Engineering ecosystem restoration: Surface water availability and bird community structure following beaver re-introduction in an arid land river system

Many land managers and restoration ecologists consider the re-establishment of beaver (Castor canadensis) as an effective and low-cost means for restoring riparian areas.  In order to assess the influence of beaver re-introduction along the upper San Pedro River in southeast Arizona, we quantified conditions along 80 kilometers of the river by measuring (1) changes in June surface water availability over an 8-year time span, and (2) extant bird abundance and community structure.  In order to compare temporal and spatial variation of water availability and beaver activity, we created GIS-based data themes using beaver census data collected regularly since the initial re-introduction in 1999, integrated with data from an annual dry-season, volunteer-based surface water mapping project.  We used standardized methods to estimate bird abundance and species richness, as well as vegetation structure at over 242 point count sampling stations.  At sites with a history of inter-annual variation in surface water availability, we found significantly less variation in water availability upstream of beaver dams as compared to similar sites with no dams.  The extent of surface water also increased upstream of new dams, even as drought and probable anthropogenic factors contributed to unusually dry conditions in the upper watershed.  We found that bird species richness and the abundance of several riparian obligate bird species were higher in areas with greater intensity of beaver activity, even after adjusting for the effects of surface water and vegetation structure.  Our results suggest that encouraging or directly initiating the re-establishment of beaver is a valuable technique, both for increasing surface water availability as well as restoring or maintaining bird communities in riparian areas situated in arid land.