Biodiversity through change
When beavers colonise a body of water, they change it permanently. Dams are built and smaller bodies of water are dammed, slides, escape tubes and dwellings are built and the amount of dead wood increases. The discharge behaviour and water temperature change in the dammed waters and new niches are found for other species. External environmental conditions such as flooding also increase the structural diversity, for example when water overflows the banks at the dammed sites. The beaver dams retain fine sediments with nutrients, which has a positive effect on the waters downstream of the beaver territories. Due to the lack of fine sediments, there are more gaps and a more open water bed.
All these changes caused by the beaver also have an impact on other species groups in the area. We had the exciting opportunity to investigate this at Chräbsbach on behalf of the Forest Department of the Canton of Aargau. A beaver family had newly settled here and a parallel monitoring programme offered the rare opportunity to document the effects of habitat changes caused by the presence of beavers on the fauna in an exemplary manner. The populations of the following species groups were monitored: amphibians, breeding birds, fish, dragonflies, butterflies and plants.
Overall, there has been a positive trend since the first surveys. What was particularly interesting, however, was that different species benefited from the conditions depending on the year. In the case of butterflies, three typical wetland species were recorded: the mourning cloak, the small fritillary and the large fritillary. In contrast, there was no significant increase in breeding birds, but more species were recorded than during the initial surveys. An increase in the number of dragonflies was also observed.
The picture for fish is somewhat mixed. Although there were fewer trout in the beaver area, the individuals present were larger and significantly heavier. As long as a stream is not permanently dammed, the trout do not suffer from the presence of the beaver. They could even benefit from the retained fine sediments, as they find more gaps in the gravel of the river bed for spawning.
These studies show by way of example that various species react positively to the newly created structural diversity and find new habitats in the landscape altered by the beaver.
