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<br />" <br /> <br />,'.~ <br /> <br />,...-......" <br /> <br />Roger <br /> <br />EFfECTS OF VARYING <br />2/ <br />A. Sleeper- I Albert <br /> <br />..11t <br /> <br />A8snAC1;,~, <br /> <br />Nest boxes, live traps, and kill tra s were used to <br />co eet ata on c pmun s utamias m nlmus and E. <br />Quaar~V~trat18J, ,aeer m1ce (Peromvscus manlculatus). <br />red-backed voles (Clethriono~ Kapper!). Microtus epp. <br />(Hlcrotul montanus and tl. lonRlc8udua) and pocket_i~: <br />pherl (ThOIllOIIY8 ulpoldes) in the San Juan Mounutn. <br />of louthw..tern Colorado. 'A late snow tree date <br />correeponaeo &0 a sn11t l~ attainment of breeding com- <br />petence to a period later In the summer for all species <br />except pocket gophers, for wllich there were no 'breeding <br />data. Deer .dee stayed sexually active until A later <br />date. folloving this delay In onset oC breeding. Delay <br />1n breeding activity was also found vithin the same <br />year on north aspects 8S compared to south aspects lor <br />Microtus app. Onset of breeding Was related to snow- <br />melt dste and initiation of plant growth. Microtus <br />.pp. changed food habits from old grovth of herbaceous <br />plant a and bark of shrubs in late winter to green <br />plant. aa BOOD as they were available. Both mark- <br />recapture live trapping and kill trap ceneus lines <br />.howed a marked decline in deer mouse and chipmunk <br />population den.ities sfter winters of heavy snowfall. <br />Peer mouee populations on Missionary Ridge (Y) were <br />most strongly related to varying snowpack (X), aa <br />described by Y - -0.05 X + 5.08. Population changes <br />of the other species in relation to snowfall were not <br />detected. Low dendty, combined with the delay in <br />breeding prevented the deer mouse population lrom <br />regaining a high density level in the first summer <br />after a winter of deep snow. Chipmunk populationawere <br />mare resilient and recovered within one summer. Deer <br />~ice were the only resular ueers of neat boxes. Aver- <br />sge deer mouse lit tar size declined from 5.5 t 0.7 at <br />birth to 3.3 t 1.0 young at weaning time. Data were <br />insufficient to compare litter survival in summers <br />following beloW' versus above average snowpacks. <br /> <br />INTRODUCTION <br /> <br />The ob ective of the small mammal ro ect was to in- <br />vest ate e e ts 0 var n anow a 0 <br />the population dynamdca whlch relate to alze ot a~ll <br />ma...l populations. The.. aapecta may show r.sponse. <br />which demonatrate not only chansea in popuXations, but <br />the more b.sic reasons for these changea. S..ll <br />ma=-al numbers fluctuate conaiderfbly. but there are <br />environ.ental reasons for these variations. and anow <br />may be an important factor. The six Jobs in this <br />project war. aimed at senaitive and investigatable <br />points of amall mammal population dynamica. <br /> <br />S~ll mammals, although seldom aeen, are a part of the <br />montane ecosystem. Small rodents are food for the <br />caTnl~ore., but the a.all Todent& aTe more important <br />as consumera of pri~ry (plant) production. This con- <br />au.ption of primary production has been estimated at <br />on.,percent (Gordzlnski et al. 1966), 1.5 to 2.8 <br />percent (Hana.on 1911). and J to ~1 percent of the <br />potential food supply (Grodz1nski 1971). Cen.rally. <br />...11 rodents have little iMpact on primary production <br />through the amount consumed, but specific food habits <br />.., conflict with man's interests. Grsnivoroua (seed <br />e.ting) rodenta may hinder Rautral reforeatatlon, 604 <br />herbivorous rodents may eat bark from seedlings, <br />shrubs, and tre.. in sufficient quantit!.. to Birdl. <br />and kill the.. plane.. The obvious detrimental dtecu <br />at small animals were emphasized 1n the past, but more <br /> <br />S N(JIIP AC. ON SHAI.L HAHMALSY <br /> <br />A. Spencer, and