Effects of wildfire of varying severity on benthic stream insect assemblages and emergence

The effects of wildfire can alter the structure of stream insect assemblages. Post-fire shifts to dominance by r-strategist taxa could drive increases in productivity of primary consumer and predatory insects, but this possibility has not been explicitly investigated. Likewise, the extent and duration of such effects might be mediated by fire severity, but this hypothesis also has not been evaluated. We report results from a comparative study that examined the mid-term (5-10 y post-fire) effects of wildfire of varying severity on stream insect assemblage composition and productivity measured in terms of benthic larval biomass and flux of emerging adults. We compared benthos and emergence in a suite of 2(nd)- to 3(rd)-order, unburned streams to those that had experienced low-severity and high-severity wildfire in wilderness watersheds of central Idaho. Reaches that experienced high-severity burn had the greatest biomass of r-strategist, generalist primary consumers that included Chironomidae, Baetis, and Simuliidae. The greatest biomass of predatory insects, such as Rhyacophila, occurred in reaches that experienced high-severity burn. Differences in composition of emerging insects were more pronounced in some time periods than others, with greatest emergence from high-severity reaches in early summer. High-severity reaches consistently had the greatest emergence flux. Reaches that experienced low-severity burn had the least emergence, and unburned reaches had intermediate emergence flux. Our results suggest that burn severity might drive differences in aquatic insect assemblages and their productivity.