 1966).
In 1961, the Colorado Department of Fish, Game and Parks established a permanent trapping grid in the area south of Far View Ruins (Fig. 1). The grid was constructed and used by Mr. Harold R. Shepherd, Senior Game Biologist, and his assistant, in the summers of 1961 and 1962, in a study concerning the effect of rodents on browse plants used by deer. The Department of Fish, Game and Parks allowed me to use the grid during 1963 and 1964, and also permitted me to use its Sherman live traps.
The grid is divided into 16 units, each with 28 stations (Fig. 2). Traps at four stations (1a, 1b, 1c, 1d) are operated in each unit at the same time, with two traps being set at each station. The traps are moved each day in a counter-clockwise rotation to the next block of four stations (2a, 2b, 2c, 2d) within each unit. The stations are arranged so that on any given night, traps in adjacent units are separated by at least 200 feet. As a result, animals are less inclined to become addicted to traps, for even within one unit they must move at least 50 feet to be caught on consecutive nights.
Fig. 2: Diagram of trapping grid for small mammals, showing units of subdivision. Trapping stations were numbered in each unit as shown in unit A.
Traps were carefully shaded and a ball of kapok was placed in each trap to provide protection against the killing temperatures that can develop inside. In spite of these precautions, mice occasionally succumbed from heat or cold. The traps were baited with coarsely-ground scratch feed.
Mammals trapped in the grid were inspected for molt, sexual maturity, larvae of botflies, anomalies, and other pertinent data. Each animal was marked by toe- and ear-clipping and then released. Four toes were used on each front foot, and all five toes were used on each hind foot; two toes were clipped on the right front foot to signify number nine. The tip of the left ear was clipped to signify number 100, and the tip of the right ear was clipped to signify 200. If 300 or more animals had been captured, the tip of the tail would have been clipped to represent number 300. A maximum of 799 animals could have been marked with this system, which was used by Shepherd. I continued with it, starting my listings with number one.
Only two mice were caught that had been marked in the previous season by Shepherd.
Live traps were operated in the trapping grid from July 9 through October 25, 1963, and from June 25 through August 21, 1964. Traps were rotated through all stations five different times (35 days) in 1963, and twice (14 days)in 1964. Approximately three man hours were required each day to service and rotate the traps to the next group of stations. By the autumn of 1964, a total of 282 mice had been captured, marked and released; these were handled 817 times. In 1963, 235 mice were caught for an average of 20 captures per day; in 1964, 47 mice were caught for an average of 9 captures per day.
Calculations of Home Range
A diagrammatic map of the trapping grid was drawn to scale with one centimeter equal to 100 linear feet. Trapping stations were numbered on the diagram to correspond with stations in the field. An outline of this drawing also was prepared to the same scale, but station numbers were omitted. Mimeographed copies of such a form could be placed over the diagrammatic map and marks made at each station where an animal was caught. A separate form was kept for each animal that was caught four or more times.
In calculating home range, it was assumed that animals would venture half-way from the peripheral stations, at which they were caught, to the next station outside the range. A circle having a scaled radius of 25 feet (half the distance between stations) was inscribed around each station on the periphery of the home range by means of a drafting compass. The estimated range for each animal was then outlined on the form by connecting peripheries of the circles. Both the inclusive boundary-strip method and the exclusive boundary-strip method (Stickel, 1954:3) were used to estimate the ranges. The area encompassed within the home ranges was measured by planimetering the outline of the drawing. At least two such readings were taken for each home range; then these planimeter values were converted into square feet.
The customary practice in delimiting home ranges on a scaled map of a grid is to inscribe squares around the peripheral stations at which the animal was trapped, and then to connect the exterior corners of these squares (Stickel, 1954:3). If the distance between stations was 50 feet, such squares would have sides 50 feet long. An easier method is to inscribe a circle having a scaled radius of 25 feet around the peripheral stations by means of a drafting compass. To my knowledge this method has not been used previously and consequently has not been tested by experiments with artificial populations.
