1. Many recent studies have quantified the relative importance of environmental variables and dispersal limitations in shaping the structure of stream communities. However, the relative importance of these factors at different spatial extents has been seldom evaluated.
2. We analysed the distribution of caddisfly species in 89 Amazonian streams in relation to stream characteristics and spatial variables representing overland dispersal routes. The streams occur in three regions that differ in spatial extent and environmental characteristics. We analysed the data using partial redundancy analysis with two predictor data sets, one environmental and one spatial, to evaluate the variation in assemblage composition. We also separated caddisflies into ‘good’ and ‘poor’ dispersers to evaluate possible differences in the responses of these two groups.
3. The environmental component explained a higher proportion of variance in assemblage composition than did the spatial component. Spatial effects were evident only when data from all three regions were analysed together, although the exclusive spatial fraction was quite low. Good dispersers responded similarly to the community as a whole, while poor dispersers were related to environmental variables only in one region and also were not related to spatial variables.
4. Caddisflies were most affected by environmental factors. The large environmental effect and small spatial effect are in accord with the use of these stream insects as good indicators of site properties and disturbances in monitoring programmes.