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Abstract Climate change is increasing the frequency, severity, and extent of wildfires and drought in many parts of the world, with numerous repercussions for the physical, chemical, and biological characteristics of streams. However, information on how these perturbations affect top predators and their impacts on lower trophic levels in streams is limited.The top aquatic predator in southern California streams is nativeOncorhynchus mykiss, the endangered southern California steelhead trout (trout). To examine relationships among the distribution of trout, environmental factors, and stream invertebrate resources and assemblages, we sampled pools in 25 stream reaches that differed in the presence (nine reaches) or absence (16 reaches) of trout over 12 years, including eight reaches where trout were extirpated during the study period by drought or post‐fire flood disturbances.Trout were present in deep pools with high water and habitat quality. Invertebrate communities in trout pools were dominated by a variety of medium‐sized collector–gatherer and shredder invertebrate taxa with non‐seasonal life cycles, whereas tadpoles and large, predatory invertebrates (Odonata, Coleoptera, Hemiptera [OCH]), often with atmospheric breather traits, were more abundant in troutless than trout pools.Structural equation modelling of the algal‐based food web indicated a trophic cascade from trout to predatory invertebrates to collector–gatherer taxa and weaker direct negative trout effects on grazers; however, both grazers and collector–gatherers also were positively related to macroalgal biomass. Structural equation modelling also suggested that bottom‐up interactions and abiotic factors drove the detritus‐based food web, with shredder abundance being positively related to leaf litter (coarse particulate organic matter) levels, which, in turn, were positively related to canopy cover and negatively related to flow. These results emphasise the context dependency of trout effects on prey communities and of the relative importance of top‐down versus bottom‐up interactions on food webs, contingent on environmental conditions (flow, light, nutrients, disturbances) and the abundances and traits of component taxa.Invertebrate assemblage structure changed from a trout to a troutless configuration within a year or two after trout were lost owing to post‐fire scouring flows or drought. Increases in OCH abundance after trout were lost were much more variable after drought than after fire. The reappearance of trout in one stream resulted in quick, severe reductions in OCH abundance.These results indicate that climate‐change induced disturbances can result in the extirpation of a top predator, with cascading repercussions for stream communities and food webs. This study also emphasises the importance of preserving or restoring refuge habitats, such as deep, shaded, perennial, cool stream pools with high habitat and water quality, to prevent the extirpation of sensitive species and preserve native biodiversity during a time of climate change.
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This content will become publicly available on February 22, 2026
Drought‐Induced Decreases in Abundance of Emergent Midge Subsidies Are Offset by Increased Body Size in a Prairie Stream
ABSTRACT Emerging aquatic insects can be an important resource subsidy for a variety of terrestrial consumers, including spiders, birds, bats and lizards. Emergence flux is influenced by a variety of abiotic and biotic variables, such as temperature, drying, and predators and these variables can also control the body size of emergent insects. Despite their importance, these variables can change rapidly during drought conditions as water temperatures rise, surface area decreases and predator densities increase.During 2018, the Konza Prairie Biological Station experienced a record drought: flow ceased in the lower reaches of Kings Creek for the first time in over 40 years of observation, leaving a series of isolated pools. We studied how the drought affected aquatic insect emergence in 12 of these pools via elevated temperatures, decreased surface area, and concentration of predators (e.g. fishes and crayfish) over a four‐week period. We returned in 2020 and sampled emergence in the same pools over 2 weeks under non‐drought conditions to compare emergence between drought and non‐drought conditions.We found three overall patterns: (1) rates of areal emergence abundance and biomass (number or mg DM m−2d−1) did not differ between drought and non‐drought conditions. In contrast, pool‐scale emergence abundance, but not biomass (number or mg DM pool−1d−1), was lower during drought conditions; (2) average midge body size was larger during the drought relative to the non‐drought conditions; (3) environmental variables (e.g. temperature, pool surface area, predator biomass) were not predictive of emergence during drought and non‐drought conditions.Fewer, but larger emergent midges (as seen under drought conditions) may represent a higher quality resource for terrestrial consumers than many smaller midges due to increased per‐capita energy yield. However, due to the overall decrease in water availability throughout the stream network, the overall emergence flux was concentrated in reaches with remaining water during the drought, making pools emergence subsidy hotspots. Overall, these contrasting responses underscore the complex nature of community responses to shifting climatic conditions.
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- Award ID(s):
- 2025849
- PAR ID:
- 10574623
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Freshwater Biology
- Volume:
- 70
- Issue:
- 2
- ISSN:
- 0046-5070
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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