Species Traits and Assemblage Structure Across Anthropogenic Environmental Gradients in Streams of Southeastern Michigan

Recent studies suggest that analyses of biological assemblages summarized by phenotypic traits (trophic, life cycle, habit, and stressor tolerance) may be more insightful than taxon-based analyses, and that landscape features, like anthropogenic land use, may act as filters that can be linked to assemblage differences when summarized by traits. Common fish species and macroinvertebrate taxa collected from 46 streams in southeastern Michigan were summarized by traits, and principal component analysis (PCA) was used to identify biological gradients. Three axes explained a majority of variation in trait characteristics for both groups of organisms. Axes that explained the most variation in fish and macroinvertebrates were interpreted as gradients in ecological integrity because each was strongly, positively weighted by intolerant taxa and trait specialists. However, only the axis summarizing macroinvertebrate integrity was correlated with taxon diversity and richness; the fish axis was not correlated with either measure. Instead, a different fish axis, which emphasized the inclusion of fluvio-lacustrine species, was correlated with diversity, implying that stream fish diversity was elevated by lentic conditions. Investigations of relationships of axes with habitat variables re-enforces the view that environmental features, acting as filters, have a strong influence on assemblage structure when traits are considered.