Obtaining an adequate and representative sample is a continuing challenge of community ecology. The present study focuseson what sample area represents adequately the structural composition of the caddisfly fauna of a riffle, at a given sampling occasion.Sixty-two Surber samples were collected from a riffle in a second-order reach of the Bernecei Stream (BörzsönyMountains, Hungary). This data set was used to estimate sample representativeness at different sample sizes (from 1 to 31 Surbersamples, 0.09 m2 - 2.79 m2) generated a re-sampling procedure. Sample representativeness was measured with mean JaccardCoefficient and Bray-Curtis Index between samples for species presence-absence data and abundance data, respectively. Wefound that a sample size of 2.25 m2 represented well (mean similarity 0.998) the species composition of the caddisfly fauna ifrare species were excluded from the analysis. In contrast, sample representativeness of species composition proved to be relativelylow (0.719) if rare species were included in the analysis. Curves of sample representativeness based on both raw-, or transformedabundance data were less sensitive to the presence of rare species and showed lower representativeness than samplerepresentativeness based on presence/absence data.

We tested the individual and joint effects of stream order, riverbed morphology (i.e. pool-riffle sequence), and season on the structural (i.e. species composition, density and richness) and functional (i.e. percent ratio of shredders, collectors, scrapers, and predators) properties of caddisfly assemblages in a closely natural headwater system (Börzsöny Mts., North Hungary). Each factor, both individually and in interaction had a significant effect on caddisfly assemblages as shown by MANOVA analyses. To clarify the effects of stream order, riverbed morphology and season on caddisflies, assemblage properties were also separately studied by ANOVAs. Percent ratio of shredders and collectors were strongly affected by all factors, whereas other attributes were not always sensitive to stream order, season and riverbed morphology. In contrast, species compositions of the reaches differing in stream order and riverbed morphology were significantly different. Our findings on the contrasting differences in shredder composition between riffles and pools may suggest that not only riparian vegetation, but also instream habitat features could shape the accumulation and retention of particulate organic matter (in streams with pool-riffle heterogeneity).