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 of _P. truei_ were compared with ranges of
adult male of _P. 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 any _P. truei_ was that of animal number 18, a
young male with an estimated home range of 133,333 square feet. This
animal was 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 of _P. 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 of _Amelanchier_, _Artemisia nova_ and _Purshia_. 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 of _P. truei_; he had a range of 80,000 square feet. The largest
range for an adult of _P. maniculatus_ was 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 3 shows 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 of _P. truei_ encompassed 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 of _P. maniculatus_ encompassed 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 of _P. maniculatus_ varied 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 of _P. truei_ was 363 feet
(S. D. 105 ft.); for 22 females of this species 326 feet (S. D. 94 ft.);
for 14 males of _P. maniculatus_ 286 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 and females differed significantly in _P.
maniculatus_, but not in _P. 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.
================+==================+=====+======+============+=========
| | | | Estimated |
Type of | Species | Sex | No. | home range | +- S. D.
Estimate | | | | in sq. ft. |
----------------+------------------+-----+------+------------+---------
Inclusive | _P. truei_ | M | 16 | 47,333 | 19,286
boundary-strip | " " | F | 22 | 40,666 | 17,566
| | | | |
| _P. maniculatus_ | M | 15 | 34,222 | 16,000
| " " | F | 5 | 51,333 | 15,913
----------------+------------------+-----+------+------------+---------
Exclusive | _P. truei_ | M | 16 | 34,333 | 13,266
boundary-strip | " " | F | 22 | 27,199 | 8,820
| | | | |
| _P. maniculatus_ | M | 15 | 26,666 | 10,180
| " " | F | 5 | 37,199 | 10,140
----------------+------------------+-----+------+------------+---------
Adjusted Length | _P. truei_ | M | 16 | 363 | 105
| " " | F | 22 | 326 | 94
| | | | |
| _P. maniculatus_ | M | 14 | 286 | 94
| " " | F | 4 | 347 | 83
----------------+------------------+-----+------+------------+---------
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 of _P. maniculatus_ traveled an
average of 151.66 feet (n = 24); young males of this species traveled an
average of 134.28 feet (n = 7). Adult females of _P. maniculatus_
traveled 170.00 feet (n = 4); no data were available for young females.
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 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
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.
[Illustration: 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 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 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 clustered in the western half of the
trapping grid, whereas the ranges of _P. maniculatus_ are 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.
[Illustration: 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
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).
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 of
_Poa 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 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-muttongrass community to the east. The next most abundant
plants in the ground cover are _Solidago petradoria_, _Penstemon
linarioides_ and _Comandra umbellata_ (bastard toadflax). _Koeleria
cristata_ (Junegrass) is as abundant as _Comandra_, but probably is less
important as a source of food for mice.
A small strip of the pinyon-juniper-muttongrass 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.
[Illustration: FIG. 5: Diagram showing the major associations 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 assume 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 passing 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.
[Illustration: 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 montanus_.
[Illustration: 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
wheatgrass), and _Koeleria cristata_ (Junegrass), 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).
[Illustration: FIG. 8: Diagram showing the third most abundant
species of plants in the ground cover of the trapping grid south
of Far View Ruins.]
MICROCLIMATES OF DIFFERENT HABITATS