DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ŠUMARSKI LIST 11-12/2013 str. 43     <-- 43 -->        PDF

in the mud. Combined with the condition of the dams and observations of any recent construction activity on them provided a good picture of whether beaver were present or not.
Materials and methods
Metoda rada
Drift samples were taken during the autumn of 1997, from August 29 to September 4. Each stream was sampled once during 4 hours, from 19.00 to 23.00 (daylight savings time). Six drift samplers (Figs. 2A, 3) were used. Three were placed downstream of the beaver dam within a distance of 1,5–4 m from the dam, depending on where most of the water had returned to the main stream bed. The other three were placed immediately upstream of the pond (Fig. 4). The samplers were held in position with iron rods driven into the stream bed.
All samples were preserved in 70% alcohol and then hand-sorted in the laboratory. Samples were weighed on a Mettler AD 160 balance after drying at 60° for 24 h. Drift density was calculated as the number of individuals in 100 m3 of water. Animals already dead at the time of the catch were not included. The animals were divided into a benthic and a pelagic group, with both free-swimming and  planktonic animals. Emerging water insects and terrestrial animals were placed in separate groups.
Benthic insects were divided into functional feeding groups according to keys in Merritt & Cummins (1996) and Merritt & Cummins (1978). Functional feeding group ratios are responsive to changes in food resource base (e.g., algae, litter, fine organics, prey) and are a useful ecological tool, indicative of various ecosystem parameters (Table 2). The following ratios were calculated: predators/total of all groups, filtering collectors/gathering collectors, shredders/total collectors and scrapers+filtering collectors/shredders+ gathering collectors.
Habitat features were measured at each site. Water temperature was measured with a digital thermometer. To measure pH, indicator paper was used (Merck Spezialindikator pH 4,0–7,0). The height, width and length of the beaver dam were measured. Water velocity was measured by timing a weighted float over a marked distance. The bottom substrate was also categorised by ocular inspection at each site.