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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001588

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breeding season ended the earliest of the 3 years.
and this was when the highest population densities
occurred. Krebs et a1. (1973) found that at high
population densities, meadow voles (Microtus pennsy!-
vanicus) stopped breeding sooner than at low popula-
tion densities. This feed-back mechanism apparently
also occurs in the deer mouse population. When popu-
lation densities were low and onset of breeding was
delayed by snowpackt the greater proportion of breed-
ing occurred later in the summer.

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- Summary

The population density of deer mice was low after the
severe winter of 1972-73. Also, breeding was delayed
by the prolonged suowpack. The population has adapted
somewhat to such environmental streSB. Thus, in re-
sponse to this stress, the breeding season lasted
longer in 1973. The majority of breeding occurred
later in the summer than in previous years when popu-
lation densities were higher. However, the extension
of the breeding season did not enable the population
to recover completely.

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4.7.3. Job 2 Population Density (Harold Steinhoff)

- Objectives

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The objectives of the population density portion of
the small mannnal study are to relate varying snowfall
to varying population density of small mannnals and to
associated population parameters ~uch as proportion
of young, fertility rates, and litter size. Ques-
tions we are asking include: (1) How does population
density of small mammals relate to varying snowpack?,
(2) How does litter size of small mammals relate to
-varying snowpack?, and (3) How does age structure of
small mammal populations relate to varying snowpack?
My hypothesis is that Calhoun population indices of
deer mice, red-backed voles, and chipmunks will de-
crease after a heavy snow year while those of montane
voles will not.

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- Procedures

Paired Calhoun census lines were run in late summer
of 1970, 1971, 1972, and 1973 in each major vegeta-
tive type available at each of the three elevational
sites in each of the three study areas, Missionary
Ridge, Wolf Creek, and Rico.

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- Findings

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Missionary Ridge -

Most small mammal populations on Missionary Ridge de-
creased markedly in 1973 after three previous years
of quite consistent increase (Table 3).

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Table 3. Population indices of small mammals in re-
lation to snow.
1970 1971 1972 1973
Missionary Ridge
Snow Course 1/, % of mean 96% 75% 78% 147%
Snow Disappearance Date 5-20 5-16 6-24
Small Mammal Pop. (C/100TN)
Deermouse 2/ 0.6 1.6 1.9 0.4
Colorado Chipmunk 0.9 1.7 2.1 0.8
Red-backed Vole 0.6 0.8 2.2 0.5
Microtus spp. 1.0 1.5 3.3 3.3
Rico
Snow Cpurse, % of mean 94% 81% 62% 140%
Small Mammal Pop. (C/100TN)
Deermouse 0.4 0.3 0.6 0.8
Colorado Chipmunk 1.3 1.3 1.5 0.7
Red-backed Vole 0.0 0.8 1.8 0.2
Microtus spp. 0.0 0.2 1.1 0.1
Wolf Creek
Snow Course, % of mean 96% 76% 78% 128%
Small- Mammal Pop. (C/100TN)
Deermouse 0..6 1.1 0.2
Colorado Chipmunk 3.1 3.4 2.3
Red-backed Vole 0.8 1.2 1.3
Microtus spp. 0.2 3.2 1.5

1/ Mean of SCS Snow Courses maximum depths, as
follows:
Missionary Ridge - Cascade, Pur~atory, Spud Mountain
Rico - Rico, Lizard Head
Wolf Creek - Upper San Juan. Wolf Creek Summit

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Deermouse (peromvscus maniculatus), Colorado
Chipmunk (Eutamias quadrivittatus), Red-backed
Vole (Clethrionomys Kapper1), M~crotus spp
(~. montanus and~. lonKicaudus.

Microtus spp. populations were the exception. They
remained at the same high level as 1972. The popula-
tion changes of deermice (Peromyscus maniculatus),
Colorado chipmunks (Eutamias Quadrivittatus, and red-
backed voles (Clethrionomvs Kapperi) wer~ strongly
inversely correlated with snow depth in these years
as can be seen in Table 3. The relation of the
deermouse population to snow-free date was tested by
analysis of covariance and found significant
(F=2206.0, F.05-l.99). The sample size of 32 will
permit a valid estimate within 20% of the true mean
with 95% probability. The smallest difference in
deermouse populations means of Missionary Ridge is
about 42%, so there is sufficient sample size to
detect easily and significant differences in popula-
tion between years. Thus the possibility is strength-
ened that there is in fact a relationship between
snow depth and small mammal populations. at least for
deermice. Each additional year of data is increas-
ingly valuable now because only one year of complete-
ly reversed results would engender almost over-
whelming doubt that the relationship actually exists.
A continued strong correlation would greatly increase
confidence in the tentative conclusions that the
relationship does exist.

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Other population parameters associated with popula-
tion change. and which may help to explain the
mechanism are shown in Table 4. The population
decline in deermice cannot be explained by reduced
natality or by differential mortality of young
because number of young per litter, proportion of
young, and fertility rate of deermice has remained

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