WMOD00258

• Applicant: Western Weather Consultants
• Project Name: Vail & Beaver Creek
• Date: 11/1/1987
• Weather Modification - Doc Type: Application

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EfFE.CTS OF .,'r_RYBiG SNC,r..'PACK 0:;

SM1.L MA..'!/

AESH.ACT

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~o~eT ~. Slee?er~/. Albert A. 5per.cer, end Harold W. Steir~off

Nest boxes. live traps, and kill traps ~ere used to
collect data on chlpmunks (Eutamias minimus and E.
quaarlvltlatlS), oeer mice (Peromyscus maniculat~s).
red-backed voles (Clethrio~.~ gapperf). Microtus spp.
(Microtus roontanus and ~. lonRi~audus) and pocket go-
Ehers (Thomomys talpoides) in the San Juan Mountains
of southwestern Colorado. A late snow free date
corresponoea to a snllt 10 attainment of breeding COm-
petence to a period later in the swr.mer for all species
except pocket gophers, for ~hich there were no "breeding
data. Deer mice stayed sexually active until a later
date. following this delay in onset of breeding. Delay
1n breeding activity was also found within the same
year on north aspects as compared to south aspects for
Microtus spp. Onset of breeding was related to snow-
melt date and initiation of plant growth. Microtus
spp. changed food habits from old growth of herbaceous
plants and bark of shrubs in late winter to green
plants a8 soon as they were available. Both mark-
recapture live trapping and.kill trap census lines
showed a marked decline in deer mouse and chipmunk
population densities after winters of heavy snowfall.
Deer mouse populations on Missionary Ridge (Y) were
most strongly related to varylog snowpack (X). as
described by Y - -0.05 X + 5.08. Population changes
of the other species in relation to snowfall were not
detected. Low density. combined with the delay in
breeding prevented the deer mouse population from
regaining a high density level in the first summer
after a winter of deep snow. Chipmunk populations were
more resilient and recovered within one summer. Deer
mice were the only regular USers of nest boxes. Aver~
age deer mouse litter size declined from 5.5 ~ 0.7 at
birth to 3.3 ! 1.0 young at weaning time. Data were
insufficient to compare litter survival In summers
follo~ing belo~ versus above average snawpacks.

INTRODUCfION

The objective of the small ma~l project was to in-
yestiRate effects ot vary~nR sno~f811 on aSDects of
the population dynamics which relate to size of small
mammal populations. These aspects may show responses
which dpmonstr;Jte not only ch;Jnges in po'pulRt1.onR. hilt:
the more basic reasons for these changes. Small
mammal numbers fluctuate consider~bly, but there are
environmental reasons for these variations. and snow
may be sn important factor. The six Jobs in this
project were aimed at sensitive and investigatable
points of small mammal population dynamics.

Small mammals, although seldom seen. are a part of the
montane ecosystem. Small rodents are food for the
.carnivores. but the small rodents are more important
as conSumers of primary (plant) production. This con-
sumption of primary production has been estimated at
one percent (Gordzlnski et al. 1966). 1.5 to 2.8
percent (Hansson 1971), and 3 to 47 percent of the
potential food supply (Grodzinski 1971). Generally,
small rodents have little impact on primary production
through the amount consumed. but specific food habits
may conflict with ~n's interests. Granivorous (seed
eating) rodents may hinder nautral reforestation, and
herbivorous rodents may eat bark from seedlings,
shrubs, and trees 1n sufficient quantities to girdle
and kill these plants. The obvious detrimental affects
of small animals were emphasized in the past, but more

recent research has brought iorth be~eficial fcnct1o~s
of small ma~ls. Grant (197~) found that grassland
small mammals had a significant positive effect on the
quantity of nitrogen in the top SOlI layer, and this
was the most likely mechanism by ~hich small ma~als
may influence primary production. Grant (1974) fur-
ther stated that the t~o major path~o:s for reintro-
duction of material into the biolobical cycle, physical
introduction of previously unavailable soil organic
matter and decomposition of fresh organic matter, are
both directly influenced by small ruammals. Thus, any
effects of varying snowfall on small mammals could
influence other components of the ecosystem.

Jncreased sno~fall may affect small mammals more than
migratory birds and large mammals. because small mam-
mals are non-migratory and are in the same area C1rcum-
annually. Large mammals can move to elevations below
the effective cloud seeding area which begins at about
2700 ~. and only the smaller, less mobile, mammals re-
main 1n tbe area of effective cloud seeding. Mammals
that weigh less than about 200 g live beneath the snow
(Prult 1958) in the space formed at the snow snd
ground interface. This subnivean environment 1s
characterized by fairly constant temperatures and
sacur~Ced air (Pruit 195]). The subnivean space is
formed by the melting-of the snow at the ground-snow
interface or by snow being supported on vegetation
(Coul1anos and Johnel.s .19&3). .

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Broad Si~nificance of Results

If deer mou~ populations are related to varying snow-
pack as indicated by the equation on page 444, in
association with Table Y. then a 30 percent increase
in snolo"PBck on Missionary Ridge could result in a
populatlon decline of ~l percent in an average year.
In a li~hter snow year and at a lower elevation the
effect would be less, perhaps a little as 25 percent
in a light snowpack year. The populatiOn thenretic~
ally could decline to zero if the snovpack were aug-
mented by 30 percent in an otherwise heavy snow year

::..t !~::'Sh el€'..:~:.~i.cr:~.

Deer mice occur naturally at elevations to 4267 m
(14,000 feet) and are presumably genetically adapted
there to heavier snowpacks. IheretoEany extirpa~d
local population would presumablY be replaced bv one
better adjusted to heavier snowpacks and the void
would be of short duration. The new population would
probably exist at a lo~er population level. however,as
indicated by data in Table 9. Also, these predictions
are based on the assumption of a linear relation
between deer mouse populations and snowpack depth.
I During the years of study no sno'Wpacks occurred at 90
to 135 percent of avera~e. Therefore the linear re-
relationship might not be an adequate description. It
could be a curvilinear. or a threshold response at a
certain level of snolo~ack.

The illlplication for deer mouse activities which inter-
fere ~ith nan, such as girdling of tree seedlings and
competition for torage with livestock and big ga~~, -
ois that these would be lessened as a result of sno~~ack
augmentation. The forester COl\ld predict that the ____
year arter a neavv snowfall ~ould be a ~ood one to
. plant trees.
J) .!..!! Steinhoff, H. I.'.. and J. D. Ives (Eds). 1976. Ecological impacts of snolo-pack augmentation in the Sa" Juan
~lounta1ns , Colorado 0 Fin.)l Report, San Juan Ecology Proj ect. Colorado State Univ. Publ.. Fert CoIl ins 0
PreSent addre~s: Dept. of Fishery and ~ildlife Biology, Colorado State Univ., Fort Collins.

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