AQUATIC PATCH CREATION IN RELATION TO BEAVER POPULATION TRENDS

The creation of aquatic patches by Castor canadensis in the boreal forest of N Minnesota was studied to determine how the population dynamics of a disturbance-causing animal are linked to rates of patch formation and growth over a period of population expansion and stabilization. Using 6 series of aerial photographs taken between 1940-1986, the authors show that the rate of patch formation as much higher during the first 2 decades of colonization than during the subsequent 2 decades. Average area of all pond sites, which included both filled and drained ponds, remained at c4 ha throughout the period, but average area of new ponds decreased significantly over time. Ponds established by 1961 constituted 75% of the total number and 90% of the total pond-site area as of 1986. When pond sites of similar age but different pond cohort (decade of establishment) were compared, average area per pond site was always significantly larger for the earlier cohort. Rate of patch formation after the first two decades of beaver colonization was probably constrained by geomorphology, which limited the availability of sites at which a beaver dam could impound a large area of water. -from Authors