Population density, colony composition, and local movements in two Sierra Nevada beaver populations

The fundamental unit of a beaver (Castor canadensis ) population is the colony, defined by Bradt as "a group of beavers occupying a pond or stretch of stream in common, utilizing a common food supply. and maintaining a common dam or dams." The authors studied two beaver populations in the central Sierra Nevada, one of which was much higher in density than the other and higher than those in most published reports. The dense population had closer spacing between colonies and more beavers per colony than the less dense population. Most significantly, more than two adults were present in most colonies of the dense population, an observation suggesting greater plasticity in social organization of beavers than is often assumed. The purposes in this note are to compare spacing and composition of colonies in these two populations, to describe differences in movement patterns related to age, sex, and especially population density, and to consider implications of these results for the social organization and population ecology of beavers.