Comparative Ecology of Pinyon Mice and Deer Mice in Mesa Verde National Park Part 4

Adult males of _P. truei_ traveled 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 of _P. 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 female _P. truei_ that traveled 750 feet between captures.

Figure 3 shows the distribution of distances traveled by mice of each species between successive captures. Since there were no demonstrable differences between age groups or s.e.xes in the distances traveled, these data represent a composite of the ages and s.e.xes 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 among _P. 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 of _P. maniculatus_ was 161 feet. Sixteen males and 22 females of _P. truei_ traveled 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 of _P. truei_ is 64,210 square feet, and that of _P. maniculatus_ is 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.

[Ill.u.s.tration: 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 of _P. truei_ and _P. maniculatus_ usually 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 range was 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 s.e.x 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 4 shows 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 of _P. maniculatus_ may be found in what appears to be _P. truei_ habitat, and _vice 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 of _P. truei_ tend to be cl.u.s.tered in the western half of the trapping grid, whereas the ranges of _P. maniculatus_ are cl.u.s.tered 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.

[Ill.u.s.tration: FIG. 4: Diagram of trapping grid south of Far View Ruins, showing the preferred habitats of _P. truei_ and _P. 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 of _P.

truei_, there should be approximately 0.781 to 9.345 individuals of this species per acre of pinyon-juniper woodland. An average home range of 29,400 24,570 square feet for males and females of _P. maniculatus_ indicates 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 4 shows that approximately 10 of the 16 units of the trapping grid are suitable habitat for _P. truei_; the remaining six units are habitat of _P. maniculatus_. From the preceding calculations of density one could expect to find between seven and 90 individuals of _P. truei_, and between five and 54 individuals of _P. maniculatus_ as 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 of _P. truei_ were caught four or more times, or in both years, and considered to be residents; six other mice were cla.s.sed as probable residents. Of _P. maniculatus_, 18 individuals were cla.s.sed as residents, and two as probable residents. Thus the trapping data for 1963 indicate that 21 individuals of _P. truei_ and 20 of _P.

maniculatus_ were 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. a.n.a.lyses 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 of _Peromyscus_ appeared to be highly individualistic in the amount of area they utilized. Some adult males of _P. truei_ covered large areas, whereas others were relatively sedentary. The same was true of young males of _P. 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 an animal's range within the grid. Far more data would be needed to correlate minor differences in vegetational a.s.sociations with sizes of ranges in different parts of the grid.

It is surprising that adults of _P. truei_ do not have larger home ranges than adults of _P. maniculatus_. _P. truei_ is 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 of _P. truei_ may explain why individuals of this species do not have larger ranges than individuals of _P.

maniculatus_. _P. truei_ has a three-dimensional home range, whereas _P.

maniculatus_ has a range that is two-dimensional only (excluding the relatively minor amount of burrowing done by each species).

VEGETATIONAL a.n.a.lYSIS OF HABITATS

Detailed maps of vegetation within the trapping grid were needed to aid in a.n.a.lyzing 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 a.n.a.lysis, four vegetational maps were prepared. One shows a.s.sociations 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 a.n.a.lyze each trapping unit, and 112 man-hours to a.n.a.lyze the entire grid.

The home range grid encompa.s.ses approximately one million square feet.

At least four different vegetational stands occur within the grid: 1) pinyon-juniper woodland with various a.s.sociations in the understory; 2) _Artemisia tridentata_ (big sagebrush), or _A. nova_ (black sagebrush); 3) _Quercus gambelii_ (Gambel oak); and 4) mixed shrubs--_Fendlera rupicola_ (fendlerbush), _Amelanchier utahensis_ (Utah serviceberry), and _Cercocarpos monta.n.u.s_ (mountain mahogany).

Flora in the ground cover is regulated, at least in part, by the canopy cover; hence different a.s.sociations 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 of _Poa fendleriana_ (muttongra.s.s), 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 tridentata_ is codominant there with _Poa fendleriana_. As far as individual plants are concerned, _Poa_ far outnumbers _Artemisia_. The next most abundant plants in the ground cover are _Solidago petradoria_ (rock goldenrod), _Chrysothamnus depressus_ (dwarf rabbitbrush), and _Penstemon 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 nova_ grows there, and pioneering plants adapted to early stages of succession are present.