To test the accuracy of this method, a "grid of traps" was constructed by using 81/2by 11 inch sheets of graph paper with heavy lines each centimeter. The intersects of the heavier lines were considered as trap stations. A "home range" of circular shape, 200 feet (4 cm.) in diameter, with an area of 31,146 square feet (0.71 acre), was cut from a sheet of transparent plastic. Another "home range" was made in an oblong shape with rounded ends. This range measured 2 by 65 centimeters (100 by 325 feet) and had an area of 32,102 square feet (0.74 acre). Each plastic range was tossed at random on sheets of graph paper for fifty trials each. The range was outlined on the graph paper, then circles having a scaled radius of 25 feet were inscribed around each "trap station" within the range. The peripheries of the inscribed circles were then connected and the estimated home range was delimited by the exclusive boundary-strip method. The estimated range was measured by planimetering, and the data were compared with the known home range (Table 2).
It was found that when calculated by the exclusive boundary-strip method, the circular home range was overestimated by 2.22 per cent. The oblong home range was overestimated by only 1.50 per cent. Stickel (1954:4) has shown that the exclusive boundary-strip method is the most accurate of several methods of estimating home ranges, and in her experiments this method gave an overestimate of two per cent of the known range. Thus, my method of encircling the peripheral stations yields results that are, on the average, as accurate as the more involved method of inscribing squares about the trap stations, and saves a great deal of time as well. My method probably yields better accuracy; a perfect circle is easily drawn by means of a compass, whereas a perfect square is more difficult to construct without a template.
It is generally understood that the estimated home range of an animal tends to increase in size with each additional capture; this increase is rapid at first, then slows. Theoretically, the more often an animal is captured, the more reliable is the estimate of its home range. Most animals, however, rarely are captured more than a few times. The investigator must decide how many captures are necessary before the data seem to be valid for estimating home ranges.
An animal must be trapped at a minimum of three stations before its home range can be estimated, and even then the area enclosed in the triangle will be much less than the actual home range. Some investigators have plotted home ranges from only three captures (Redman and Selander, 1958:391), whereas others consider that far more captures are needed to make a valid estimate of range (Stickel, 1954:5).
Table 2—Summary of Data from Experiments in Calculating Home Ranges for an Artificial Population.SeriesNo. of trialsTrap spacing in ft.Shape of rangeActual area ofrange in ft.Calculated area ofrange by exclusiveboundary-strip method± S. D.A5050Circular31,14631,7829,600B5050Oblong32,10232,5839,466
Table 2—Summary of Data from Experiments in Calculating Home Ranges for an Artificial Population.
In my study, 161 individuals ofP. trueiwere caught from one to 13 times each. The estimated home ranges of 10 individuals ofP. truei, each caught from eight to 13 times, were plotted and measured after each capture from the fourth to the last. The percentage of the total estimated range represented by the fourth through tenth captures was, respectively, 52, 65, 73, 85, 88, 93, and 96 per cent.
Ninety-seven individuals ofP. maniculatuswere caught from one to 10 times each. For five individuals that were each caught from seven to 10 times, the percentage of total estimated range represented by the fourth through seventh captures was, respectively, 59, 69, 85, and 93 per cent.
The above percentages do not imply that the true home range of individuals of these species can be reliably estimated after seven or 10 captures; the average percentages do, however, indicate a fairly rapid increase inknown size of home ranges between the fourth and tenth captures. The estimated home ranges ofP. maniculatustended to reach maximum known size at about seven captures, whereas the estimated ranges ofP. trueitended to attain maximum known size after nine or more captures. The controversy over the number of captures of an individual animal required for a reliable estimate of its home range was not settled by my data.
I initially decided to estimate home ranges for animals caught five or more times and at three or more stations. Of the 282 animals caught and marked, only 48 were caught five or more times. Because of the small numbers ofP. maniculatusthat were caught five or more times, I wanted to determine whether mice caught four times had an estimated range that was significantly smaller than that of mice caught five times. Eight individuals ofP. maniculatuswere caught four times each, and it seemed desirable to use the data from these mice if such use was justified. Data from the 48 mice caught five or more times were used for this testing.
By means of a T-test, I compared the estimated ranges of those 48 mice following their fourth capture with ranges estimated after the fifth capture. The results did not demonstrate significant differences between the two sets of estimates; therefore, I decided to use data resulting from four or more captures, and at three or more stations.
Table 3shows estimations of the home ranges of males and females of each species ofPeromyscus. When the inclusive boundary-strip method is used, the area encompassed by the range tends to be larger than the area of the same range when estimated by the exclusive boundary-strip method. Stickel (1954:4) has shown that the inclusive boundary-strip method overestimates the home range by about 17 percent.
