SPATIAL VARIATION IN THE DISTRIBUTION OF STREAM INVERTEBRATES - IMPLICATIONS OF PATCHINESS FOR MODELS OF COMMUNITY ORGANIZATION

1. Variation in abundances of stream fauna across a range of spatial s cales has been neglected, vet such variation can have important implic ations for experimental and descriptive work. We tested the hypothesis that the abundances of stream animals living on stones vary between s ites within the same stream order, riffles within the same site and gr oups of stones within the same riffle. We also examined how epilithon and water depth and velocity varied over these spatial scales. 2. Thir ty stones were sampled from each of three sites in a fourth-order sect ion 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 thre e stones within 0.5 m 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 densit ies, 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 betw een riffles and groups of stones. Of the thirty-five most abundant tax a, three-quarters (twenty-seven) varied over one or more of the spatia l scales, with variation between riffles being particularly common. DE CORANA analyses confirmed that two of the three sites had riffle fauna s that were dissimilar to each other, whereas the third site had riffl e faunas that were very similar. Significant spatial variation was obs erved also in water velocities and depths and chlorophyll a and organi c biomass concentrations. 4. Overall, we have demonstrated that signif icant variation in faunal densities may occur over small spatial scale s, such as those represented by groups of stones. Moreover, such varia tion was not consistent between taxonomically or functionally related species within the assemblage. 5. The results emphasize the need for s tream ecologists to be aware of the ramifications of such variation. N eglect of small-scale variation has produced spatially confounded desi gns in both surveys and experimental studies. Additionally, the assump tion that variation over small scales does not occur or is trivial see ms to have been responsible for the virtual enshrinement of certain la rge-scale models of community organization.