Post-fire debris flows delay recovery and create novel headwater stream macroinvertebrate communities

Wildfire is one of the most important global agents of disturbance affecting terrestrial and riparian vegetation. Post-fire vegetation changes can alter stream resource pathways and cause channel reorganization and sediment-laden debris flows. Yet, little is known about macroinvertebrate community recovery following wildfire and debris flows and how these communities fit into the broader stream community mosaic. We examined the effects of wildfire and debris flows on relative resource availability and macroinvertebrate assemblages at 31 streams in Idaho, USA using a space-for-time study design. Wildfire and debris flows had no apparent effects on resource standing crop. However, macroinvertebrate communities among unburned, burned, and debris flow streams were quite different. Compared to unburned streams, biomass and density were higher at streams which experienced debris flows ~ 10 years post fire, but exhibited the near-complete absence of macroinvertebrates at streams with more recent debris flows. Stream macroinvertebrate communities impacted by debris flows were distinct compared to unburned and burned streams which did not experience debris flows. When found, differences in macroinvertebrate biomass, density, richness, and community structures were largely due to the incidence of debris flows. Debris flows removed the riparian vegetation, slowing its recovery, cascading to affect macroinvertebrate community structure into the long term.