Transformation of benthic communities in forest lowland streams colonised by Eurasian beaver Castor fiber (L.)

Beavers are an exception among animals in terms of the scale of environmental transformations they achieve. This study investigated primary environmental factors influencing the occurrence of aquatic invertebrates in lowland streams inhabited by the Eurasian beaver. The study was conducted in two forest streams inhabited by beavers, and in an uninhabited stream. In streams inhabited by beavers, the study covered seven ponds. Sections with flowing water were also analysed downstream and upstream of the ponds. Benthos and water samples were collected at each site. Dissolved oxygen (DO) concentration and saturation were the only physicochemical parameters that indicated decreases in water quality in beaver ponds. The benthic communities of different beaver ponds were similar. The taxa that exerted the greatest influence on the similarity of the invertebrate fauna in the ponds were Oligochaeta and Chironomidae. Ostracods were also abundant in the ponds, whereas they were few in the flowing sections. Mayflies (Cloeon) and caddisflies belonging to the family Phryganeidae were also closely associated with the ponds. Caddisflies (Plectrocnemia and Sericostoma), mayflies (Baetis) and stoneflies (Nemourella and Leuctra) exhibited the highest correlation with DO concentrations, which is typical of flowing sections, and avoided stream fragments dammed by beavers. Bivalvia (Pisidium) were also abundant in each of the streams along the flowing sections. The highest number of taxa and greatest taxonomic diversity was observed in sections flowing below the beaver ponds. The engineering activity of beavers transformed the studied lowland streams, resulting in the development of rheophilic and stagnophilic communities of aquatic invertebrates, in free-flowing and dammed sections, respectively.