Lutra 46(2)_Hartman_2003

Irruptive population development of European beaver (Castor fiber) in southwest Sweden

European beavers (Castor fiber) have been reintroduced to many areas within their former range. The resulting populations are still in a phase of population growth and range expansion. From a management point of view it is of interest to understand the pattern of population development that these populations are, or will be, exhibiting. Based on data from two surveys of a province in southwestern Sweden, I have earlier proposed that reintroduced beaver populations may exhibit an irruptive pattern of development, possibly as a result of overutilization of resources and lack of predators. The aim of this study was to see if a repeated study, twelve years after the previous, would support or question the proposed pattern of population development. Data from three surveys of the same province were used. The overall population density for the total area had increased from 0.10 colonies/ km2 in 1976, to 0.19 in 1987, and to 0.21 in 1999. However, when population density of local areas with time passed since colonisation was related, a peak in density (mean: 0,34 colonies/km2) after 25 years was revealed. Dividing the data into groups, areas colonised more or less than 25 years ago, and beaver population density decreased or increased since the previous survey (1987), showed that a negative change in population density was significantly more common in areas colonised more than 25 years ago. The results support the proposed pattern of population irruption in the studied beaver population.