Analysis of Home Range by Inclusive Boundary-Strip Method
When all age groups were considered, the ranges of 16 males ofP. trueiaveraged 20,000 to 80,000 square feet (ave. 47,333; S. D. 19,286). The sizes of home ranges were not significantly different (P > 0.05) between adult and subadult (including juveniles and young) males.
All females ofP. truei(22) had ranges encompassing 16,666 to 83,333 square feet (ave. 40,666; S. D. 17,566). Sizes of home ranges between adult and non-adult females did not differ significantly. The mean range of adult males ofP. trueidid not differ from that of adult females (P > 0.05).
Fifteen males ofP. maniculatushad ranges of 16,666 to 66,666 square feet (ave. 34,222; S. D. 16,000); six adult males had ranges of 33,333 to 53,333 square feet (ave. 38,666). Sizes of home ranges of adult and non-adult males of this species did not differ significantly.
Five females ofP. maniculatushad ranges of 33,333 to 76,666 square feet (ave. 51,333; S. D. 15,913); of these, four adults had ranges of 33,333 to 53,333 square feet (ave. 45,000). Sizes of home ranges of adult males of this species did not differ (P > 0.05) from those of adult females.
The ranges of adult males ofP. trueiwere compared with ranges of adult male ofP. maniculatus; likewise the ranges of adult females of each species were compared. In each case no difference was demonstrable in sizes of ranges between the species.
The largest home range of anyP. trueiwas that of animal number 18, a young male with an estimated home range of 133,333 square feet. This animalwas caught only five times, and his home range appeared unusually large in relation to that of other young males of this species; hence some of the widely-spaced sites of capture probably represent excursions from the animal's center of activity, rather than the true periphery of his range. These data were, therefore, not used in further computations. Stickel (1954:13) pointed out the advisability of removing such records from data to be used in calculations of home range.
Number eight had the largest home range of any female ofP. truei; she was captured ten times, and had a range of 83,333 square feet. The vegetation within her range was pinyon-juniper woodland with understories ofAmelanchier,Artemisia novaandPurshia. Most of her home range was in the western half of unit H, but extended into parts of units D, I, G and N.
The largest home range for adult males of either species was number three ofP. truei; he had a range of 80,000 square feet. The largest range for an adult ofP. maniculatuswas 66,666 square feet (Table 3).
Analysis of Home Range by Exclusive Boundary-Strip Method
Stickel (1954:4) has shown that under theoretical conditions the exclusive boundary-strip method is the most accurate of several methods of estimating home range. This method overestimates the known range by only two percent.
Table 3shows a comparison of home range calculations obtained for each species, when calculated by inclusive and exclusive boundary-strip methods.
The data for males and for females of each species were compared in the same manner as in the inclusive boundary-strip method. The ranges of 16 male individuals ofP. trueiencompassed 14,000 to 56,666 square feet (ave. 34,333; S. D. 13,266); of these, the ranges of 10 adult males were from 23,333 to 53,333 square feet (ave. 39,733). Twenty-two females of this species had ranges of 13,333 to 50,000 square feet (ave. 27,199; S. D. 8,820). Eighteen adult females had the same extremes, but the average size of range, 28,000 square feet, was larger. Sizes of home ranges of males and females did not differ significantly.
The ranges of fifteen males ofP. maniculatusencompassed 13,333 to 46,666 square feet (ave. 26,666; S. D. 10,180). Of these, six adults had the same extremes in range, but an average size of 31,440 square feet.
The ranges of five females ofP. maniculatusvaried from 28,000 to 53,333 square feet (ave. 37,199; S. D. 10,140). All but one of these females were adults. The sizes of home ranges of males and females did not differ significantly. No differences were found when ranges of adult males, or adult females, of both species were compared.
Adjusted Length of Home Range
The adjusted length of the range also can be used as an expression of home range. In this method, one-half the distance to the next trapping station is added to each end of the line drawn between stations at either end of the long axis of the range (Stickel, 1954:2).
