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ŠUMARSKI LIST 11-12/2013 str. 46     <-- 46 -->        PDF

The ratio filtering collectors/gathering collectors (Fig. 7) was significantly higher downstream the dam, but for the other ratios no significant differences were noted.
Discussion
Rasprava
Beaver activity certainly alters the habitat and makes it potentially very unique (McDowell and Naiman 1986). The drift of the downstream sites is likely to receive animals from all these different beaver altered habitats – pond, dam, and downstream reach – and perhaps also from the area upstream the pond since drift distances can be quite long (Allan 1995).
Drift density of benthic species: Our first prediction, that the density of drifting benthos would be lower downstream the dams, was upheld. The significantly higher drift density of benthic species in the upstream part comes largely from more Ephemeroptera. This could be a result of: 1) the ponds trapping drifting individuals 2) reduced entry to the drift from the beaver pond or 3) reduced entry to the drift from the area downstream the dam. McDowell & Naiman (1986), Nummi (1989) & Sprules (1940) found that larval densities of Ephemeroptera decreased in the over-dammed river bed. There is no absolute correlation between the benthic community and the drift fauna. Certain genera can be present in high numbers but contribute very little to the drift, and vice versa (Bailey 1966). Sjöberg (unpublished.data) found Ephemeroptera in the ponds, but of the genera Cloëon that prefers slow-running waters and possibly is not a drift-prone taxon. The third explanation is not likely since Smith et al. (1991) found higher densities of Ephemeroptera downstream the pond. Sjöberg (unpublished data.) could detect no difference in Ephemeropteran density at the downstream site. Reduced entry from the pond caused by reduction and/ or change to genera which drift little is a possible explanation for the lower Ephemeropteran drift densities. If, however, all the Ephemeroptera that enter the pond would pass through, there would be no difference in drift density. Some trapping and maybe more importantly, predation from fish in the pond is also likely to occur and contribute to the result.