WMOD00071

• Applicant: Western Weather Consultants
• Project Name: Vail Pass
• Date: 11/1/1984
• Weather Modification - Doc Type: Application

- Implication of the Effects of Cloud Seeding on Small
Kamm.a.ls

Whether cloud seeding is good or bad is a value judg-
ment that 1nvolves a complex system. Since deer mice,
showed the greatest relationship with snowpack, I
will use this species as an example of the complexity.
Deer mice primarily eat seeds during the winter
(Johnson 1962. Vaughan 1974). Although deer ~ice occur
mainly on the south aspects. they do invade north
aspects when population densities are high; where,
because of their seed eating habits. they could po-
tentially interfere with natural reforestation of
conifers. ~eavy snowpack results in a reduction of
deer mice, so no deer mice are on the north aspects
and foresters might then consider cloud seedin ood.
eer m1ce ee pr marily on insects during the summer,
and with the bulk of the deer mouse population on the
south aspects. there could be a reduct10n or nerD1-
vorous insects and thus more tor age ~ould rema1n tor
elk and cattle. Heavy snowpaCk results ~n a reductIon
of deer mice. so ranchers and hunters could consider
cloud seeding bad. This example has presented two
sides to the issue of cloud seeding in relation to
deer mice. yet even this 1s a ~~mplification~beca~se
there are many other interactions that could be con-
sidered before making judgements. Yhen consid~r1ng
all the species of small mammals. the system beco~s
even more complex. Many interactions were not studied
during the course of this project. Decisions made
solely On the results of tbis project could be errone-
ous. because of the limited scope of this project.

Extrapolation of results in ,my study to other areas
of the montane ecosystem might not be appropriate.
Vaughan (1969) studied montan~ small mammals in
northern Colorado. His data sbow different popula~ion'
trends in relation to snowpack. although he did not
quantify bis snowpack data.

Sum:c.ary

Substantial home range data were obtained on deer mice
during the snow free period of the year. During the
firs~ 3 months after 6no~lt. deer mouse home range
size was highly correlated (r--O.90) with population
size. Hypothesis A (Summertime home range will be
unaffected by snowfall) would be accepted for deer
mice. because of the relationship of home range to
population size. Snowpack only has an Indire~t 'effect
on home range through population size. Data were.
insufficient to make conclusions on Hypothesis A for
the other four species, but I expect the hypothesis
would be accepted if sufficient data were available.

Hypothesis B (Increased snowfall vill increase mortality
rates in the winter of occurrence and the summer follow-
ing) would not be accepted for any species. because
of its two parts. There are data to answer the first
part.. but the phrase "and the summer following" makes
the hypothesis too broad to be testable. because
summer and winter mortality appear co be affected by
different variables.

Density estimates for five species of small mammals
were obtained during each live trapping period. Sample
sizes varied greatly. with deer mice providing the
best data and long-tailed voles providing the least
reliable data. Deer mouse population size had a
strong negative relationship with spowpack. and chip-
munk population size had 8 weak negative relationshiP
with snowpack. The other three species reacted to
variables other than snowpack. thus obscuring any
relationships with snowpack. Annual variability in
snowpack is probably an important factor in allowing
an animal population to recover from any effects of a
heavy sno~ year, whether these effects are positive
or negative.

Short term effects will be temporary. and long tere
effects ~ould probably be associated ~ith any v~getation
changes. Each species has some~hat different habitat
preferences, so it is possible that each species will
respond 1n a different way on a long term basis.
Habitat preferences for ~ach small mammal species are:
Deer mice - SOuth aspects with sparse herbaceous
vegetation; montane voles - south aspects. probably
with dense herbaceous vegetation; red-backed voles -
conifer forest; loog-tailed voles - clear-cut areas
on north aspects; chipmunks - clear-cuts. near the
edge of the conifer forest.

-Significance of the Study

Breeding of all the small herbivorous mammals in the
San Juans with the possible exception of gophers is
inhibited by the presence of snow cover Bnd the be~in-
in of breeding is correlated with time of snowmelt.
Thus it should be possible to predict the e ect 0
increased ~nowtall due to weather modification on the
timing of the initiation of the breeding season pro-
vided the effect of the increased snow on duration
of the snowpack can be predicted. Since air tempera-
.tures have an equally significant role in determining
the date of sno~t, it would be necessary to con-
sider that factor also. The result will necessarily
be an approximation with rather vide limits of uncer-
tainty..

The actual effect on the small mammals themselves will
be more difficult to assess. A shortened season will
decrease the summer recruitment of new individuals
into the population,. Because the total nWllber of
litters is small and the time required to rear each
litter is large in relation to the total length of the
reproductive season and because of the factor of
synchronization of breeding among femal~8. it aeems
possible that the effect of the limitation will be
somewhat greater than a s~le proportion of days
delayed relative to the days required to rear one
litter. If the delay r~sults in w@aning of any
proportion of first litters after the cutoff point
(not precisely known) the reduction in recruitment
viII be more significant because those young will
'probably not reproduce until the following year. The
delay will exert ita most significant influence on
those species whose primary habitata lie on south
aspects.

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Tbe total number of births in the summer reproductive
season is only one component in the dynamics of:
populations. howevet, and the other components. summer
survival of young and winter mortality at least in the
sample of years we were Observing. vere more signifi-
cant in the ultimate determination of population size
than number of y~ung born. Food and feeding habits
as we have indicated above are very significapt fact-
ors in survival of young Bnd mortality as well as
influencing natality directly.

Broad Si~nificant of Results

Since pocket gopher populations do not appear related
to varying snowpack. 1t seems un11kelY that they would
be affected by snowpack augmentation. However. 11
later lying snow consistently resUTted 1n increased
numbers ot winter cas~s. the impact at gophers on the
grassland 1n br1og1ng subsoil to the surtace and spread-
ing it over a larger area would be greater w1th the
~reased 6no~vack. This corresponds'~ell with
Hoeing:: in the alpine tundra ecosySte.m. Th~ r~t(' u!"
sell r.1.:'v€"IT!ent is not thought so p;reat at. the lc\,;er
elo?vctions and gel;tler slopes of the foreslO eccs;-s=.(';:t';,
so the ultimate 1mpact ot t.h1S ettect at poc~et ~ophers
would not be as great 10 the torest.