Van Sickle, J. and R.M. Hughes, 2000. Classification strengths of ecoregions, catchments, and geographic clusters for aquatic vertebrates in Oregon. J. N. Am. Benthological Soc.19(3):370-384.
The usefulness of ecoregions and catchments (hydrologic units) as bases for classifying aquatic vertebrate assemblages in western Oregon was compared using samples collected by electrofishing from 137 wadeable stream sites distributed evenly throughout the region. The classification strengths of these regionalizations were also compared with neutral-model classifications that were based either on intersite proximities in geographic distance or on intersite similarities in the sampled vertebrate assemblages. The strength of each classification was assessed by the extent to which average within-class assemblage similarities exceeded the average similarity between classes. Mean similarity dendrograms were used as a concise graphical comparison of between- and within-class similarities for alternative classifications, whether they were specified a priori or constructed by clustering For each a priori classification, a permutation test of the no class structure hypothesis was performed Classification strengths were assessed using the Sorenson-Dice (presence / absence) and Bray-Curtis (relative abundance) similarity measures, applied to both species-level and family-level assemblage characterizations, as well as for the Bray-Curtis measure applied to a set of 5 assemblage metrics that were designed to reflect stream impairment For all 5 measures of assemblage similarity, ecoregions had higher classification strengths than did large catchments, and large catchments had about the same strength as a stream-order classification. A catchment classification with 1 of the 3 catchments split into 2 ecoregions separated assemblages as strongly as ecoregions alone. A neutral-model classification based solely on geographic site proximity classified assemblages with about the same strength as ecoregions. Another neutral-model classification of sites, based solely on their sampled assemblages, was at least twice as strong as any of the geographic classifications Intermediate strength was seen in site groupings derived front a composite measure of between-site assemblage and geographic dissimilarities. Our results suggest that ecoregions and large catchments do indeed have utility for classifying stream vertebrate assemblages. However, much of their classification strength may be a result of spatial autocorrelation effects, rather than ecological factors that determine their particular boundaries. Our similarity analyses also suggest that geographic partitions can be expected to account for only a minor portion of the total variation seen in stream vertebrate assemblages across a large region.