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| ŠUMARSKI LIST 11-12/2013 str. 39 <-- 39 --> PDF |
Effects of beaver dams on invertebrate drift in forest streams Utjecaj dabrovih brana na drift beskralješnjaka u šumskim potocima Andreas Redin and Göran Sjöberg Abstract We aimed to assess the effects of beaver dams on the invertebrate drift fauna in five central Swedish boreal forest streams. Each stream was sampled once during the autumn, with drift traps placed upstream and downstream of the beaver dams. Drift densities (numbers/100 m3 water) were calculated. The invertebrates were determined, dried and weighed. No significant differences were noted in total drift densities or in the drift densities of pelagic species. The drift densities of benthic species were higher upstream of the dam, mainly because Ephemeroptera were more abundant in the upstream part. No significant difference was observed in diversity or dry weight. The functional feeding group ratio: filtering collectors/gathering collectors was significantly higher downstream of the dam. Key words: stream invertebrates, drift fauna, beaver dam, beaver pond Introduction Uvod Eurasian beaver was reintroduced to Sweden beginning in 1922. By early 1990´s the population had increased to over 100 000 individuals and beaver again occurred over a large part of the country’s landscapes (Hartman 2011). The construction of dams by beavers alters the ecosystem in many ways (Naiman et al. 1988; Rosell et al 2005; Baskin et al. 2011, Zav’yalov 2011). Also, the cutting and decay of trees causes a substantial input of wood to the water, and zones with a more open canopy are created (Naiman et al. 1986). The alteration of the ecosystem by beaver is larger in low order streams (Naiman et al. 1986). In beaver ponds, typical running-water invertebrate taxa may be replaced by a community more similar to lakes or slow-running water (Sprules 1940; McDowell & Naiman 1986; Naiman et al. 1988; Rosell et al 2005). In central Sweden, typical pond taxa like Dytiscidae and Corixidae were abundant in high numbers in the pond (Rosell & Pedersen 1999). Sprules (1940) in Ontario, Canada, and Nummi (1989) in Finland found that larval densities of Ephemeroptera, Trichoptera and Plecoptera decreased in the over-dammed river bed, but the number of Chironomidae increased. The site immediately downstream of a beaver dam in USA exhibited lower Plecoptera and Trichoptera densities than upstream, but the total invertebrate, Diptera, Ephemeroptera and predator densities were higher immediately downstream of the beaver dam (Smith et al. 1991). Beaver activities influence community functioning by increasing the absolute importance of collectors and predators, while decreasing the importance of shredders and scrapers at impounded sites. The dams can be an important |