|
<br />~,....,..--~.,.
<br />
<br />n
<br />
<br />~,-
<br />
<br />-'.' .,~
<br />
<br />-
<br />
<br />f ';-,,1,.
<br />
<br />EFfECTS Of VARYr~G SNQWPACKION S~~4L,~/
<br />Roger A, Sleeper~/. Albert A. Spencer. and Harold W. Steinhoff
<br />
<br />ABSTRACT
<br />
<br />Nest boxes, live traps, and kill traps ~ere used to
<br />collect data on ch~pmunks (Eutamias mlnimus and E.
<br />quaar1V1t~a~1s), aeer m~ce (Peromyscus maniculatus),
<br />red-backed voles (Clcthrionomys ~aDDer1). Microtus spp.
<br />(Microtus montanus and~. lonRicaudus) and pocket go-
<br />phers (Thomomys talpoidcs) in the San Juan Mountains
<br />of southwestern Colorado. A ~ate snow free date
<br />corresponuea to a sn~tt 1n attainment of breeding com-
<br />petence to' a period later in the summ~r for all species
<br />except pocket gophers, for which there were no 'breeding
<br />data. Deer mice stayed sexually active uncil a later
<br />'date, following'this delay in onset of breeding. Delay
<br />in breeding activity was also Eound wIthin the same
<br />year on Qorth\~sp~cts as compared to south aspects for
<br />Microtus spp.; 'Onset of-breeding was related to anow-
<br />melt data,and 1nit1ati~n of plant growth.' Microtus
<br />.pp. ,changed,fQ9d'.habits from old growth of herbaceous
<br />planta aud .bark-fof .shrubs in late wInter. to green
<br />plAnts aS~.Q~n,a. they ,were availAble. Both,mark-
<br />recapture.liv~~trappin& and kill trap census ,lines
<br />showed . aarked decline in deer mouse and chipmunk
<br />populacfoR'deaait1es .fter winter_ of heavy .nowfall.
<br />Deer mouse populations on Missionary Ridge (Y) were
<br />most strongly related to varying aDowpack (X). 8S
<br />described bY;,Y_..-':~O~.05 X + 5.08. Population changes
<br />of the other' .species 'in relation to snowfall were noC
<br />detected. Law density. 'combined,with the delay in
<br />breeding'prevented the deer mouse population fr~
<br />regaining a high density level in the fint sUllDer
<br />after. winter. of. deep snow. Chipmunk populations were,
<br />IllOre resUient-and recovered with1.n one summer. Deer
<br />.ice were tbe only regular, users of neat boxes. Aver":
<br />age deer mouse.1itter<size declined from 5.5 t 0.7 at
<br />birtb 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 snowpacka.
<br />
<br />INTRODUCTION
<br />
<br />The objective of the small mammal project was to in-
<br />vestiRate effects ot varv1DR snowfall on aSDects of
<br />tbe population dynamics which relate to size of small
<br />~l populations. These aspects may show responses
<br />which demonstrate not only changes in population_, but
<br />the more basic, reasons for these change.. Small
<br />mammal numbers fluctuate considerfbly. but there are
<br />environmental reasons for these variations, and 800W
<br />may be an important factor. The six Jobs in this
<br />project were aimed at sensitive and investigatable
<br />pointB of small mammal population dynamics.
<br />
<br />Small mammals, slthough seldom Been. are a part of the
<br />montane ecosystem. Small rodents are food for the
<br />.carnivores. but the small rodents are more important
<br />as consumers of primary (plant) production. This con-
<br />sumption of primary production has been estimated at
<br />one percent (Gordzinski et al. 1966). 1.5 to 2.B
<br />percent (Hansson 1971). and 3 to ~7 percent of the
<br />potential food supply (Crodzinski 1971). Generally,
<br />small rodents have little impact on primary production
<br />through the amount consumed. but specific food habits
<br />may conflict with man's interests. Granivorous (seed
<br />eating) rodents may hinder nautral reforestation. and
<br />herbivorous rodents may eat bark from seedlings.
