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 native 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.
Drought disturbances can have strong but variable effects on aquatic communities and little is understood about the impacts of drought, fragmentation and habitat reconnectivity on the dynamics of intermittent stream fish metacommunities. We performed two experiments using outdoor stream mesocosms to test the effects of drought‐mediated connectivity and habitat heterogeneity on realistic stream fish assemblages at local (pool/patch) and regional (stream unit) scales under non‐drought versus drought conditions (Exp1) and under drought conditions with ‘pulse’ flow connectivity versus ‘non‐pulse’ (Exp2). Survivorship, These experiments demonstrate that stream fish assemblages responded strongly to reduced flows and fragmentation, and that metacommunity dynamics were structured by differential emigration and immigration rates and directionality among heterogeneous and variably connected pool habitats.
- NSF-PAR ID:
- 10245125
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Freshwater Biology
- Volume:
- 61
- Issue:
- 4
- ISSN:
- 0046-5070
- Page Range / eLocation ID:
- p. 533-548
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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