We examined the ability of 5 a priori landscape classifications (3 sizes of catchments, ecoregions, and life zones) to partition observed variation in the composition of stream invertebrate assemblages. Analyses were based on 254 unimpaired sites from montane streams in California and 1912 sites from several regions of the United States. Estimates of classification strength (CS) were used to measure how well classificationsp artitionedv ariationi n compositiona mong sites. CS was measured as the difference between overall weighted mean within-class compositional similarity (V) and mean between-classs imilarity (B). We compared CSs of the 5 a priori classificationsw ith post hoc classificationo f sites based on clustera nalysis of the [...]