<br />shrubs. and trees 1n sufficient quantities to girdle
<br />and kill'these plants. The obvious detrimental affects
<br />of small animals were emphasized in the past. but more
<br />
<br />recent nises["ch has brought forth beneficial functions
<br />of small mammals. Grant (1974) found that grassland
<br />small mammals had a significant positive effect on the
<br />quanticy of nitrogen in the top soil layer, and this
<br />was the most likely mechanism by ~hich small mammals
<br />may influence primary productlon. Grant (l97q) fur-
<br />ther stated that the t~o major pathyays for reintro-
<br />duction of material into the biolOGical cycl~, physical
<br />introduction of previously 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 sno~fall on small mammals could
<br />influence other components of the ecosystem.
<br />
<br />Increased snowfall may affect 81'll411 mammals more than
<br />migratory birds and large mammals. because small mam-
<br />mals are non-migratory and are 1n the same area circum-
<br />annually. Large mammal., can move to elevations below
<br />the effective cloud .eeding area which begins at about
<br />2700 m. and only the smaller. 1es8 mobile. mammals re-
<br />main in tbe area of ' effective cloud seeding. Mammals
<br />that weigh less ~han about 200 g live beneath the snow
<br />(pruit 1958) io tbe _pace formed at the snow and
<br />ground interface. This. 8ubnLvean environment is
<br />characterized by fairly constant temperatures and
<br />saturated air (Pru1t'19~7). The subnivean space is
<br />formed by 'the melting'ol, the snow at the ground-snow
<br />interface or by snow.be~ aupported on vegetation
<br />(CouJ.1anos and Johnel:..'1963). .
<br />
<br />~.sec+lt>"::s
<br />
<br />O....itfed
<br />
<br />Broad SiKnificance of Results
<br />
<br />If deer moW2 populations are related to varying snow-
<br />pack as indicated by the equation on page 444. in
<br />association with Table 9. tben a 30 percent incrp8se
<br />in anowpack on Missionary Ridge could. result in a
<br />population decline at )1 percent 1n an ave["age year,
<br />In a liKhter snow year and at,a lower elevation the
<br />effect would be less. perhaps a little as 25 percent
<br />in a light snowpack year. The population theoretic~
<br />ally could decline to zero if the snowpack were aug-
<br />~nted by 30 percent 1n an otherwise heavy snow year
<br />at high elevations.
<br />
<br />Deer mice occur naturally at elevations to 4267 m
<br />(l~,OOO feet) and are presumably genetically adapted
<br />there to heavIer snowpacks. lheretoeany ext1rpa~ed
<br />local population would presumably be replaced bv one
<br />better adjusted to heavier snowpacka and the void
<br />would be of short duration. The new population would
<br />rohabl exist at a lower population level, ho~ever.as
<br />indicated by data 1n Table so. t ese ,predictions
<br />are based on the assumption of a linear relation
<br />I bet~een deer mouse populations and snowpsck depth.
<br />During the years of study no snowpacks occurred at 90
<br />to 135 percent of average. Therefore the linear re-
<br />relationship might not be an adequate description. It
<br />could be a curvilinesr, or a threshold response at a
<br />certain level of snowpack.
<br />
<br />The implication for deer mouse activities vhich inter-
<br />fere with man. such as girdling of tree seedlings and
<br />competition for forage with livestock and big game, -
<br />.is that 'these would be lessened as a result of snowpack
<br />augmentation. The forester could predict that the ~-
<br />year atter a heavy Bno~fall would be a ~ood one to
<br />I plant trees.
<br />In Steinhoff I H. W.. and J. D. Ives (Eds). 1976. Ecolog1cal impacls of snowpack augmentation In the San Juan
<br />MOuntainst Colorado. Final Report, San Juan Ecology Project. Colorado State Univ. Publ., Fort Collins.
<br />Present.address: Dept. of Fishery and Wildlife Biology. Colorado State Univ., Fort Collins.
<br />
<br />11
<br />~. 2 '
<br />1,-'
<br />
<br />437
<br />
<br />I A-8d-5
<br />
|