The average length of home range for 15 males ofP. trueiwas 363 feet (S. D. 105 ft.); for 22 females of this species 326 feet (S. D. 94 ft.); for 14 males ofP. maniculatus286 feet long (S. D. 94 ft.); and for four females of this species 347 feet (S. D. 83 ft.). The mean lengths of range of males andfemales differed significantly inP. maniculatus, but not inP. truei. However, no difference was demonstrable in mean sizes of ranges between males, or between females, of the two species.
Distance Between Captures
The distance between captures has been used by several investigators as an index of the extent of home range. More short than long distances tend to be recorded when traps are visited at random, and when inner traps of the range are more strongly favored (Stickel, 1954:10).
Table 3—Summary of Data for Estimated Home Ranges of Mice from a Wild Population.Type of EstimateSpeciesSexNo.Estimated home range in sq. ft.± S. D.Inclusive boundary-stripP.trueiM1647,33319,286„„F2240,66617,566P.maniculatusM1534,22216,000„„F551,33315,913Exclusive boundary-stripP.trueiM1634,33313,266„„F2227,1998,820P.maniculatusM1526,66610,180„„F537,19910,140Adjusted LengthP.trueiM16363105„„F2232694P.maniculatusM1428694„„F434783
Table 3—Summary of Data for Estimated Home Ranges of Mice from a Wild Population.
It is important to know approximately how far mice travel in one night. The distances traveled between captures on successive nights were calculated for all mice. Even animals caught most frequently usually were caught only once or twice on successive nights. Data from animals caught less than four times, and hence not usable for calculations of home range, could be used in calculating the distance between captures on successive nights. Thus the data were sampled in a more or less random manner for each species.
The mean distance traveled between captures on successive nights was determined for adult and non-adult animals (juvenile, young and subadult) of both sexes. Adult males ofP. maniculatustraveled an average of 151.66 feet (n = 24); young males of this species traveled an average of 134.28 feet (n = 7). Adult females ofP. maniculatustraveled 170.00 feet (n = 4); no data were available for young females.
Adult males ofP. trueitraveled an average of 169.47 feet (n = 38); and young males traveled 159.44 feet (n = 18). Adult females of this species traveled 155.71 feet between captures (n = 35), while young females traveled 140.66 feet (n = 15).
The means were tested for differences in the distances traveled between young and adult males and between young and adult females of each species, as well as between males and between females of opposite species. In all cases, there were no demonstrable differences in the distance traveled between captures.
One of the more striking journeys between captures was that of number 59, a juvenal male ofP. maniculatus, which traveled 1,070 feet between captures on July 16 and 17, 1963. The route between the two capture sites was over the most rugged part of the trapping grid. This datum was excluded from further calculations. The only other animal that approached this distance was a young femaleP. trueithat traveled 750 feet between captures.
Figure 3shows the distribution of distances traveled by mice of each species between successive captures. Since there were no demonstrable differences between age groups or sexes in the distances traveled, these data represent a composite of the ages and sexes of each species. They show 101-125 feet to be the most prevalent of the distances traveled by both species, and 51-75 feet to have a higher percentage of occurrence amongP. maniculatus. These distances indicate that if an animal was trapped on successive nights, it tended to be trapped within the same unit of the grid. It would have been necessary for an animal to travel 200 feet or more in order to be caught in traps in an adjoining unit of the grid.
The distance between captures also was calculated by the more customary method of averaging the distances between sites of capture, regardless of the time intervening between captures.
Only data from mice caught four or more times were used because these individuals probably had home ranges in the study area, whereas those caught fewer than four times may have been migrants.
The mean distance between captures (n = 95) for 15 males and five females ofP. maniculatuswas 161 feet. Sixteen males and 22 females ofP. trueitraveled an average of 143 feet between captures (n = 248). For purposes of comparison, these average distances between captures could be considered as radii of the estimated home ranges. When the range for each species is calculated by considering average distance between captures as the radius of the estimated home range, the average range ofP. trueiis 64,210 square feet, and that ofP. maniculatusis 81,392 square feet. Both of these estimations are larger than those made by the inclusive and exclusive boundary-strip method (Table 3), and smaller than those calculated by using adjusted length of range as the radius.
Since it is known that ranges of some animals tend to be longer than wide (Mohr and Stumpf, 1966), calculations of estimated range based on average distance between captures probably are more accurate than those based on adjusted length of range.
