Ecological relationships between stream communities and spatial scale: implications for designing catchment‐level monitoring programmes

Summary 1. Stream communities are structured by factors acting over multiple spatial and temporal scales. Identifying what factors are driving spatial patterns in stream communities is a central aim of ecology. 2. Here we used two large European data sets of fish, invertebrates, macrophytes, benthic diatoms and environmental data in two stream groups (lowland and mountain) to determine the importance of variables at different spatial scales (geographical, regional, local) on community structure. 3. Both geographical position and ecoregion were selected first in canonical correspondence analysis (CCA), clearly showing the broad spatial gradients covered in the data set. Secondary predictors (after accounting for spatial and/or ecoregion effects) were similar between stream groups and among the four organism groups. In particular, conductivity and N concentration were strong predictors reflecting catchment land use. 4. Using partial CCA, we assessed the individual importance of the three spatial scales on the community structure of the four organism groups in the two stream groups. The majority of among‐site variability (22–29%) was accounted for by local scale variables (e.g. water chemistry and substratum type), with regional and spatial variables accounting 11–13% and 5–6%, respectively. Our findings indicate that the four organism groups are responding similarly to the different levels of spatial scale, implying much redundancy which should be consider when implementing studies of bioassessment.