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<br />-.Implication .of the Effects of Cloud Seeding on Small
<br />.....,..ls
<br />
<br />Whether cloud seeding is good or bad is a value judg-
<br />ment that invo~ves a comp~ex syatem. S1nce deer mice
<br />shoved the greatest relationship with snovpack, I
<br />will use this species 8S an exa~ple of the complexity.
<br />Deer mice primarily. eat seede during the winter
<br />(Johnson 1962~ Vaughan 1914). Although deer mice occur
<br />mainly on the south aspeccs, th~y do invade north,
<br />. aspects when population densities are high. where,
<br />becaus~ of their seed eating habits, they eould po-
<br />tentially interfere with natural reforestation of
<br />conifers. ~eavy snovpack results in a reduction of
<br />deer mice. so no deer mice are on the north aspects
<br />and foresters mi he then consid~r cloud se@din ood.
<br />er m ce ee pr mari y on nseets during the summer.
<br />and with the.bulk of the deer mouse population on the
<br />south aspects, 'there coul<1 De a reductlon or neroJ.-
<br />'.vorous 'insects and.thus more torage wou~d rema1n tor
<br />elk and cattLe. ~eavy snowpac~ re8~ts ~n a,reauct~on
<br />of deer mice. so ranchers' and hunter. coqld.consider
<br />cloud seeding bad. 'ThiA e~~).haa.pres~t.d two .
<br />sides to, the i._sue of cl~u~. '~~~c1~.y'. .~. ,r~~~~~o,:,.' ~2
<br />deer atee;'yet:even thi. i. a:~~.plif1c&tion b.c.ua.
<br />there are maay othGr'1n~eract1oQ' that could bo con-
<br />sidered before.maklng judgement.., When considering
<br />all .tb...., ape<:tu.: ~f,;....l~:' :........1.. the ..yat~ becomes
<br />,evtlD.....IIl9~..;:co:~lex'l." ':*117 :;l.nc.r&Cc~ were not .tu~ied
<br />durinif~be~cOUl'.e, 'of .thi. .proj.ct....;~'J)ec:1a10D. aade
<br />"'solely on' the _r.~ultl'.o~-"-,tb~. .p_roject could be err,?ne-
<br />. ous, because ~;~.:.t:~e'l~~ .eope.',of.t~. project.
<br />
<br />Ext~ap018tion, of ~esults in ,MY .tudy to other ~reas
<br />of the montane. ecosyecCDI adght not be appropriAte.
<br />Vaughan (1969) studied IDOQt.~e ...11......18 1n'" .
<br />northern Colorado. 1118 data show dUferent popula:c:ion'
<br />trends in relation to anovpaclt, although he" did not
<br />quantify his anOvpaclt data. . '
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<br />Summary
<br />
<br />Substantial home range data were obtained on d~er mice
<br />during the snow free period of the year. During the
<br />firs: 3 months after 8nowmelt, deer mouse home range
<br />size vas highly correlated (r--O.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. Soowpack ooly has an ~d1rect effect
<br />on home range through population size. Data'were.
<br />insufficient'to,make conclusioQ8 on Hypothesis A for
<br />the'other four-species. but I expect the hypotbesis
<br />would be accepted if sufficient data were available.
<br />
<br />Hypothesis B (Increased snowfall viII incre,se mortality
<br />rates io the vinter of oecurrence and the summer follo~
<br />in g) 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 btoad to be testable, because
<br />summer and winter mottality appear to be affected by
<br />different variables.
<br />
<br />Density estimates fot five speeiee of small mammals
<br />were obtained during each live trapping period. Sample
<br />si%es varied greatly, with deer mice provid1ng the
<br />best data and long-tailed voles providing the least
<br />reliable data. Deer mouse ,population size had a
<br />strong negative relationship with soowpack. and chip-
<br />munk population size bad 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 1s probably an important factor in allowing
<br />an animal population to recover from any effects of a
<br />heavy sno~ year. whether these effects are positive
<br />
<br />or negative.
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<br />Short term effects will be temporary, aod long. term
<br />effects would prObably be a.~oci.ted with any vegetation
<br />changes. Each species has somewhat different habitat
<br />preferences, 80 it is possible that each species ~ill
<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 veget8tionj red-backed voles -
<br />COnifer forest; long-tailed voles - elear-cut areas
<br />on north aspectsj chipmunks - clear-cuts, near the
<br />edge of the Conifer foreat.
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<br />-SigoJflcance of the Study
<br />
<br />Breed!" of all the small herbivorous mammals 1n the
<br />San Juans with the possible exception 0 gophers is
<br />'inhibited by the presenc~ of snow cover and the beKin-
<br />in of breedin is 'correlated with time of snowmelt.
<br />Thus it shou d pass e-to pre_ ct. tee ect 0
<br />increased sno~tall' due to.weather modification on the
<br />'tiMdng of the initiat1on'of the breeding season pro-
<br />'vlded the effect of the lncrea.ed gnow on duration
<br />of the snowpatk can be predicted. S1nce air tempera-
<br />.tur.. have an equally .1~~f~c.nt.rol. 1~ determining
<br />the date ot .nowmQlt, :1t would be Dace.Barr to con-
<br />:, e1dflr that factor alao. ,:'The result lI1ll necessarily
<br />,be an 8pprO~t~on with" rather wide limits of uncer-
<br />tainty.' '-. '.. ..'-:;. . .. .,'"
<br />';".;:';::?',";,.,",~:", '
<br />"' ~~' -.:"'):
<br />The actual effect .on' dM!~'Small 'Il8amaals tbemselVfe8 will
<br />be more dHr1cu.lt to aBBess. A shortened season will
<br />decrease the 8ummer.recruitwent'of'new.individuals
<br />1nto the population'; Becau.e the total number of
<br />Utters is small and'the..t1M requitf.!;d to rear each
<br />.litter 1s large in relation. to the total length of the
<br />reproductive 8eaeon and.becauae of, the factor of
<br />synchronhation of breeding: aDliOng females. it Beams
<br />POSsible that the effect of the i1a1t.ti~n will be
<br />so~what greater tban a 8~le proportion-of days
<br />delayed relative to the days required to rear one
<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 />vill be more significant because those young will
<br />probably not reproduce until the following year. The
<br />delay will exert iTs 'ItK)at significant influence on
<br />those species "hose prima~ habitat. lie on south
<br />aspects.
<br />
<br />The total number of births in the summer reproductive
<br />season 1s only one component in the dynamics of
<br />populations, however. and the other Component8, Bummer
<br />survival of young and winter mortality at least :tn 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 s1gnlficapt fact-
<br />ors in survival of young and mortality as well as
<br />influencing natality directly.
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<br />Since pocket gopher popul.tion. do nnt appear related \
<br />to varying sooYpack, it seems unlikely that they wouLd I
<br />be attected by snowpack augmentation. However, it
<br />later lying Bno~ consistently reeUIiid in increased I
<br />numbers ot w1nter casts. the impact at gophers on the
<br />grassland 1n bringing'subsd11 to the surtace and spread- I
<br />iog it over a lsrger area would be greater with the I"
<br />the increased 6no~~8ck. This correspQnds.vell ytth
<br />finti1ngs in the alpine. tundra ecosystem. The nH(" tl:
<br />soil ~0vernent 1s not thought so great at the lc~e,
<br />elev..tion", and gel:tler slopes of the [orc"" ecc5:~s~,:::::.:...:_
<br />'so thE< ultl.mate impact at thiS et:1ect 01 pocket goprl(.rr.
<br />would not be. as great 1n the rorest;.
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<br />Broad SiRn1fic8nt of Results
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