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<br />- Implication of the Effects of Cloud Seeding on Small <br />Mammals <br /> <br />Whether cloud seeding is good or bad is a value judg- <br />ment that invoLves a compLex system. ~lnce deer mice <br />showed the greatest relationship with snowpack, 1 <br />will use this species as an example of the complexity. <br />Deer mice primarily eat seeds during the winter <br />(Johnson 1962. Vaughan 1974). Although deer mice occur <br />mainly on the south aspects. they do invade north <br />aspects when population densities are highi where, <br />because of their seed eating habits. they could po- <br />tentially interfere with natural reforestation of <br />conifers. ijeavy snowpack results 1n a reduction of <br />deer mice, so no deer mice are on the north aspects <br />and foresters might then consider cloud seed in ood. <br />er ce ee pr mari y on insects during the summer, <br />and with the bulk of the deer mouse population on the <br />south aspects. there could be a reduction or nerOl- <br />vorous insects and thus more torage would remain for <br />elk and cattLe. Heavy snowpack reSultS in a reduction <br />of deer mice. so ranchers and hunters could consider <br />cloud seeding bad. This example has presented tva <br />sides to the issue of cloud_ see~ing 1~relat~on t? <br />deer mice. yet even this is a ~~11fication because <br />there are many other interactions that could be con- <br />sidered before making judgements. When considering <br />all tbe specIes of small lD8.JIIZlI8la. the system becomes <br />even-<tiore co~lex. Many interactions were not studied <br />durina the cour.. of tbis project. Decisions made <br />solely on the results of this project could be errone- <br />ous. because of tbe limited scope of this project. <br /> <br />Extrapolation of results in ~ study to oth~r areas l <br />of the montane ecosystem might not be appropriate. <br />Vaughan (1969) studied !DOntan,e small 1D81111!o81s in <br />northern Colorado. His data show different population' <br />trends in relation to snowpack. although he did not <br />quantify his Bnavpack data. __-'-_ <br /> <br />Summary <br /> <br />Substantial home range data were obtained on deer mice <br />during the snow free period of the year. During the <br />firs~ 3 months after snowmelt, deer mouse home range <br />size was highly correlated (r--G.90) with population <br />size. Hypothesis A (Summertime home range will be <br />unaffected by snowfall) would be accepted for deer <br />mice, because of the relationship of home range to <br />population size. Snowpsck only has an indirect 'effect <br />on home range through population size. Data were. <br />insufficient to make conclustons on Hypothesis A for <br />the other four species. but 1 expect the hypothesis <br />would be accepted if sufficient data were available. <br /> <br />Hypothesis B (Increased snowfall will in~~ease mortality <br />rates in the winter of occurrence and the summer follow- <br />ing) would not be accepted for any species, because - <br />of its two parts. There are data to answer the first <br />part, but the phrase "and the summer following" makes <br />the hypothesis too broad to be testable. because <br />summer and winter mortality appear to be affected by <br />different variables. <br /> <br />Density estimates for five species of small mammals <br />were obtained during each live trapping period. Sample <br />sizes varied greatly I with deer mice providing the <br />best data and long-tail~d voles providing the least <br />reliable data. Deer mouse population size had a <br />strong negative relationship with s~owpackl and chip- <br />munk population size had a weak negative relationship <br />with snowpack. The other three species reacted to <br />variables other than snowpack, thus obscuring any <br />relationships with snowpack. Annual variability in <br />snowpack is probably an important factor in allowing <br />an animal population to recover from any effects of a <br />heavy snow year, whether these effects are positive <br />or negative. <br /> <br />Short teTln effects will be te1llporary. and long term <br />effects would probably be associated with any vegetatien <br />changes. Each species hss somewhat different habitat <br />preferences. so it is possible that each species will <br />respond in a different way on a long term basis. <br />Habitat preferences for each small mammal species are: <br />Deer mice - south aspects with sparse herbaceous <br />vegetation; montane voles - south aspects, probably <br />with dense herbaceous vegetationi red-backed voles - <br />conifer forest; long-tailed voles - clear-cut areas <br />on north 8spectsi chipmunks - clear-cuts, near the <br />edge of the conifer forest. <br /> <br />-Significance of the Study <br /> <br />Breedin~ of all the small herbivorous mammals in the <br />San Juans with the possible exception of gophers is <br />inhibited b the resence of snow cover and the be in- <br />in of breedin i6 -correlated with time of snowmelt. <br />Thus it shou d be possi Ie to pre ict the e ect 0 <br />increased snowfall due to weather modification on the <br />timing of the initiation of the breeding season pro- <br />vided the effect of the increased snow on duration <br />of tbe snowpack can be predicted. Since air tempera- <br />.tures have an equally significant role in determining <br />the date of sno~tJ it would be necessary to con- <br />sider that factor also. The result will necessarily <br />be an approximation with rather wide limits of UDcer- <br />tainty. <br /> <br />The actual effect on the-small mammals themselves will <br />be more difficult to assess. A shortened seaSOn will <br />decrease the summer recruitment of new individuals <br />into the population. Because the total number of <br />litters is small and the time required to rear. each <br />litter is large in relation to the total length of the <br />reproductive season and because of the factor of <br />synchronization of breed1ns among females. it 8eems <br />possible that the effect of the limdt.tl~n will be <br />somewhat greater than a 8i~le proportion of days <br />delayed relstive to the days required to rear Dne <br />litter. If the delay results in weaning of any <br />proportion of first litters after the cutoff point <br />(not precisely known) the reduction in recruitment <br />will be more significant because those young will <br />probably not reproduce until the fQllowing year. The <br />delay will exert its most significant influence on <br />tbase species whose primary habitats lie on south <br />aspects. <br /> <br />t <br />I <br /> <br />, <br /> <br />The total number of births in the Summer reproductive <br />season is pnly one component In the dynamics of <br />populations, however. and the other components. summer <br />survival of young and winter mortality at least in the <br />sample of years we were observing. were more signifi- <br />cant in the ultimate determination of population size <br />than number of young born. Food and feeding habits <br />as we have indicated above are very significapt,fact- <br />ors in survival of young and mortality as well 8S <br />influencing natality directly. <br /> <br />Broad Si~nificant of Results <br /> <br />Since pocket gopher populations do not appear related <br />to varying snowpack, it seems unlikely that they would <br />be affected by snowpack augmentation. However. it <br />later lying snow consistently resUTted in increased I <br />numbers ot winter cast,s, tne impact ot goptlers on tne i <br />grassland in bringing subsoll to tne 5urtace and spread- I~ <br />ing it over a larger area would be greater with t~e <br />the increased sno~~ack. This corresponds ~ell wich <br />findings in the alpine tundra ecosystem. The T;:tf' 0: <br />se11 r.h")vement is not thought so great at the lU\,o;c:r . <br />elev;:;dons and gel.tler slopes of the fores!; eccsys:'c;:Js, <br />'so Lhe ulLimate impact at th1S eifect 01 pocket garners <br />would not be as ~reat 1n tne forest. <br />