Variability in stream macroinvertebrates at multiple spatial scales

1. We intensively sampled 16 western Oregon streams to characterize: (1) th e variability in macroinvertebrate assemblages at seven spatial scales; and (2) the change in taxon richness with increasing sampling effort. An analy sis of variance (ANOVA) model calculated spatial variance components for ta xon richness, total density, percent individuals of Ephemeroptera, Plecopte ra and Trichoptera (EPT), percent dominance and Shannon diversity. 2. At the landscape level, ecoregion and among-streams components dominated variance for most metrics, accounting for 43-72% of total variance. Howeve r, ecoregion accounted for very little variance in total density and 36% of the variance was attributable to differences between streams. For other me trics, variance components were more evenly divided between stream and ecor egion effects. 3. Within streams, approximately 70% of variance was associated with unstru ctured local spatial variation and not associated with habitat type or tran sect position. The remaining variance was typically spit about evenly betwe en habitat and transect. Sample position within a transect (left, centre or right) accounted for virtually none of the variance for any metric. 4. New taxa per stream increased rapidly with sampling effort with the firs t four to eight Surber samples (500-1000 individuals counted), then increas ed more gradually. After counting more than 50 samples, new taxa continued to be added in stream reaches that were 80 times as long as their mean wett ed width. Thus taxon richness was highly dependent on sampling effort, and comparisons between sites or streams must be normalized for sampling effort . 5. Characterization of spatial variance structure is fundamental to designi ng sampling programmes where spatial comparisons range from local to region al scales. Differences in metric responses across spatial scales demonstrat e the importance of designing sampling strategies and analyses capable of d iscerning differences at the scale of interest.