Four species — ^vagrant shrews, water shrews, northern 

 pocket gophers, and Great Basin pocket mice — were 

 trapped only in the ungrazed habitat. In 1989, only the 

 deer mouse was caught on the grazed site. 



Estimated small mammal density was almost a third 

 higher in the grazed habitat (table 3). Total biomass 

 values, however, were similar between the grazed and 

 ungrazed plots. Also, small mammal species richness 

 and our estimates of small mammal species diversity 

 were larger within the exclosure. Each of the six species 

 recorded during the study was trapped in the ungrazed 

 habitat. Only two species were trapped in the grazed 

 habitat. 



Deer mice were the most frequently trapped small 

 mammal in both the grazed and ungrazed habitats 

 (table 3). Naive density (Wilson and Anderson 1985) 

 on the grazed plot was more than twice that on the un- 

 grazed plot. Most of the deer mice were trapped in the 

 sagebrush/upland community type that occupied the 

 slopes and terraces adjoining the riparian zone. They 

 were trapped infrequently in mesic herbaceous and mat 

 muhly/hummock communities that made up the riparian 

 habitat. Brown (1967a) also trapped deer mice more com- 

 monly in areas distant from water as compared to those 

 adjacent to water. 



The deer mouse is one of the most widespread and gen- 

 eralized of all North American rodents (Baker 1968). It 

 is Idaho's most common mammal (Larrison and Johnson 

 1981). They are found in diverse habitats including 

 swamps, waterways, forests, grasslands, and deserts, and 

 among rocks and cliffs (Larrison and Johnson 1981). It 

 occupies a variety of plant successional stages (Thomas 

 1979). Higher densities on the grazed plot at Summit 

 Creek suggest a tolerance by the deer mouse of habitats 

 with a low, sparse herbaceous layer. Samson and others 

 (1988) found deer mice frequently associated with low 

 values of grass and litter cover as well as the presence 

 of shrubs. 



Others have reported contradictory results when com- 

 paring the abundance of deer mice in grazed versus un- 

 grazed habitats. Kauffman and others (1982) found more 

 deer mice in eastern Oregon riparian habitats after late- 

 season grazing (late August to mid-September) than in 

 ungrazed riparian habitats. But by late summer of the 

 following year, and before grazing, the species composi- 

 tion of small mammal communities was not significantly 

 different between grazed and ungrazed plots. Similarly, 

 Moulton (1978) reported a positive response by deer mice 

 to grazing in a cottonwood {Populus sargentii) riparian 

 habitat in eastern Colorado. Samson and others (1988) 

 also found deer mouse densities consistently higher on 

 grazed pastures. Conversely, Rucks (1978) reported fewer 

 deer mice in grazed versus ungrazed riparian communi- 

 ties. Hanley and Page (1982) found a positive response 

 by deer mice to grazing in mesic habitats and a negative 

 response in dry habitats. 



Unlike the deer mouse, highest densities of the mon- 

 tane vole occurred in the ungrazed area (table 3). Four 

 times as many montane voles were trapped on the un- 

 grazed plot than on the grazed plot. Most were trapped in 

 streamside habitats with the frequency of capture highest 



in mesic herbaceous communities. None were trapped in 

 the sagebrush/upland community type. Montane voles 

 occur most commonly in moist, weedy, or brushy areas 

 near water at the edge of grasslands (Larrison and 

 Johnson 1981). The importance of vegetative cover to the 

 montane vole has been well documented (Brovm 1967a; 

 O'Farrell and Clark 1986). Grass seems to be a desirable 

 component of the habitat (Randall and Johnson 1979). 

 In eastern Oregon, high pregrazing populations of mon- 

 tane voles were either drastically reduced or eliminated 

 aft«r late-season grazing (Kauffman and others 1982). 



Vagrant shrews and water shrews, both scarce on the 

 study plots, were trapped only on the ungrazed area 

 (table 3). Captures were irregular and consisted of only 

 one or two animals in each trapping period. All were 

 caught near the stream in mesic herbaceous communities. 

 Vagrant shrews prefer moist, grassy habitats (Spencer 

 and Pettus 1966), but they occur in a variety of other 

 habitats including forests and shrublands (Brown 1967b). 

 Water shrews are typically found along edges of swift- 

 flowing streams with rocks, logs, crevices, and overhang- 

 ing banks (Beneski and Stinson 1987). Kauffman and 

 others (1982) reported reduced populations of the vagrant 

 shrew in postgrazing environments in eastern Oregon. 



Other species of small mammals were either trapped 

 or observed on the Summit Creek study site. The Great 

 Basin pocket mouse, a species that generally occurs in 

 arid and semiarid habitats (Verts and Kirkland 1988), 

 was trapped only on the ungrazed plot (table 3). It was 

 caught at a single location in the sagebrush/upland com- 

 munity type where giant wildrye {Elymus cinereus) was 

 codominant with scattered individuals of sagebrush and 

 rabbitbrush. Mounds of the northern pocket gopher were 

 evident throughout the area, but it was trapped only in 

 the ungrazed habitat. Columbian ground squirrels (Sper- 

 mophilus columbianus) were occasionally seen on the 

 study area, especially early in the season. Mink {Mustela 

 vison) and muskrats (Ondatra zibethicus) were rarely 

 observed and only in the ungrazed habitat. 



REFERENCES 



American Ornithologists' Union. 1983. Check -list of North 

 American birds. 6th ed. Washington, DC: American 

 Ornithologists' Union. 877 p. 



Ames, Charles R. 1977. Wildlife conflicts in riparian man- 

 agement: grazing. In: Johnson, R. Roy; Jones, Dale A., 

 tech. coords. Importance, preservation and manage- 

 ment of riparian habitat: a sjTnposium; 1977 July 9; 

 Tucson, AZ. Gen. Tech. Rep. RM-43. Fort Collins, CO: 

 U.S. Department of Agriculture, Rocky Mountain For- 

 est and Range Experiment Station: 49-51. 



Baker, Rollin H. 1968. Habitats and distribution. In: 

 King, John A., ed. Biology of Peromyscus (Rodentia). 

 Spec. Publ. 2. LawTence, KS: American Society of Mam- 

 malogists: 98-126. 



Beneski, John T., Jr.; Stinson, Derek W. 1987. Sorex 

 palustris. Mammalian Species No. 296. Shippensburg, 

 PA: American Society of Mammalogists. 6 p. 



Brown, Larry N. 1967a. Ecological distribution of mice 

 in the Medicine Bow Mountains of Wyoming. Ecology. 

 48(4): 677-680. 



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