A zone of woodland, where _Artemisia nova_ replaces _A. tridentata_ as an understory codominant with _Poa fendleriana_, borders the pinyon-juniper-muttongra.s.s community to the east. The next most abundant plants in the ground cover are _Solidago petradoria_, _Penstemon linarioides_ and _Comandra umbellata_ (b.a.s.t.a.r.d toadflax). _Koeleria cristata_ (Junegra.s.s) is as abundant as _Comandra_, but probably is less important as a source of food for mice.

A small strip of the pinyon-juniper-muttongra.s.s community with an understory of _Artemisia nova_ and _Purshia tridentata_ (bitterbrush) adjoins the above area to the east (Figs. 5-8). _Solidago petradoria_, _Balsamorrhiza sagittata_ (balsamroot), and _Comandra umbellata_ are the three most abundant plants in the ground cover. The terrain slopes eastward from this zone into a large drainage.

[Ill.u.s.tration: FIG. 5: Diagram showing the major a.s.sociations of understory and overstory vegetation in a trapping grid located south of Far View Ruins, Mesa Verde National Park, Colorado.]

As the forest floor begins to slope into the drainage, the ground becomes rocky and shrubs a.s.sume more importance in the understory. Most of this shrubby zone is on the slope; on the western side this zone abuts pinyon-juniper woodland, and on the eastern side is bordered by _Artemisia tridentata_ in the sandy bottom of the drainage. Shrubs become more abundant and pinyon and juniper trees become less abundant as one approaches the drainage. In the vegetation maps, this brushy zone is delimited on the east by a heavy line pa.s.sing vertically through the middle of the grid (Figs. 5-8). The codominant shrubs in the understory of this zone are _Amelanchier utahensis_, _Artemisia nova_ and _Purshia tridentata_. The three most abundant plants on the ground are _Artemisia ludoviciana_, _Chrysothamnus depressus_ and _Penstemon linarioides_.

The drainage occupies most of unit N and parts of Units I, J and M. Unit N is at the head of the drainage; the ground slopes rapidly southward and the bottom of the drainage in unit J is approximately 50 feet lower than in unit N. The canopy cover of the drainage is _Artemisia tridentata_ (Fig. 5). The same three plants that are most abundant in the ground cover of the slope are also most abundant in the drainage.

[Ill.u.s.tration: FIG. 6: Diagram showing the most abundant species of plants in the ground cover of the trapping grid south of Far View Ruins.]

The eastern slope of the drainage is covered with oak chaparral (_Quercus gambelii_); this zone occupies parts of units J, L, M, and P.

_Artemisia ludoviciana_, _Solidago petradoria_, and _Viguiera multiflora_ (goldeneye), are the most abundant plants of the ground cover.

Mixed shrubs (_Amelanchier_, _Cercocarpos_, and _Fendlera_) form large islands in the oak chaparral, in units K, L and P. The brushy areas of oak and mixed shrub give way at the top of the slope to pinyon-juniper forest with an understory of _Artemisia nova_ and _Purshia tridentata_.

The three most abundant plants in the ground cover of the shrub zones are _Solidago petradoria_, _Balsamorrhiza sagittata_, and _Comandra umbellata_. The eastern part of unit O has _Amelanchier utahensis_ in the understory, in addition to _Artemisia nova_ and _Purshia tridentata_ (Fig. 5). The northeastern corner of unit O is in pinyon-juniper woodland with an understory of _Cercocarpos monta.n.u.s_.

[Ill.u.s.tration: FIG. 7: Diagram showing the second most abundant species of plants in the ground cover of the trapping grid south of Far View Ruins.]

There are two relatively pure stands of sagebrush in the grid: one is in unit N, and the other in unit F and part of unit G. As figures 5 to 8 show, unit N has a relatively pure stand of _Artemisia tridentata_ (big sagebrush), with _Artemisia ludoviciana_, _Agropyron smithii_ (western wheatgra.s.s), and _Koeleria cristata_ (Junegra.s.s), being most abundant in the ground cover. _Artemisia tridentata_ and _Artemisia nova_ form the overstory in unit F and part of G. The three most abundant plants in the ground cover there are _Chrysothamnus depressus_, _Solidago petradoria_, and _Penstemon linarioides_ (Figs. 6-8).

[Ill.u.s.tration: FIG. 8: Diagram showing the third most abundant species of plants in the ground cover of the trapping grid south of Far View Ruins.]