DIGITALNA ARHIVA ŠUMARSKOG LISTA
prilagoðeno pretraživanje po punom tekstu



ŠUMARSKI LIST 11-12/2013 str. 48     <-- 48 -->        PDF

Drift density of pelagic species: The second prediction, that the amount of pelagic invertebrates would be higher downstream of the dam, was not supported. In the ponds, typical running water taxa may be replaced by pond taxa (Sprules 1940; Nummi 1989; McDowell & Naiman 1986). Since we did not take any insect samples from the ponds per se we cannot say for sure that lentic taxa were abundant. In only one of our five streams was the dam made quite recently (Lötbergstjärnbäcken). In the other, older, ponds it is likely that a more lentic fauna has developed. No overall significant difference was, however, observed in drift densities of pelagic species between upstream and downstream sites. There may be suitable habitats for lentic invertebrates in slow-running parts and in lakes further upstream of the sampling point above the dam. Although we tried to find beaver dams far from lake outlets there might still be some lake effects in our results. It is likely that the distance to wetlands and lakes upstream overrides the local production of pelagic fauna in the pond itself (see Table 1).
Plecoptera: The third prediction, that drift of Plecoptera would be reduced downstream of the dam, was not supported by a statistically-significant difference. The results thus do not agree with the idea that the reduction of Plecoptera in the benthos of ponds and reservoirs (Sprules 1940; Ward & Stanford 1980; McDowell & Naiman 1986; Nummi 1989) and in the area downstream the dam (Ward & Stanford 1980; Smith et al. 1991) would reduce the Plecopteran drift densities.
Functional feeding groups: The fourth prediction, that the functional feeding group ratio filtering collectors to gathering collectors should be higher in the sites downstream the dams, was supported. The wood and debris dam created by beaver are often a suitable habitat for filtering collectors (Clifford et al. 1993). Streams below reservoirs are characterised by a predominance of filter-feeding Trichoptera and Simuliidae (Ward & Stanford 1980). FPOM is deposited in large amounts in the benthos of the beaver dam (Naiman et al. 1986). Gathering collectors that feed on FPOM in deposit are likely to have their food resource decreased in the downstream area.
Conclusions
Zakljuèci
Drift sampling is rapid and relatively easy and appears to be a useful method for studying effects of beaver activities on the stream community. Although it does not directly mirror the composition of benthos, it can be seen as an indicator of the conditions, and also gives a direct picture of food availability for salmonid fish.
Although the numbers of streams we investigated was not large, the functional group approach and analysis of trophic group ratios (Merritt & Cummins 1996) is, we suggest, a promising avenue for future studies, and allows testing of a priori hypotheses as we do here.
There was no general trapping of drifting animals in ponds created by beaver, so the area downstream the dam can have a rather rich drift. The drift densities of benthic animals are however reduced downstream of beaver dams, and beaver ponds in this case seem to have the same general effect on drift as large water power station reservoirs (Gönczi et al. 1986).
The drift differed considerably among the studied streams. The effects on the invertebrate drift are likely to differ considerably with the size of the stream, as well as the size and age of the dam. Finally, the condition of the dam and the distance to the nearest upstream lake may also impact the composition of the drift downstream.
Acknowledgements
Zahvale
We wish to thank Stig and Christine Redin, who helped with parts of the field work.
References
Literatura
Allan, J.D, 1995: Stream ecology. Structure and function of running waters, Chapman & Hall, 388 p., London        
Allan, J.D., E. Russek, 1985: The Quantification of Stream Drift, Can. J. Fish. Aquat. Sci., 42: 210–215.
Bailey, R.G., 1966: Observations on the nature and importance of organic drift in a Devon river, Hydrobiol., 27: 353–67.
Baskin, L.M., Novoselova, N.S., S.L. Barysheva, 2011: Landscape level habitat selection by beavers and the long-lasting effects of beaver settlements, Restoring the European beaver: 50 years of experience, Pensoft, 195–204. Sofia
Baxter, R.M., 1977: Environmental Effects of Dams and Impoundments, Ann. Rev. Ecol. Syst., 8: 255–283.
Clifford, H.F., Wiley, G.M., R.J. Casey, 1993: Macroinvertebrates of a beaver-altered stream of Alberta, Canada, with special reference to the fauna on the dams, Can. J. Zool,. 71: 1439–1447.
Elliott, J.M., 1967: Invertebrate drift in a Dartmoor stream, Arch. Hydrobiol., 63: 202–37.
Elliott, J.M., 1970: Methods of sampling invertebrate drift in running water, Ann. Limnol., 6: 133–59.
Everest, F.H., D.W. Chapman, 1972: Habitat selection and spatial interactions by juvenile chinook salmon and steelhead trout in two Idaho streams, J. Fish. Res. Board Can., 29: 91–100.
Gönczi, A.P., Henricsson, J., G. Sjöberg, 1986: Fisheries management in river reservoirs. Final report from the project Management of Fisheries in Hydropower Reservoirs, Part 1. Institute of Freshwater Research, Drottningholm, 115 p., Sundsvall (In Swedish)