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t . , - Implication of the Effects of Cloud Seeding on Small Mammals Whether cloud seeding is good or bad is a value judg- ment that involves 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 mice occur mainly on the south aspects, they do invade north aspects ~hen population densities are hi&h; vhere, 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 seedlnK ~ood. Deer mice feed primarily on insects during the summer, and vith the bulk of the deer mouse population on the south aspects, there could be a reduct10n ot nero~- vorOU$ insects and thus more torage would remain tor elk and cattle. Heavy snovpaCK results ~n a reductlon of deer mice, so ranchers and hunters could consider cloud seeding bsd. This example has presented two sides to the issue of cloud seeding in relation to deer mice. yet even this Is'a 6implifi~~tion beca~se there are many other interactions that could be con- sidered before making judgements. When considering all the species of small mammals, the system becomes even more complex. Many interactions vere not studied during the course of this project. Decisions made 90Iely on the results of this project could be errone- ous. because of the limited scope of this project. Extrapolation of results in ,my study to other ~reas I of the montane ecosystem might not be appropriate. Vaughan (1969) studied montane small ttlatllal41s in northern Colorado. 8i$ data show different population trends in relation to $nowpack, although he did not "'''''. ." -~"' ,,,,. J. Su=at'y Substantial home range data were Obtained on deer mice during the snow free period of the year. During the firs~ 3 months after snowmelt. 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 indirect effect on home range through population size. Data were. insufficient to make conclusions on Hypothesis A for the other four species, but 1 expect the hypothesis would be accepted if sufficient data were available. Hypothesis B (Increased snowfall will incre~se 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 to 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 soowpack, and chip- munk population size had a weak negative relationship '\.lith sno'olpack. 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 snow year. '\.Ihether these effects are positive or negative. . Short term effects viII be temporary. and long term effects would probably be associated with any vegetation changes. Each species has somewhat different habitat preferences. so it is possible that each species will respond in a different vay on a long term basis. Habitat preferences for each small mammal species are: Deer mice - south aspects with sparse herbaceous vegetation; montane voles - south aspects, probably ~ith dense herbaceous vegetation; red-backed voles _ conifer forest; long-tailed voles - clear-cut areas on north aspects; chipmunks - clear-cuts, near the edge of the conifer forest. -SigDjfi~bnce of the Study Breeding of all the small herbivorous ma~ls In the San Juans with the possible exception of gophers is inhibited b the resence of snow cover and the be in- in of breeding is.correlated with time of snowmelt. Thus it should be possible to predict the effect 0 increased snovfall due to weather modification on the timing of the initiation of the breeding season pro- vided the effect of the 1nereased snow on duration of the snovpack can be predicted. Since ai~ tempera- "tures hav~ an equally significant role in determining the date of snowmelt, it ~uld be necessary to con- sider that factor also. The result viII necessarily be an apprd~imation vith rather ~~de limits of uncer- tainty. The actual effect .on the s~ll 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 number 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 seems possible that the effect of the limitation will be somewhat greater than a si~ple proportion of days delayed relative to the days required to rear one litter. If the delay rEsults in weaning of any proportion of first litters after the cutoff point (not precisely known) the r~duct10n in recruitment will be more significant because those young will probably nOt reproduce until the folloving year. The delay w1ll exert its most stgnificant influence on those species whose primary habitats lie on south aspects. The total number of births in the summer reproductive season is onlY one component in the dynamics of populations, however. and the other components. summer survival of young and winter mortality at least in the sample of years we vere observing. were more signif1- cant in the ultimate determination of population size than number of y~ung born. Food and feeding habits as ~e have indicated above are very significapt fact- ors in survival of young and mortality as well as influencing natality directly. r i , Broad Significant of Results Since pocket gopher populations do not appear related to varying $oowpack, ~t seems un11kely that they would be affected by snowpack au~ntation. However. it later lying snOW consistently resUTted in increased numbers at winter cas~s. the 1mpact at gophers on the grassland i1'l; bn..nging s\)bso1.1 to the surtace and spread- ,. ing it over a larger area ~ould be greater wlth ~~ the increased sno~~ack. This corresponds ~ell ~~th fincil1g= in the alpine tundra ecosyStt.!l:. Tnt' r~t(' <): sell ~~ve~~~t 1s not thought so great at the l~~~~ elev,,-,tions and gel;tler slopes of the !OrBSJ;'(:t:C!O;'$:'~'~'s. so th('" ultitrlate impact at thls et1ect at pocket gcpnt-rs would not be as great 1n the torest.