Usually the estimated home ranges were not symmetrical, and did not resemble oblongs or circles in outline. Rather, the ranges tended to follow parts of vegetational zones. Since trapping grids are geometrical in form, there is a tendency among investigators to consider home ranges of animals as conforming to geometrical design. This may or may not be the true situation;telemetric studies on larger animals indicate that home ranges do not conform to geometrical design. At present there is a poverty of knowledge concerning methods for determining the precise home ranges of small mammals. Telemetry appears to offer an unlimited potential for studies of this kind.
Fig. 3: Graphs showing the distribution of distances between stations at which mice were captured on successive nights in Mesa Verde National Park. Graphs for each species represent records of both males and females.
Individuals ofP. trueiandP. maniculatususually do not have mutually exclusive home ranges. When the home ranges for all females or males of one species are drawn on a single map of the trapping grid, almost every one of their ranges overlaps with the range of at least one other mouse. In some instances, the home range of an individual overlaps ranges of several other individuals. In extreme cases an animal's range lies completely within the estimated boundaries of another individual's range. Such an enclosed rangewas always that of a juvenile or of a young animal. However, an adult may have more than half of its range overlapping with that of another adult of the same sex and of the same, or different, species.
In general, the two species tended to be restricted to certain areas of the trapping grid where the respective habitats were more favorable for their needs.Figure 4shows the parts of the trapping grid utilized by each species. Of course there is overlap in the areas utilized by each species; a few individuals ofP. maniculatusmay be found in what appears to beP. trueihabitat, andvice versa. In such cases, an inspection of the vegetation usually reveals an intermediate type of habitat—for example, an open sagebrush area in pinyon-juniper woodland—that is habitable for either or both species.
The ranges ofP. trueitend to be clustered in the western half of the trapping grid, where ranges ofP. maniculatusare clustered in the eastern half of the grid (Fig. 4). The vegetation of the grid and the preferred habitats of each species are discussed in following chapters.
On the basis of the sizes of estimated home ranges, it is possible to compute the approximate number of individuals of each species that occur in each acre of appropriate habitat.
Fig. 4: Diagram of trapping grid south of Far View Ruins, showing the preferred habitats ofP. trueiandP. maniculatus.
On the basis of an average home range of 30,206 ± 25,545 square feet (one standard deviation) for both male and female individuals ofP. truei, there should be approximately 0.781 to 9.345 individuals of this species per acre ofpinyon-juniper woodland. An average home range of 29,400 ± 24,570 square feet for males and females ofP. maniculatusindicates that the density of this species is between 0.807 and 9.018 animals per acre in mixed shrub or shrub and sagebrush types of vegetation.
Figure 4shows that approximately 10 of the 16 units of the trapping grid are suitable habitat forP. truei; the remaining six units are habitat ofP. maniculatus. From the preceding calculations of density one could expect to find between seven and 90 individuals ofP. truei, and between five and 54 individuals ofP. maniculatusas residents within the 22.95 acres of the trapping grid. The higher estimates of density appear to be large enough to compensate for any overlapping of home ranges.
The calculation of density of each species within the trapping grid is dependent upon the precision with which the home ranges of individuals can be estimated. At this time, home ranges of small rodents can not be measured with great precision, therefore any such calculations are, at best, only approximations. This does not imply that estimations of home range are of little value; however, calculations of density, using home ranges as a basis, tend to amplify the variance inherent in the data. This amplification is reflected in the wide range between low and high limits of the densities for each species within the trapping grid.
In order to check on the accuracy of the above calculations, an estimate of density was made for each species on the basis of trapping data. Trapping records kept for each animal were checked for the year 1963. More data on home ranges were obtained in that year due to higher population densities than in 1964. If an animal was caught four or more times in 1963, it was considered to be a resident; animals caught in both 1963 and 1964 were considered to be residents even if caught fewer than four times. Mice caught three times, with at least a month elapsing between the first and third captures, were considered to be probable residents. Other animals caught three or fewer times were considered to be migrants.
In 1963, 15 individuals ofP. trueiwere caught four or more times, or in both years, and considered to be residents; six other mice were classed as probable residents. OfP. maniculatus, 18 individuals were classed as residents, and two as probable residents. Thus the trapping data for 1963 indicate that 21 individuals ofP. trueiand 20 ofP. maniculatuswere residents of the trapping grid. These estimates lie well within the estimated limits of density of each species, as calculated from data on home range while taking into account the relative proportions of available habitat for each species within the trapping grid. Analyses of trapping data indicate that the density of each species probably is overestimated by calculations of density based on home range data.
