Spatial variation in the distribution of stream invertebrates: implications of patchiness for models of community organization

SUMMARY 1 Variation in abundances of stream fauna across a range of spatial scales has been neglected, yet such variation can have important implications for experimental and descriptive work. We tested the hypothesis that the abundances of stream animals living on stones vary between sites within the same stream order, riffles within the same site and groups of stones within the same riffle. We also examined how epilithon and water depth and velocity varied over these spatial scales. 2 Thirty stones were sampled from each of three sites in a fourth-order section of the Taggerty and Steavenson Rivers, south-eastern Australia. At each site, fifteen stones were taken from each of two riffles; within each riffle, the fifteen stones were collected as five groups of three stones within 0.5m of each other. For each stone, we measured water depth and velocity, took samples of epilithon to estimate chlorophyll a concentrations, organic biomass and bacterial and algal cell densities, and collected all macroinvertebrates. 3 Hierarchical analyses of variance revealed that species richness did not vary over any of the spatial scales, but densities of individuals varied substantially between riffles and groups of stones. Of the thirty-five most abundant taxa, three-quarters (twenty-seven) varied over one or more of the spatial scales, with variation between riffles being particularly common. DECORANA analyses confirmed that two of the three sites had riffle faunas that were dissimilar to each other, whereas the third site had riffle faunas that were very similar. Significant spatial variation was observed also in water velocities and depths and chlorophyll a and organic biomass concentrations. 4 Overall, we have demonstrated that significant variation in faunal densities may occur over small spatial scales, such as those represented by groups of stones. Moreover, such variation was not consistent between taxonomically or functionally related species within the assemblage. 5 The results emphasize the need for stream ecologists to be aware of the ramifications of such variation. Neglect of small-scale variation has produced spatially confounded designs in both surveys and experimental studies. Additionally, the assumption that variation over small scales does not occur or is trivial seems to have been responsible for the virtual enshrinement of certain large-scale models of community organization.