Quantifying the relative importance of how local (environmental or niche‐based) and regional (dispersal‐related or spatial) processes regulate the assembly of communities has become one of the main research avenues of community ecology. It has been shown that the degree of isolation of local habitats in the landscape may substantially influence the relative role of environmental filtering and dispersal‐related processes in metacommunities. Dendritic stream networks are unique habitats in the landscape, where more isolated upstream sites have been predicted to be primarily structured by environmental variables, while more central mainstem rivers by both environmental and spatial variables (hereafter the network position hypothesis, NPH). However, the NPH has almost exclusively been tested for stream macroinvertebrates, and therefore its predictions warrant confirmation from multiple taxa. We examined the validity of the NPH for benthic diatoms, macrophytes, macroinvertebrates and fish in the Pannon Ecoregion, Hungary. Following the NPH we predicted a clear dominance of environmental over spatial variables in headwaters, and a larger effect of spatial variables in rivers compared to headwaters. We tested these predictions using variance partitioning analyses separately for the different taxa in headwater and in riverine habitats. We found large differences in the explained community variance when the impact of environmental (physical and chemical) and spatial (overland and watercourse distance) variables for various taxa was studied. In general, total explained variance was lower for the more passively dispersing plant taxa than for animal taxa with more active dispersal in both streams and rivers. However, similar to other studies, the total explained variance was low for both headwater streams and rivers. Community structure of diatoms could be best explained by both environmental and spatial variables in streams, whereas their community structure could not be explained by either variable group in rivers. The significance of environmental and spatial variables depended on the distance measure (overland versus watercourse) in the case of macrophytes. Community structure of macroinvertebrates could be explained by environmental variables in streams and by both environmental and spatial variables in rivers. Moreover, variation was explained by different predictors when macroinvertebrate taxa were divided into flying and non‐flying groups, suggesting the importance of dispersal mode in explaining community variation. Finally, community structure of fishes could be explained by both environmental and spatial variables in streams and only by environmental variables in rivers. In conclusion, we found no clear evidence of the NPH in our multi‐taxa comparison. For example, while patterns in macroinvertebrate communities seem to support the NPH, those in fish communities run counter with the predictions of the NPH. This study thus shows that different taxa may behave differently to isolation effects in stream ne...

Self Organizing Maps (SOMs) are increasingly popular methods in processing highdimensional ecological data, however, their potentials are not yet fully utilized. It was our objective to prove evidence on an unknown advantage of the SOMs which we aimed to test using data on the spatial distributional patterns of gammarids. Quantitative samples and a wide spectrum of environmental data were obtained from the catchment area of two of the largest side tributaries of the Tisza River. Distributional patterns and habitat preference of three Gammarus species were described by Self Organizing Map methods and regression tree analysis (CART) on spatial and temporal scale. Using SOMs helped us to bring out distinctions in our data and enhance the differences, thus making them easier to recognize and A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPTalso, with their help, we were able to model the relations of the species to habitat types nonexistent among our samples. According to the analysis Gammarus roeselii preferred low altitudes, high conductivity, fine substrate, deep actual mean depth and dense plant coverage;G. fossarum preferred rocky stream beds, high altitude, lower temperatures and little actual mean depth; while G. balcanicus preferred coarse substrate, little or no plant coverage and low temperature. SOMs improved the correlations that proved to be highly useful: besides their use to display complex data in a perspicuous way they have other advantages in bringing out existing relationships in data otherwise difficult to detect.