Habitat filtering and adult dispersal determine the taxonomic composition of stream insects in an urbanizing landscape

1. Suitability of the local habitat (‘habitat filtering’) and dispersal between stream reaches determines the composition of insect communities, and urban land use may affect both processes. While urban streams are often poor habitats for insects and dispersal between them is often hindered, conservation and restoration activities generally focus solely on the local (in-stream) environment. 2. We determined whether in-stream habitat filtering (a ‘local’ process) or dispersal between reaches (a ‘regional’ process) controlled assemblage composition in a landscape subject to ongoing urban development (‘urbanizing’). We compared models incorporating geographic distance between sites, environmental dissimilarity, and land-use/land-cover attributes of dispersal pathways in an attempt to explain the dissimilarity of stream insect assemblages. Distance and land-use/land-cover attributes were characterised along both overland (straight line) and corridor pathways. 3. Both in-stream habitat filtering and dispersal affected assemblage composition, but habitat had a stronger influence. Overland distance was a better predictor of assemblage dissimilarity than corridor distance, implying stream insect dispersal occurs across catchment boundaries as well as along stream corridors. The best model incorporated land-use/land-cover attributes along dispersal pathways, which supported the idea that urban land-use in the terrestrial environment mediates dispersal. 4. Conservation and restoration strategies for streams in urbanizing landscapes that focus on local habitat quality and ignore dispersal are missing a potentially important mechanism affecting insect assemblage composition. While the primary focus should be on maintaining or improving the local habitat, potential dispersal pathways should also be considered.