Males and females of both species ofPeromyscusappeared to be highly individualistic in the amount of area they utilized. Some adult males ofP. trueicovered large areas, whereas others were relatively sedentary. The same was true of young males ofP. truei, although the younger males tended to have smaller ranges than adult males. Most pregnant or lactating females, of both species, tended to use smaller areas for their daily activities than did non-pregnant or non-lactating females. There were notable exceptions to this generality, for some lactating females had exceptionally large home ranges.
Size of home range apparently was not influenced by the location of ananimal's range within the grid. Far more data would be needed to correlate minor differences in vegetational associations with sizes of ranges in different parts of the grid.
It is surprising that adults ofP. trueido not have larger home ranges than adults ofP. maniculatus.P. trueiis the larger, more robust animal, capable of rapid running and occasional saltatorial bounding; individuals of this species can traverse large areas with ease. The semi-arboreal nature ofP. trueimay explain why individuals of this species do not have larger ranges than individuals ofP. maniculatus.P. trueihas a three-dimensional home range, whereasP. maniculatushas a range that is two-dimensional only (excluding the relatively minor amount of burrowing done by each species).
Vegetational Analysis of Habitats
Detailed maps of vegetation within the trapping grid were needed to aid in analyzing distribution of mice within the grid. In preparing such maps, I recorded all plants within a 25 foot radius of each trapping station. The dominant and codominant plants in the overstory (trees or shrubs) were noted at each station. Next the three most abundant plants other than the dominant and codominants were rated for each station, where possible. Finally a listing was made of all remaining species of plants.
On the basis of this analysis, four vegetational maps were prepared. One shows associations of dominant overstory and understory plants. Individual maps are devoted to the first, second and third most abundant plants in the ground cover within the trapping grid (Figs.5-8). Approximately seven man-hours were required to analyze each trapping unit, and 112 man-hours to analyze the entire grid.
The home range grid encompasses approximately one million square feet. At least four different vegetational stands occur within the grid: 1) pinyon-juniper woodland with various associations in the understory; 2)Artemisia tridentata(big sagebrush), orA. nova(black sagebrush); 3)Quercus gambelii(Gambel oak); and 4) mixed shrubs—Fendlera rupicola(fendlerbush),Amelanchier utahensis(Utah serviceberry), andCercocarpos montanus(mountain mahogany).
Flora in the ground cover is regulated, at least in part, by the canopy cover; hence different associations of pinyon-juniper woodland and each of the stands mentioned above have different plants, or a different distribution of the same kinds of plants, in their ground cover.
Units A, B, E, and parts of D and G in the western third of the grid are in pinyon-juniper woodland (Fig. 5). A relatively pure understory ofPoa fendleriana(muttongrass), is typical of such woodland on the middle parts of the mesas. Woodland on the western third of the grid differs somewhat in that, when the area occupied by each plant is considered,Artemisia tridentatais codominant there withPoa fendleriana. As far as individual plants are concerned,Poafar outnumbersArtemisia. The next most abundant plants in the ground cover areSolidago petradoria(rock goldenrod),Chrysothamnus depressus(dwarf rabbitbrush), andPenstemon linarioides(penstemon), in that order.
In unit E there is a large depression, about 200 by 60 feet, created by removal of soil (Fig. 8).Artemisia novagrows there, and pioneering plants adapted to early stages of succession are present.
A zone of woodland, whereArtemisia novareplacesA. tridentataas anunderstory codominant withPoa fendleriana, borders the pinyon-juniper-muttongrass community to the east. The next most abundant plants in the ground cover areSolidago petradoria,Penstemon linarioidesandComandra umbellata(bastard toadflax).Koeleria cristata(Junegrass) is as abundant asComandra, but probably is less important as a source of food for mice.
A small strip of the pinyon-juniper-muttongrass community with an understory ofArtemisia novaandPurshia tridentata(bitterbrush) adjoins the above area to the east (Figs.5-8).Solidago petradoria,Balsamorrhiza sagittata(balsamroot), andComandra umbellataare the three most abundant plants in the ground cover. The terrain slopes eastward from this zone into a large drainage.