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

4.7.1 Introduction

4.7 SMALL MAMMALS (Drs. Albert Spencer & Harold Steinhoff, Roger Sle per)

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Small mammals of the forest are seldom seen by the
casual visitor so their importance in forest ecosystems
is easily overlooked. Their biomass often exceeds
that of the larger more obvious mammals. Add to this
the faster energy turnover rate characteristic of
smaller animals and the consumption of plants by small
mammals may greatly exceed consumption by larger ones. ~
The result is competition with larger game and domes-
tic mammals of direct importance to man. Also, small
mammals girdle tree seedlings a~d eat tree seeds, thus
interfering with reforestation. This is particularly
a problem in the San Juans where spruce must be clear-
cut to reduce wind throw and resultant natural regener-
ation is poor. Forest officers routinely check small
mammal populations before reseeding cut-over areas,
and spend extra effort on control if populations above
about 6 per hundred trap nights.

Thus effects of varying snowfall on small mammal popu-
lations is of interest to resource managers of the
San Juans regardless of the question of cloud seeding.
If increased snow hurts small mammal populations,
Project Skywater is of economic aid to the timber
industry. Of course, the inverse 'lso is true.

The objective of the small mammal project is to
investigate effects of varying snowfall on aspects
of the population dynamics which, ~e1ate to size of
small mammal 'populations. These aspects may show
responses which demonstrate not only changes in small
mammal populations, but the more basic reasons for
those changes. Small mammal populations fluctuate
~onsiderab1y, but there are ~nvironmenta1 reasons for
these variations. If snow is related (or is not
related) to the fluctuations, we expect to find this
out. The six Jobs in this project are aimed at
sensitive and investigable points of small mammal
popu~ation dynamics.

4.7.2. Job 1. Litter Size and Survival
(Roger Sleeper)

- Objective

To relate snowfall to the timing and survival of
litters of small mammals.

- Procedures

Weekly examination of the 196 nest boxes provided in-
formation on litter survival. Young were marked with
ear tags when about 2 weeks old. Nursing young were
aged to the nearest week. Live trapping with Sherman
live traps provided information on winter survival.

- Findings

The size of the deer mouse (Peromyscus maniculatus)
population was very small in the summer of 1973, and
thus there were very few observations of deer mice in
the nest boxes on the study area. There was only one
litter of young found in a nest box during 1973, so it
was not possible to compare litter survival for the 3
years. Discussion of litter survival for' 1971 and
1972 appears in the previous annual report (Teller,
Ives, and Steinhoff, 1973).

Over-winter survival was very low during the winter of
1972-73. Only one deer mouae was captured during the
first live-trapping period in June, 1973, and this
animal was not previously marked. None of the 108

de~r mice marked in the summ r of 1972 were recaptured
in_ 1973. The mortality duri g the winter of 1972-73
was so high. there was a ver small probability of any
deer mouse surviving. In 19 2, seven deer mice recap-
tured from those marked in 1 71 were all born prior to
mip-Ju1y, 1971. This indica ed that there was either
preferential survival for 01 er animals or that
younger animals had a greate tendency to disperse
from the area. Due to the h gh mortality in the win-
ter of 19.72-73, no more info 'mation was obtained in
1973 to determine which of t ese two mechanisms might
be- operating.

There were some data availab
of litter production for the
consist of estimates of the
and dates when females were
ing. The information on lac
not as good as the data on p
some subjectivity is involve
and late in lactation period
ulied to compare the breedins
on birth dates of litters wa

The data on birth dates of
indicates breeding commenced
1971 and 1972, but breeding
The majority of litters were
yeara. No information was

Table 1. Number of litters

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e to compare the timing
3 years. These data
irth dates of litters,
ound pregnant or lac tat-
ation and pregnancies is
rturition dates, because
during early pregnancies
Thia information was
seasons, because data
not obtained in 1973.

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tters (Table 1)
about the same time in
nded earlier in 1972.
produced in July both
ai1ab1e for 1973.

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om each week.

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June Ju1 Auaust ~
Week
Year .! 2 3 4 1 ; 3 4 1 2 3 4 1
1971 "2 I "2 "2 I "3 "2 "3 "2
1972 2 2 2 2 2 1 2
1973 1

Observations of pregnant fe .ales are presented in
Table 2. The number of pre nant females is related
to population size, so the elative distribution of
pregnancies over the summer was used_ to compare
trends in timing of breedin Observations were not
begun until July in 1971. regnant deer mice were
found early in June, but no late in the summer
during 1972. In 1973, bree ing apparently began
later and ended later, beca se the pregnancies are
distributed more toward the end of the summer.
Breeding in 1971 also conti ued longer than in 1972,
but not as long as in 1973.

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Table 2. Percent of female observed pregnant each
week of each year

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June Ju1 August ~
Week
Year .! 2 1 i 1 1- 4 .! 2 3 4 1
1971 "8 i'l i'l25 "8 i'l
1972 2 28 1 7 19 19 6
1973 10 10 10 30 40

The deer mouse breeding sea on began earlier in 1972
than in 1973. The delay in 1973 was probably due to
the prolonged snowpack. Hi ckley (1966) found that
extracts from ge~inating s eds enhanced testes
development in montane vole (Microtus montanus).
The snowpack may have restr cted deer mouse breeding
because of the delay in ini iation of plant growth.
The breeding season was shi ted more towards the end
of the summer in 1973 than n 1971 or 1972. The 1972

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