Resistance and resilience of stream insect communities to repeated hydrologic disturbances after a wildfire

Vieira, Nicole; Clements, William; Guevara, Lynette; Jacobs, Brian

1. Wildfires are often followed by severe, sediment-laden floods in burned catchments. In this study, we documented resistance and resilience of stream insect communities to repeated post-fire flash floods in a ‘burned stream’. We employed a before-after-control-impact (BACI) design, where communities in comparable reaches of a burned stream and a reference stream were sampled from 2 years before, to 6 years after, a crown wildfire in north-central New Mexico. 2. The first 100-year flood following the 1996 Dome wildfire reduced total insect density and taxon richness to near zero in the burned stream. Despite showing low resistance, density returned rapidly to pre-ire levels because of colonization by simuliids, chironomids and the mayfly Baetis tricaudatus. In general, taxa that were generalist feeders (collectors) with strong larval dispersal dominated communities in early post-fire years with repeated, moderate flash floods. 3. Taxon richness and community composition were less resilient to post-fire hydrologic disturbances. Taxon richness did not recover until floods dampened 4 years after the fire. Despite hydrologic recovery, composition in the burned stream still differed from pre-fire and reference stream compositions after 6 years post-fire. A unique assemblage, dominated by taxa with strong larval or adult dispersal, was established after flash floods abated. Specialist feeders (shredders and grazers) that were common in pre-fire years were reduced or absent in the post-fire assemblage. 4. Community succession in the burned stream was explained by the interaction between species traits, geographic barriers to colonization and hydrologic conditions after the fire. Comparable changes in insect density, taxon richness, community composition and trait representation were not found in the reference stream, providing strong evidence that repeated post-fire flash floods shaped community responses in the burned stream.