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EFFECTS OF VARYING SNQWPACK O~ SMALL ~I ABSTRACT 0' Roger A. SleepEo'~' , Albert A. Spencer, and Harold W. Steinhoff The implication for deer mouse activities yhich inter- fere with nan, such as girdling of tree seedlings and competit~on tor tor age with l~vestock and big game, ~ .is that these would be lessened as a result of snowpack augmentation. The forestet could predict that the - year arter a heavy snowfall would be a ~ood one to plant trees. In Steinhoff, H. W.. dud J. D. Ives (Eds). 1976. Ecological impacts of snovpack augmentation in the San Juan MOuntains, Colorado. Final Report, San Juan Ecology Project. Colorado State Univ. Publ., Fort Col11ns. Present address: Dept. of Fishery and Wildlife Biology, Colorado State Univ., Fort Collins. Nest boxes, live traps, and kill traps were used to collect data on chipmunks (Eutamias minimus and E. quaor~v~ttat~s), Oeer mice (Peromyscus maniculat~s), red-backed voles (Clethriono~ Kapperi), Microtus spp. (Microtus montanus and ~. 10nRicaudus) and pocket go- Ehers (Thomomys talpoides) in the San Juan Mountains of southwestern Colorado. A late snow free date corresponoeo to a sn~It ~n attainment of breeding com- petence to a period later in the summer for all species except pocket gophers, for which there were no .breeding data. Deer mice stayed sexually active until a later date, following this delay in onset of breeding. Delay in breeding activity was also found within the same year on north aspects as compared to south aspects for Hi~rotus spp. Onset of breeding was related to snow- melt date and initiation of plant growth. kicrotus spp. changed food habits from old growth of herbaceous plants and bark of shrubs in late winter to gre~ plants as soon as they were available. Both mar~- recapture live trapping and kill trap census lines showed a IlUlrked decline in deer tDOuse and chipmWlk population densities after winters of heavy snowfall. Deer mouse populations on Missionary Ridge (Y) were tDOst strongly related to varying snowpack (X), as described by Y - -0.05 X + 5.08. Population changes of the other species in relation to snowfall were not detected. Low density, combined with the delay io breeding prevented the deer mouse population from regaining a high density level in the first summer after a winter of deep snow. Chipmunk populations were more resilient and recovered within one summer. Deer mice were the only regular users of nest boxes. Aver- age deer mouse litter size declined from 5.5 ~ 0.7 at birth to 3.3 ~ 1.0 young at weaning time. Data were insufficient to compare litter survival in summers following below versus above average soowpacks. INTRODUCTION The objective of the small mammal project was to io- veStiRate effects ot varvloR snowfall on aSDects of the population dynamics which relate to size of small mammal populations. These aspects may show responses which demonstrate not only changes in populations, but the more basic reasons for these changes. Small mammal numbers fluctuate consider~bly. but there are environmental reasons for these variations, and snow may be an important factor. The six Jobs in this project were aimed at sensitive and investigatable points of small mammal population dynamics. Small mammals, although seldom seen, are a part of the montane ecosystem. Small rodents are food for the carnivores. but the small rodents are more important as consumers of primary (plant) production. This con- sumption of primary production has been estimated at one percent (Gordzinski et al. 1966), 1.5 to 2.8 percent (Hansson 1971), and 3 to 47 percent of the potential food supply (Grodzinski 1971). Generally, small rodents have little impact on primary production through the amount consumed, but specific food habits may conflict with man's interests. Granivorous (seed eating) rodents may hinder nsutral reforestation. and herbivorous rodents may eat bark from seedlings, shrubs, and trees in sufficient quantities to girdle and kill these plants. The obvious detrimental affects of small animals were emphasized in the past, hut more 11 ,'?:..' -11- recent research has brought fortn beneficial functions of small mammals. Grant (1974) found that grassland small mammals had a significant positive effect on the quantity of nitrogen in the top soil layer, and this was the most likely mechanism by which small ma~~als may influence primary production. Grant (1974) fur- ther stated that the two major pathways for reintro- duction of material into the biological cycle, physical introduction of previously unavailable soil organic matter and decomposition of fresh organic matter, are both directly influenced by small rr~mmals. Thus, any effects of varying snowfall on small mammals could influence other components of the ecosystem. ~ncreased snowfall may affect small mammals more than migratory birds and large mammals, because small mam- mals are non-migratory and are in the same area circum- annually. Large mammals can move to elevations below the effective cloud seeding area which begins at about 2700 m, and only the smaller, less mooile. mammals re- main in the area of effective cloud seeding. Mammals that weigh less than about 200 g live beneath the snow (Pruit 1958) in the space formed at the snow and ground interface. This subnivean environment is characterized by fairly constant temperatures and saturated air (Pruit 1957). The subnivean space is formed by the melting-of the snow at the ground-snow ioterfece or by snow being supported on vegetation (Cou11anos and Johnels 1963). -See+lon-s o""itIeJ ~~j Broad Si~nificance of Results If deer mou~ populations are related to va pack as indicated by the equation on page association with Table 9, then a 30 percen in snowpack on Missionary Ridge could res populat10n decline of 51 percent 1n an ave In a lixhter snow year and at a lower elev effect would be less, perhaps a little as in a light snowpack year. The population ally could decline to zero if the snowpack mented by 30 percent in an otherwise heavy at high elevations. rying snow- 444. in t 1nCrE'8Se ult 1n a rage year. atioo the 25 percent thenretic~ were aug- snow year Deer.. mice occur naturally at elevations to 4267 m (14,000 feet) and are presumably genetically adapted there to heavier snowpacks. TheretoEany extirpa~ed local population would presumably be replaced- bv o~ better adjusted to heavier snowpacks and the void would be of short duration. The new population WOUld probably exist at a lower population level, however,as I indicated by data in Table 9. Also, these predictions are based on the assumption of a linear relation between deer mouse populations and snowpack depth. During the years of study no sno~acks occurred at 90 to 135 percent of average. Therefore the linear re- relationship might not be an adequate description. It could be a curvilinear, or a threshold response at a certain level of snowpack. 437 A-8d-5