Beaver in the drainage basin: an ecosystem engineer restores wetlands in boreal landscape

Abstract

Wetland and deadwood loss have had a profound effect on boreal aquatic and terrestrial ecosystems and their biodiversity. Deadwood-dependent species are one of the most endangered organism groups in the world, while amphibians on the other hand excellently represent the ecological state of wetlands. The boreal region contains a large proportion of the world’s wetlands, which have undergone two major alterations during the last 500 years: first the extirpation of beavers and secondly draining during the 20th century. Beavers are well-known ecosystem engineers of the Northern Hemisphere. They modify their surroundings by damming water systems. Damming raises flood waters into the surrounding riparian forest and changes environmental conditions both on land and in water. Ecosystem processes are altered when beavers turn a lotic water system into a lentic one, but the alteration is also evident when beavers modify initially lentic water systems. Organic matter and nutrients are transferred into a wetland from beaver-felled trees and vegetation killed by flooding. The amount of dissolved organic carbon increases during the first three impoundment years, which enhances the growth of aquatic vegetation and the abundance of phyto- and zooplankton, thereby also increasing invertebrate abundances. Luxuriant vegetation and ample plankton and invertebrate populations facilitate frogs, which become abundant in beaver wetlands. The moor frog in particular favours beaver-created wetlands. Flooding and beavers kill trees, producing high amounts of deadwood. The riparian forests of beaver wetlands include much higher deadwood levels than wetlands without beavers. Increased deadwood creates substrate resources for deadwood-dependent species. Snags are a typical deadwood type in beaver wetlands. Calicioids are deadwood-dependent species particularly specialised in inhabiting standing deadwood. The comeback of beavers has aided the restoration of wetlands and deadwood. Beaver wetlands can be seen as carbon and biodiversity hot spots that increase the heterogeneity and hydraulic connectivity of the boreal landscape.