No beaver were recorded in the early history of the Sierra Nevada, Present beaver populations are derived from introductions during the past fifty years. A series of studies at Sagehen Creek has afforded a very complete record of a beaver population from the initial introduction in 1945 through 1970. An initial phase of rapid increase and expansion was based on a large standing crop of aspen. When the aspen was depleted the primary food base changed to willow, with an attendant decline in the population. Lifelong histories of marked animals demonstrated that mated pairs stay together for life. There was no evidence of polygamy.
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