Wildfire and drying legacies and stream invertebrate assemblages

Abstract

Climate change is engendering droughts and wildfires in many ecosystems, especially those in Mediterranean climates. Yet, there is little information on the long-term responses of stream invertebrates in Mediterranean climates to wildfire, particularly during and after extreme drought. To examine the effects of drought and wildfire on stream invertebrate assemblages, we sampled stream reaches in southern California with surface water in 2016, at the end of a 5-y drought, and stream reaches in 2017, after flows resumed or increased, including sites within (burned), outside (unburned), and downstream of footprints for fires occurring 8 to 10 y ago. Spatial and temporal variation in invertebrate assemblages were attributed to hydrological regimes, including stream drying during the drought, but there were few fire-legacy effects. At the end of the drought, invertebrate assemblages occupying remaining standing water with high solute, temperature, and benthic organic matter levels were dominated by tolerant, lentic taxa. After flows returned in spring 2017, all sites, including those that were dry in 2016, were dominated by quick-colonizing riffle taxa. Invertebrate densities increased into the subsequent summer at the same time that flows declined and temperatures and conductivities increased; however, sensitive cool-water taxa were more abundant at perennial, shaded sites, whereas tolerant, warm-water taxa were more abundant at shallow, open sites that had dried the year before. We observed negative relationships between riparian burning extent vs canopy cover (weak) and coarse particulate organic matter levels (strong); however, invertebrate assemblage structure was similar in basins that did and did not burn. Although invertebrate populations were severely reduced at 1 site whose basin had burned the year before, invertebrate assemblages had largely recovered after 10 post-fire y, and fire-legacy effects were dwarfed by hydrological variation, particularly stream drying during the drought. Our data also suggest the importance of protecting water supplies and riparian vegetation for perennial, shaded headwater reaches, which provide refuges from drought and wildfire for native biodiversity.