Adaptation of beavers (Castor fiber) to extreme water level fluctuations and ecological implications
We have observed how beavers (Castor fiber) cope with water fluctuations in the Millingerwaard, part of the Gelderse Poort, a floodplain system of the river Rhine in the Netherlands. Beavers were introduced here in the mid 1990s. After severe losses in the first years the population grew gradually, up to about 60 animals in 2004. The narrow floodplain causes extreme fluctuations in water levels, normally between 6-7 m on an annual basis. During a series of floods in the 1990s it was observed that the beavers could cope with these fluctuations, constructing special lodges on higher ground within a few days. During the extreme dry summer of 2003 most beaver habitat in the Millingerwaard dried up completely. The beavers constructed burrows in the banks of a sand pit as safe homes to survive the drought. These events show that beavers readily adapt to periods of extreme flooding and drought. Each event stimulates the settlement of beaver territories at new locations. The concentration of beaver activity in periods of extreme drought may heavily influence the landscape. In the Millingerwaard concentrated beaver activity in the summer of 2003 resulted in more open alluvial forests.