Abstract
Based on canonical correspondence analysis (CCA) ordination of the relative caddisfly abundance of 79 streams (5-15 m in width), the state of Michigan appears to be composed of three distinct regions of caddisfly biological diversity corresponding to the Upper Peninsula, northern Lower Peninsula, and southern Lower Peninsula. The analysis further determined latitude, watershed gradient, and anthropogenic disturbance as important variables influencing distributional patterns. Overall species richness was higher and trophic functional group composition was as predicted by the river continuum concept in the predominately forested Upper Peninsula and northern Lower Peninsula regions. The northern Lower Peninsula region in particular was dominated by shredders. Overall species richness was lower in the agricultural southern Lower Peninsula region, and streams were dominated (>75% of specimens) by filtering collectors. Similar patterns of biological diversity and important environmental variables were determined when combining the Michigan streams with 87 similar streams from throughout Minnesota into a larger CCA ordination. Overall, five regions of caddisfly biological diversity are now established in the two states: northern Great Lakes, northern forested, southern agricultural, southeastern Minnesota, and northwestern Minnesota.