Urbanization impacts aquatic and terrestrial stages of caddisflies (Trichoptera) to reduce richness of larval assemblages headwater streams

Human activities in and around streams can lead to lower in-stream habitat and water quality that impact in-stream, larval stages of stream insects and lead to lower community richness. Human activities in upland areas can also decrease survival and reproduction of adults, but is rarely considered a potential cause of low in-stream diversity. We surveyed adult and larval caddisfly (Trichoptera) assemblages at four urban and four rural headwater streams to determine the importance of in-stream impacts to larvae versus terrestrial impacts to adults for the observed low diversity of larval assemblages in the urban streams. Several species of adult caddisflies were found at urban streams where their larvae were absent. This suggests that poor in-stream conditions prevent larvae from surviving. However, both larvae and adults of other species of caddisflies typical of rural headwaters were absent from some urban headwaters. This suggests that impacts to the terrestrial, adult stage is likely preventing migrations to these urban sites, and this lack of migrants serves as another contributor to low larval diversity in urban streams. Patterns of adult assemblage composition were explained by regional land-use around urban streams and by species traits.