Beaver Habitat Use and Impact in Truckee River Basin, California

Abstract

Stepwise logistic regression was used to identify factors important for habitat use by beavers (Castor canadensis) on streams. Increasing stream width and depth and decreasing gradient had the strongest positive effects on habitat use; food availability variables added little explanatory power. Some abandoned colony sites appeared to have been located on physically unsuitable habitat, whereas others appeared to be physically suitable sites abandoned due to resource depletion. The fact that few unused or uncolonized reaches were misclassified as suitable habitat suggests that suitable habitat has been saturated. Impact of beaver on woody plants was assessed for 8 forage species. Local extinction of quaking aspen (Populus tremuloides) and black cottonwood (P. trichocarpa) occurred on 4-5% of stream reaches. Willow (Salix spp.) showed good vigor despite heavy use in most reaches. J. WILDL. MANAGE. 51(4):794-799 Multivariate wildlife-habitat relationships models are often used to identify actually or potentially suitable habitat for various species, or to identify those factors that are important in habitat selection (Shugart 1981). Retzer (1955), Slough and Sadlier (1977), and Howard and Larson (1985) have used quantitative techniques to relate beaver occurrence, persistence, and density to various physical and vegetative characteristics. Wildlife managers have long been concerned with the loss of aesthetic value and impacts on stream channels due to destruction of riparian vegetation by beaver (Bump 1941, Munther 1981). Hall (1960) reported that beaver will deplete aspen in the immediate vicinity of a colony. However, there has been little quantitative documentation of the role of these animals in the loss of riparian vegetation (Kindschy 1985). Our objectives were to: (1) develop a model to identify the physical and vegetative characteristics of habitat colonized by beavers, and validate the model on a set of stream reaches not used in model development; (2) use the model results to suggest causes for colony abandonment, and evaluate whether beaver have saturated suitable habitat during the 40 years since introduction; and (3) describe the impact of beavers on woody vegetation. This research was supported by the Calif. Dep. Fish and Game and the Calif. Agric. Exp. Stn. Proj. 4326-MS. J. S. Slaymaker assisted in most of the fieldwork. M. L. Morrison, R. A. Lancia, S. H. Jenkins, and W. M. Block provided helpful criticisms of the manuscript. STUDY AREA The Truckee River Basin lies within Sierra, Nevada, Placer, and El Dorado counties, California, and Washoe County, Nevada. Our study covered the Truckee River and its tributaries from the confluence with Deer Creek downstream to Verdi, Nevada. This encompassed an area of approximately 600 km2, with 153 km of streams ranging from 1,485 to 2,750 m in elevation. Beavers were introduced into the area during 1938-46 and have since established colonies throughout the basin (P. Beier and R. H. Barrett, unpubl. data). The dominant vegetation of the area is mixed conifer forest with an overstory of white fir (Abies concolor); Jeffrey (Pinus jeffreyi), ponderosa (P. ponderosa), Washoe (P. washoensis), and lodgepole (P. contorta) pine; and a shrub component including greenleaf (Arctostaphylos patula) and whiteleaf (A. viscida) manzanita, snowbrush (Ceanothus velutinus) and squawcarpet ceanothus (C. prostratus), pale serviceberry (Amelanchier pallida), rose (Rosa spp.), and Sierra gooseberry (Ribes roezlii). The mixed conifer forest is replaced in higher elevations by a forest dominated by Shasta red fir (Abies magnifica), western white pine (P. monticola), and lodgepole pine, with squawcarpet as a dominant shrub; and in lower elevations by open stands of Jeffrey pine and an understory including big sagebrush (Artemisia tridentata), antelope bitterbrush (Purshia tridentata), and herbaceous plants. Stream banks were characterized by deciduous riparian vegetation consisting of aspen, cottonwood, willow, mountain alder (Alnus in-