Colonization of stream macroinvertebrates onto artificial substrates was examined in Sagehen Creek, California, during 1993. Substrate size and presence or absence of pupal cases of the caddisfly Anagapetus (Glossosomatidae) were examined as possible influences on macroinvertebrate colonization. Macroinvertebrates were collected from substrates after 60 d of incubation and identified to the lowest feasible taxonomic unit. Dry weight of the filamentous green alga Cladophora also was determined for each substrate.

A general linear model indicated a significant positive relationship between both substrate size and presence of pupal cases on richness (P < 0.001). There was no significant effect of cases on the E5 evenness index, which supports the passive sampling model as a mechanism giving rise to the effect of cases on richness. A multivariate general linear model indicated a significant positive effect of Anagapetus cases on Micrasema bactro, Epeorus, and Rhyacophila sp. 1, and of substrate size on Hydropsyche occidentalis and Epeorus. Neither cases nor substrate size had a significant effect on Cladophora density (P > 0.05). There were significant negative correlations between Cladophora density and both Shannon Hʹ and Jʹ indices. There were significant positive relationships between Cladophora density with Baetis, Simulium, and Chironomidae density. Combined main effects of substrate size, Anagapetus pupal cases, and Cladophora have dynamic influences on this macroinvertebrate fauna. This study demonstrates the complex interplay of biotic and abiotic factors that determine macroinvertebrate distribution and abundance in this system.