Harold W. Steinhoff <br /> <br />recent research haa brought forth beneficial funct:tons <br />of smRll mammals. Grant (1974) found that graseland <br />small mammals had a significant positive effeCt on the <br />quantity of nitrogen In the top soil layer, and this <br />was the most likely mechanism by which small mammals <br />may influence primary production. Grant (1974) fur- <br />ther Stated that the two major pathwaya for reintro- <br />duction of material into the biological cych. physicaL <br />introduction ot previoualy unavailable soil organic <br />matter and decomposition of fresh organic matter. are <br />both directly influenced by small mammals. Thus, any <br />effects of varying snowfall on small mammals could <br />influence otheT components of the ecosystem. <br /> <br />~creased snowfall may affect small mammals more thsn <br />migratory birds and large mammals. becauee small mam- <br />mals are non-mi rstor and are In the same area clrcum- <br />annually. Largll!! mamma . can move to e evationsbefow' <br />the effective cloud seeding area which begins at about <br />2700 m. and only the smaller, le88 mobile, mammals re- <br />main in the area of effective cloud seeding. Mammals <br />that weiSh les8 than about 200 g live beneath the snow <br />(Pruit 1958) in the apace formed at the snow and <br />ground interface. Thill subntvean environment is <br />characterized by fsir1y constant temperatures dnd <br />saturated air (Pruit 1957). The subnivesll space is <br />formed by the melting ot the snow st the ground-sno.... <br />intetfac.e or by anoW' being supported on vege.tation <br />(Couliano. and Johnels 196)). <br /> <br />-.$e,,+lon-s <br /> <br />0... itreJ <br /> <br />Broad Slgniflcanc~ of Reaults <br /> <br />If deer mou~ populations sre related to vsry~ng ann~- <br />'pack 8. indicated by the equation on page 444, in <br />aSsociation with Table 9. then a 30 peTcent lncr~a~~~ <br />in snowpack on Missionary Ridge could result in B <br />population decl1ne ot ,)1 percent in an aversge yea_~.. <br />I~ a liRhter snOw year and at a lower elevation th~ <br />effect would be 1e.a. perhapa a little as 25 2!~!~t <br />in a light snowpack year. The population thenrettcr <br />ally could decline to zero if the snowpack were ilIlR'" <br />mented by 30 percent in sn otherwise heavy snow year <br />at high elevations. <br /> <br />Deer mice occur naturally at elevations to 4267 m <br />(14,000 feet) and are presumably genetically adapted <br />there to heavier snowpacka. TheretOIB any exE'lrpihd <br />local population would preiumablv be replaced by one' <br />better ad usted to heavier Bno acks and the void <br />wou d e 0 B ort urat on. T e new popu ation would <br />robabl exist at a lower opulation level, however,as <br />indicated by ata n Ta e 80. t eae predictions <br />are based on the assumption of a linear relation <br />between deer mouse populstions and snoW'pack depth. <br />During the years of study no 8no~scks occurred at 90 <br />to 135 percent of averaRe. TherefoTe the linear re- <br />relationship might not be an adequate description. It <br />could be a curvilinear, or a threahold response at a <br />certain level of anowpack. <br /> <br />The implication for deer mouse scthicies which inter- <br />fere wLth man, such as girdling of tTee seedlings and- <br />competition tor tora e with 11vestock and bi arne ~ <br />y t at t eae wou e e.s.ne aa a reau t 0 snovpack <br />augmentation. The forester could predict that the ... - <br />year aIter a neaVV snowfall would be a aood nnp. ~o <br />. Dlant Creel. . <br />11 In Steinhoff, H. W.. and J. D. Ives (Eds). 1976. Ecological imp.cl8 of snowpack. augmentation in the San Juan <br />. - lfounta1na, Colorado. Final Report, San Juan Ecology Project. Colorado State Univ. Publ.. Fort Collins. <br />Ili!1 rTeeen~,address: Dept. of Fishery (Hid \.ll1dlife Biology, Coloradu Stall'! Vnlv., Fort Collins. <br />,I 431 <br /> <br />~2 <br /> <br />A-8d-5 <br />