Ecosystem Engineers: Beaver Ponds

The beaver population of the Kabetogama Peninsula is very dense in comparison to other areas of North America. Both biotic (forage availability) and abiotic (topography, geology, hydrology) factors contribute to this dense population. Historical (1927–2003) and contemporary aerial photos were used to map beaver ponds, beaver meadows, and other features altered by beaver dam construction. The objectives of the study were to relate the extent and type of beaver works to the beaver population present. A total of 1009 pond sites were identified within the 302 km2 Kabetogama Peninsula where flooding by beaver dams had discernibly altered the vegetation. Cover types ranged from open water to wetland forests, depending on the depth and duration of flooding. The sites were not continuously occupied by beavers over time, but were frequently recolonized. On average, each 100 beaver colonies increased the proportion of the landscape covered by open water by 2.15%. In addition, 16 of 21 permanent lakes on the Kabetogama Peninsula had lake outlet beaver dams. Collapse of the Shoepack Lake outlet dam in 2001 released an estimated 2.16 million kL of water and drained approximately 2/3 of the lake’s pre-collapse area. Beaver dams built in the first several decades of peninsula recolonization (1940–1948) created the largest ponds with the greatest potential for expansion, implying that beavers are able to optimize dam location.