WSP00036

• File Number: 8200.750
• Description: San Juan River General
• State: CO
• Basin: San Juan/Dolores
• Water Division: 7
• Date: 3/1/1975
• Author: BOR
• Title: San Juan Ecology Project - Interim Progress Report - March 1975
• Water Supply Pro - Doc Type: Report/Study

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4.7.4. Job 3. Home RanRe (Roger Sleeper)

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- Objectives

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Home range data will be used to determine whether size
of home range of deer mice varies with snowfall. The
trapping procedures used to obtain data on home range
will also provide information on population size,
longevity and distributional shifts in individual deer
mice. These data will also sssist in interpretation
of Calhoun population index data.

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- Procedures

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Live trapping was conducted for 6 days each month
during the snow free period of the year. Nest boxes,
Sherman live traps and pitfall traps were used to
capture small mammals. Animals were marked with ear
tags and released.

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- Findings

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The size of home range can change and thus the dis-
tance animals may be moving to a trap line will change
(Brant, 1962). Determining home range would be a use-
ful technique to estimate the actual area from which
animals were captured during a trapping period. Brant
(1962) found that average distance between consecutive
captures of an individual deer mouse was a good esti-
mate of the distance deer mice move to a trap line.

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Average distance, for deer mice in.my study area, can
vary with population size and time of year (Fig. 1,2).
During early and mid-summer, average distance is
inversely related to population density. Late in the
summer, average distance is low, apparently indepen-
dent of population density. The low values for
average distance late in the summer may be due to an
abundance of food or decreased activity at the end of
the breeding season.

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Population density was calculated from the effective
area trapped and the population size. The effective
area trapped was determined by extending the perimeter
of the trapping grid outward a length of average
distance. The population size was determined by the
Schnabel index method for mark - recapture studies.

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Population density estimates over the snow free
periods of the study. are presented in Figure 3.
Densities were relatively high during the first two
years of the study, but there was a marked reduction
in numbers sometime during the fall and winter of
1972-73. Deer mice do not undergo cyclic f1uctations
in numbers as do some microtine rodents (Terman, 1968).
Therefore, such a drastic reduction in the population
may be related to the weather conditions to which the
deer mice were exposed. Controlling factors of
natural small mammal populations are difficult to
determine, but weather conditions are a possibility
(Ehrlich, 1972; Fuller et al., 1969).

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Cumulative precipitation for approximately one month
prior to the initiation of snowpack are represented by
the dashed lines in Figure 3. There were 2.43 decime-
ters of precipitation in October 1972 versus 0.96 dm
in October 1971. Some of this precipitation may have
fallen as snow, but snow does not usually begin
accumulating before late October. The heavier fall
rains in 1972, coupled with cold temperatures during
that period of the year could have adversely affected
the deer mouse population. Also, the snowpack during
the winter of 1972-73 was greater and lasted longer
than in the 1971-72 winter. Hansson (1971) theorized
that granivorous rodents, such as deer mice~ may ap-

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90 Gl ~
80
~ 70
00
"
.
w 60
.
~
. 50
0
"
.
w
00 40 (;l
OM
"
.
"' 30
.
~
.;; 20
1972
10 (;) 1973
May Jun Ju1 Aug Sep
Figure 1. Average distance between consecutive
captures of deer mice.

90
80
~ 70
00
~
w 60
.
~
. 50
0
~
w 40
00
OM
"
. 30
"'
.
"
.
.;; 20
10

e

o

(\)

10

20

30 40

80

50

60 70

Population Size (Number of animals)

Figure 2. Average distance between consecutive
captures of deer mice at different
population sizes.

proach a food shortage in late winter or early spring
before insects or new seeds are abundant. The pro-
longed snowpack, with the subsequent delay in plant
growth, could have produced such a food shortage.
Prolonged anowpack or heavy rains during the early
fall may have contributed to the decline of the deer
mouse population. However, no conclusions can be
made, because there has only been the one heavy snow
year during the course of this study.

The monthly population density estimates (Fig. 3) show
the population growth rates for 1972 and 1973. The
population density increased at a slower rate in 1973,
when the density was very low. This may be indicative
of a sigmoid growth curve, but also at these low
densities, any mortality would substantially decrease

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