Dam Site and Vegetation Selection by Reintroduced Beaver (Castor canadensis) on the San Pedro Riparian National Conservation Area, Arizona

Beaver (Castor canadensis) were reintroduced on the San Pedro Riparian National Conservation Area (SPRNCA) after having been extirpated. The two most obvious effects from this keystone species, dam construction and herbivory, have not been studied on the SPRNCA. The purpose of this research was to determine any preference for dam sites, vegetation preferences, or if beaver may travel longer distances for preferred food or dam materials. Individual dams remained or were rebuilt in the same location on average every 2.4 years. Dam building was disproportionate to availability of sites, with beaver more commonly building dams at the confluence of the San Pedro River and tributaries. The size of areas with impacts to vegetation from beaver herbivory averaged 0.34 ha (2.5 ac), containing an average of 40.7 trees. Beaver exhibited a preference for smaller-sized cottonwood (Populus fremontii) trees over larger cottonwood trees and smaller or larger Gooddings willow (Salix gooddingii), both in terms of whether trees were eaten and left standing or felled, and distance beaver travelled from water to the tree. Management implications include continued research on effects of beaver, management of invasive plant species, use of cottonwood genotypes with high tannin levels during restoration projects, further augmentation of beaver, use of beaver-dam analogues, and continued closure to hunting and trapping of beaver.