Food web simplification in a highly urbanized stream

Urbanization has been shown to reduce invertebrate species richness and diversity in stream ecosystems.  We determined how this decreased richness and diversity affects stream food webs.  We sampled invertebrates and food resources for invertebrate community assessment and stable isotope analysis (13C and 15N) of food webs in a forested and an urban stream during spring, summer, and autumn 2008.  As expected, food webs in the urban stream contained fewer invertebrate taxa, and taxa present in the urban stream were typically pollution tolerant.  Functional feeding group analysis revealed that the relative abundance and biomass of filter feeder and collector taxa was greater in the urban stream, while shredder abundance was greater in the forested stream.  As a result, trophic pathways using terrestrial leaf litter were less important in the urban stream, and this was offset by a higher utilization of epilithic periphyton in the urban stream.  Additionally, food webs in the urban stream were noticeably simplified and typically only contained two trophic levels with virtually no predator taxa.  In contrast, the forested stream contained several predator taxa from a wide range of taxonomic groups.