Comparison of geographic classification schemes for Mid-Atlantic stream fish assemblages

Understanding the influence of geographic factors in structuring fish assemblages is crucial to developing a comprehensive assessment of stream conditions. We compared the classification strengths (CS) of geographic groups (ecoregions and catchments), stream order, and groups based on cluster analysis of fish assemblage data from 200 wadeable streams in the Mid-Atlantic Highlands. We 1st calculated intersite similarity indices (Bray-Curtis on relative abundance; Dice-Sorensen on presence/absence), then compared overall mean within-group similarities ((W) over bar) with among-group similarities ((B) over bar). We used subsets of the data to test CS from: 1) 31 reference sites defined on chemistry and habitat criteria, and 2) 21 samples from 8 sites that had been resampled within and between years to estimate the maximum similarity expected for any of the classifications. We assessed the strength of each classification by determining the degree to which (W) over bar was greater than (B) over bar. Sites classified by taxonomic clusters had higher CS than did sites grouped by stream order, US Geological Survey 4-digit Hydrologic Unit Code (HUC) catchments, and ecoregions. Except for taxonomic clusters, the CS values were greater when all sites were used in the analysis than when only reference sites were used. The mean similarities for the revisits were 2 to 3 times greater than for all other classifications. We used nonmetric multidimensional scaling as an alternative approach to detecting geographic structure in the data. We found little separation of ecoregion or catchment groups except at very broad spatial scales. The relatively weak CS of any of the geographic groups suggests that the interaction of complex zoogeographic patterns and a long history of human disturbance has masked any fine-scale structure of regional fish assemblages.