Bird Populations in and 

 Adjacent to a Beaver Pond 

 Ecosystem in Idaho 



Dean E. Medin 

 Warren P. Clary 



INTRODUCTION 



Beavers {Castor canadensis) alter riparian-stream 

 ecosystems through their woodcutting and dam- 

 building activities. The resultant habitats can be 

 beneficial to some forms of wildlife and detrimental 

 to others (reviewed in Hill 1982; Jenkins and 

 Busher 1979). But to date, little information is 

 available on the importance of beaver pond habitats 

 for nongame birds. This study compared breeding 

 bird populations and bird community organization 

 between a willow {Salix spp.)-dominated beaver 

 pond habitat and an adjacent nonwillow riparian 

 habitat on Summit Creek in east-central Idaho. 



Beavers built both a primary dam and several 

 secondary dams in a previously unoccupied section 

 of Summit Creek in the summer and fall of 1979 

 (Lewis 1989). During the year of study (1989), 

 about one-third hectare of surface water was im- 

 pounded by the dams. Both the beaver pond com- 

 plex and the adjacent site were protected from live- 

 stock grazing for the previous 14 years by a large 

 (122-ha) fenced exclosure. The exclosure, con- 

 structed in late 1975, is on public lands admini- 

 stered by the Bureau of Land Management, U.S. 

 Department of the Interior. 



STUDY AREA 



The study site was 41 km north of Mackay in east- 

 em Custer County, ID. Elevation is about 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 pre- 

 cipitation 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). Microrelief in many parts 

 of the riparian area is hummocky, with soils high 

 in total salts (USDA SCS 1987). Except for the 

 beaver pond complex, the riparian area was seldom 

 more than 50 to 100 m wide. 



Four major vegetation community types were de- 

 fined on the study site. These were: willow/mesic 

 herbaceous, sagebrush {Artemisia spp.)/upland, mat 

 muhly {Muhlenbergia richardsonis)/hummock, and 

 mesic herbaceous. The upland communities occu- 

 pied the gentle slopes and terraces that bordered 

 both the beaver pond complex and the nonponded 

 area. The willow/mesic herbaceous community type 

 was found only in the immediate vicinity of the bea- 

 ver pond. The mat muhly/hummock, and mesic 

 herbaceous communities were considered elements 

 of the riparian zone and were found on both the 

 willow and nonwillow study sites. 



Upland vegetation was shrub-steppe. The most 

 common shrubs were low sagebrush (A. arbuscula), 

 threetip sagebrush (A. tripartita), and green rabbit- 

 brush {Chrysothamnus viscidiflorus). The under- 

 story included Sandberg's bluegrass {Poa sandber- 

 gii), bluebunch wheatgrass {Agropyron spicatum), 

 and long-leaf phlox {Phlox longifolia). The willow/ 

 mesic herbaceous community characterized the 

 beaver pond complex. Geyer willow (S. geyeriana), 

 bilberry willow (S. myrtillifolia), Bebb willow (.S. 

 bebbiana), and water birch {Betula occidentalis) 

 formed the tall-shrub overstory. 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 blue- 

 grass (P. pratensis). Hummocky areas, dominated 

 by mat muhly and thick-spiked wheatgrass (A. 

 dasystachyum), were generally located in an inter- 

 mediate position between the streamside communi- 

 ties and the sagebrush uplands. 



METHODS 



Two 9-ha plots, one in the upper (westernmost) 

 section of the exclosure and the other in the beaver 

 pond (downstream) area, were censused for breed- 

 ing birds using the Williams' spot-map method 

 (International Bird Census Committee 1970). Plot 

 locations were selected to best represent both the 



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