Colonization potential of caddisflies (Trichoptera) in an urban headwater stream

Mitigation and restoration efforts to increase diversity of stream insect communities often focus on improving in-stream habitat quality for larvae. However, watershed urbanization may impact the adult stage and prevent colonization of restored streams. We compared the assemblage of adult Trichoptera at an urban headwater stream, with low larval richness, to larval populations found locally (in the main-stem downstream) and regionally (in other rural headwaters) to assess the potential sources of adult immigrants and to assess differences in life history characteristics between taxa that did and did not migrate to that urban headwater. We hypothesized that the majority of migrants to the urban headwater were also found in the main-stem and that those species that migrate from distant rural headwaters have life history characteristics indicating high dispersal ability. We found that the assemblage of adult Trichoptera at the urban headwater was not as taxa poor as the larval assemblage there, and adult taxa migrating to the urban headwater were mostly those found in the larger main-stem downstream. Headwater specialist species within the region that did not make it to the urban stream had life history characteristics indicative of poor dispersal ability. Thus, spatial proximity of source populations likely control immigration of adult caddisflies in urbanized watersheds more so than dispersal ability of individual taxa.