Dissimilarity of stream insect assemblages: effects of multiple scales and spatial distances

In addition to the effects of environmental conditions, biotic assemblages may exhibit spatial structure depending on the scale of study. We tested whether the dissimilarity of stream insect assemblages is related to two types of spatial distances (stream corridor and overland distance), and evaluated the relative importance of diversity components at multiple spatial scales. Field data included assemblages of Ephemeroptera, Plecoptera, and Trichoptera found in 16 streams in four microbasins. We evaluated the relationship of the dissimilarity of assemblages with the distance types, using Mantel tests. In addition, we evaluated the relationships among the diversity components at multiple spatial scales, using additive partitioning analysis. The biological dissimilarities were correlated only with the geographical distances. Additive partitioning showed that the values of richness observed in the beta 1 (among Surber), beta 2 (among riffles), beta 3 (among streams), and beta 4 (among microbasins) were higher than those expected. The highest variation of the richness was found in beta 3 (30.6%). We conclude that stream faunas are distributed in patches all over the studied spatial extent, causing a weak relationship of biological dissimilarity with distance but important beta components when compared to a completely homogenous distribution of the fauna.