Influence of beaver dam density on riparian areas and riparian birds in shrubsteppe of Wyoming

Abstract

Through dam-building activity, beavers (Castor canadensis) play an important role in creating pond and wetland habitat for bird communities. Their impact may be intensified in semiarid landscapes and may increase with increasing dam density on a stream. Our objective was to examine relationships between dam density, riparian area characteristics, and the riparian bird community in a semiarid landscape. In 2002 and 2003 we surveyed riparian birds, riparian area characteristics, and the number of dams along 1.2-km sections of 11 streams in sagebrush steppe regions of Wyoming. We categorized the riparian bird community into 2 assemblages based on their affiliation with terrestrial or aquatic riparian habitats. Average width of the woody riparian zone, average riparian shrub height, and percent cover of emergent vegetation all had significant positive relationships with dam density, but percent cover of ponded water did not. Species richness and abundance of all riparian birds, and of the terrestrial assemblage, increased significantly with increasing woody riparian zone width. In contrast, richness and abundance of the aquatic assemblage were significantly positively influenced by cover of emergent vegetation and ponded water. When we accounted for riparian area characteristics, we found that total species richness, total abundance, and aquatic assemblage abundance were each positively correlated with dam density, suggesting that dam density is related to other riparian characteristics selected by birds. Our results suggest increasing dam-building activity may be important in creating favorable riparian conditions for a rich and abundant bird community in semiarid regions.