Small Mammals of a Beaver 

 Pond Ecosystem and Adjacent 

 Riparian Habitat in Idaho 



Dean E. Medin 

 Warren P. Clary 



INTRODUCTION 



Beaver {Castor canadensis) are important regula- 

 tors of aquatic and terrestrial ecosystems, with 

 pervasive effects far beyond their food and space 

 requirements (Naiman and others 1988), Dam 

 building creates habitats that are important for 

 some forms of wildlife, particularly fish, waterfowl, 

 and browsing mammals (Jenkins and Busher 1979). 

 But there are few quantitative assessments of the 

 importance of beaver pond habitats for nongame 

 wildlife. This study compared small mammal 

 populations and community organization between 

 a willow-dominated beaver pond ecosystem and an 

 adjacent non willow riparian habitat on Summit 

 Creek in east-central Idaho. 



Beavers built both a primary dam and several 

 secondary dams in a previously xmoccupied section 

 of Summit Creek in the summer and fall of 1979 

 (Lewis 1989). During our study, about 0.3 ha of 

 surface water was impounded by the dams. Both 

 the beaver pond complex and the adjacent site were 

 protected from livestock grazing for the previous 

 14 years by a large (122-ha) fenced exclosure. The 

 exclosure, constructed in 1975, is on public lands 

 administered by the Bureau of Land Management, 

 U.S. Department of the Interior. 



STUDY AREA 



The study site was 41 km north of Mackay in 

 eastern Custer County, ID. Elevation is approxi- 

 mately 1,975 m. Summit Creek, a tributary of the 

 Little Lost River, originates from springs and flows 

 through a broad, basinlike valley bounded on the 

 east by the Lemhi Range and on the west by the 

 Lost River Range. Regional climate is semiarid. 

 Average annual precipitation at Mackay (elevation 

 1,797 m) is 247 mm, with peaks in May and June. 

 The growing season is short, averaging less than 

 100 days at Mackay (USDC NOAA 1982). Micro- 

 relief in many parts of the study site is hummocky, 

 with soils hi^ in total salts (USDASCS 1987). 

 Except for the beaver dam pond complex, the ripar- 

 ian zone is seldom more than 50 to 100 m wide. 



Four major vegetation community types were 

 defined on the study site. These were: willow {Salix 

 spp.)/mesic herbaceous, sagebrush (Artemisia spp.)/ 

 upland, mat muhly {Muhlenbergia richardsonis)/ 

 hummock, and mesic herbaceous. The willow/mesic 

 herbaceous community type was found only in the 

 immediate vicinity of the beaver pond. Sagebrush/ 

 upland communities occupied the gentle slopes and 

 terraces that bordered both the beaver pond com- 

 plex and the adjacent areas. The other two types — 

 mat muhly/hummock, and mesic herbaceous — were 

 elements of the riparian zone and were found on 

 both the beaver pond site and adjacent sites. 



Upland vegetation was shrub-steppe (West 1983). 

 The most common shrubs were low sagebrush (A. 

 arbuscula), threetip sagebrush (A. tripartita), and 

 green rabbitbrush {Chrysothamnus viscidiflorus). 

 The understory included Sandberg's bluegrass (Poa 

 sandbergii), bluebunch wheatgrass (Agropyron 

 spicatum), and long-leaf phlox (Phlox longifolia). 

 Geyer willow (S. geyeriana), bilberry willow (S. 

 myrtillifolia), Bebb willow (S. bebbiana), and water 

 birch (Betula occidentalis) formed the tall-shrub 

 overstory of the willow/mesic herbaceous commu- 

 nity. The understory included a wide variety of 

 graminoids and forbs. The stream was closely 

 bordered by clumped mesic herbaceous communities 

 dominated by beaked sedge (Carex rostrata), water 

 sedge (C. aquatilis), Baltic rush (Juncus balticus), 

 and Kentucky bluegrass (Poa pratensis). Hum- 

 mocky areas, dominated by mat muhly and thick- 

 spiked wheatgrass (Agropyron dasystachyum), were 

 generally located in an intermediate position 

 between the streamside communities and the 

 sagebrush uplands. 



METHODS 



Two 1.7-ha trapping grids, one in the upper 

 (westernmost) section of the fenced exclosure and 

 the other in the beaver pond area, were established 

 to sample small mammal populations. Grid loca- 

 tions were selected to best represent both the beaver 

 pond complex and the adjacent nonponded riparian 

 habitat. Each grid measured 225 by 75 m and 



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