DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ŠUMARSKI LIST 11-12/2013 str. 40     <-- 40 -->        PDF

substratum for certain taxa. In Alberta, Clifford et. al. (1993) found larger proportions of simuliid larvae on the dam than at the main stream sites indicating that beaver dams may be important for maintaining a lotic (flowing water) fauna in slow moving, low gradient streams. McDowell & Naiman (1986) found greater density and biomass in beaver ponds compared to riffles in spring and summer, but found no difference in autumn. Sprules (1940) found fewer insects (both number and taxa) emerging from a beaver pond compared to the same area when it was a preimpounded riffle. Naiman et. al. (1988) on the other hand found that the total number of species in beaver ponds appear to be similar to those in the natural stream channel.
Stream-dwelling organisms are often transported downstream in the water column in substantial numbers, and this phenomenon has been called drift (Allan 1995). The drift is composed of benthic animals, emerging or emerged insects on the water and planktonic species (Bailey 1966). The high numbers of drifting planktonic animals from lake outlets is well known (Allan 1995). The input of terrestrial animals in the drift can be substantial. Of the benthic fauna, Ephemeroptera, some Diptera and some Plecoptera and Trichoptera are the most common, in roughly that order. At a given time there is only a small percentage of the bottom fauna (less than 0.01 %) found in the water column above a unit area of bottom (Allan 1995; Elliott 1967). Drifting invertebrates are important food for some fish species (Allan 1995). Since pools serve as depositional regions, one might think that pools (and ponds created by beaver) could serve as a major brake on drift. Bailey (1966) claimed that pools trapped drift. Waters (1962) could see that drift out of the pool was about 20% less than drift of the riffles, suggesting some trapping in the pools, but other studies (e.g. Elliott 1967) give very little support to the proposition that pools trap drifting individuals.
Our purpose was to study how beaver activity affects the invertebrate drift of small boreal streams in central Sweden. This is an aspect of beaver activity that so far has been given very little attention. The following predictions were made: