Wildfire and drying legacies and stream invertebrate assemblages

Trout and invertebrate assemblages in stream pools through wildfire and drought

“…Although we are concerned that our temperature measurements were made at only one time in each year, we also found no relationship between trout presence and the invertebrate thermal index, and long-term continuous temperature monitoring in two of our streams showed that average maximum temperatures approximated 20°C (Frazier, 2017). Although stream temperatures are often related to riparian canopy cover in this region (Cooper et al, 2015(Cooper et al, , 2021, we restricted our analyses to reaches with intact riparian vegetation. Isaak et al (2010) reported increases in stream temperatures with increased atmospheric temperatures and wildfires and decreased flows in a mountain stream network, but projected little impact on rainbow trout populations because of minimal effects on the extent and distribution of well-connected thermal habitat suitable for this species.…”

“…By contrast, our results suggest that bottom-up effects and relationships between abiotic and biotic variables accounted for the structure of the detrital-based food web with shredder abundance being positively related to leaf litter mass, which, in turn, was positively related to riparian canopy cover and negatively related to flow. Although there were direct effects of post-fire floods and drought on invertebrate assemblages via impacts on flow, sediment, and/or hydrochemical conditions (first axis in NMS analyses, see also Cooper et al, 2015Cooper et al, , 2021McMahon et al, 2023;Silins et al, 2014), changes in invertebrate and amphibian assemblages after disturbance also partly were owing to the local extirpation of trout, which led to increases in tadpoles and OCH taxa within a year or two after disturbance events, with possible repercussions for invertebrate primary consumers depending on disturbance impacts on algal resource levels (Cooper et al, 2015).…”

“…Our results show that post-fire scouring floods and prolonged droughts can extirpate sensitive species, such as the southern California steelhead trout, the top aquatic predator in southern California streams (Cooper, 1988;Cooper et al, 2015;National Marine Fisheries Service, 2012. The survival of trout and other sensitive species through these disturbances depends on protecting or restoring refuge habitats at the stream network scale, such as deep, shaded, cool stream pools with high habitat and water quality or, additionally in the case of anadromous steelhead, suitable estuarine and marine habitats (Alvarez & Dagit, 2019;Bogan et al, 2019;Cooper et al, 2021;Dagit et al, 2017;Kaylor et al, 2019;Lennox et al, 2019;National Marine Fisheries Service, 2012, 2023Robson et al, 2013). Further, the resiliency of sensitive species and stream communities to disturbances is inhibited by the construction and maintenance of barriers (e.g., dams and road crossings) that block salmonid migrations and intercept the downstream drift of stream organisms, impeding the recolonisation of disturbed habitats (Kukuła & Bylak, 2022).…”

“…By affecting flow conditions, sediment and solute inputs or accumulation, and riparian shading, wildfire and drought can have many impacts on stream communities and food webs (Gresswell, 1999; Lake, 2011; Verkaik et al., 2013). For example, post‐fire floods and sedimentation, and drying during drought, can greatly reduce most stream organisms with subsequent initial re‐colonisation by small, vagile species with short life cycles, and can increase light, nutrient, primary producer, and algivore levels and decrease coarse particulate organic matter (CPOM) and shredder levels, particularly where riparian vegetation is damaged or destroyed (Bixby et al., 2015; Cooper et al., 2015, 2021; McMahon et al., 2023; Silins et al., 2014). Although previous studies have focused on the effects of fire and drought on physical–chemical conditions in streams and, in turn, stream communities, there is less information on how these disturbances affect top predators and their impacts on the rest of the community (Ledger et al., 2013; McIntosh et al., 2018; Rodríguez‐Lozano et al., 2015; Woodward et al., 2016).…”

“…Arrows point from driver to response variables and double-headed arrows represent associations between variables. Although we have found a strong relationship between canopy cover and algal biomass in past studies (Cooper et al, 2015(Cooper et al, , 2021, those studies included sites where the canopy cover was reduced by riparian fire, whereas only shaded streams with intact riparian vegetation were examined in this study (canopy cover range: 65%-97%, mean = 89%). conditions would be more abundant in trout than troutless streams (sensitive mayfly, caddisfly, and snail taxa with small-medium body sizes and short life cycles; Figure 1b [Hemphill & Cooper, 1984]).…”