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 classed as probable residents. Of P. maniculatus, 18 individuals were classed 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. 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 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 associations 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 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), or A. nova (black sagebrush); 3) Quercus gambelii (Gambel oak); and 4) mixed shrubs—Fendlera rupicola (fendlerbush), Amelanchier utahensis (Utah serviceberry), and Cercocarpos montanus (mountain mahogany).