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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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Enter the Mosaic: Aquatic‐Terrestrial Reciprocal Fluxes and Food Webs Are Dynamically Interdependent Across Space and Through Time
ABSTRACT Decades‐old research describes dynamic interdependence among aquatic and terrestrial food webs, leading to calls for integrating cross‐ecosystem linkages with landscape ecology to evaluate dynamics of spatially‐subsidised food webs. Though development of meta‐community theory has suggested that such spatial dynamics may help sustain biodiversity, empirical data remain limited. In northern Yellowstone National Park, over a century of terrestrial wildlife dynamics, including the extirpation and subsequent reintroduction of wolves, have contributed to a habitat mosaic in which stream‐riparian ecosystems are dominated by either woody or herbaceous vegetation. In the context of this habitat mosaic, we addressed the overarching questions: (1) Are habitat mosaics associated with spatial and temporal variation in reciprocal fluxes and linked food webs and (2) how do biodiversity, organism traits and species interactions influence, and are they influenced by, that spatial and temporal variation?From 2019 to 2021, we intensively sampled eight headwater streams to characterise reciprocal fluxes of aquatic and terrestrial invertebrates and the patterns of potential responses by fish, birds, bats and spiders. We evaluated sites individually as well as how they contributed to a meta‐community.We found that local stream‐riparian ecosystems contributed to a mosaic in which reciprocal fluxes of invertebrates among local patches were asynchronous and tracked by both aquatic and terrestrial consumers in ways mediated by organism traits. Within sites, aquatic and terrestrial invertebrate fluxes were seasonally asynchronous with each other, but these patterns varied from site to site. Across the mosaic, comparisons of daily aquatic insect emergence varied from 25% to 167% among streams and did so variably throughout the year, revealing asynchronous dynamics created at the meta‐community scale. Daily inputs of terrestrial invertebrates were similarly asynchronous across the mosaic, varying from 14% to 170%. These asynchronies were positively correlated with invertebrate beta diversity and associated with varying riparian vegetation, stream temperature, and flow regimes. In turn, in situ consumers tracked the allochthonous invertebrate prey in ways that were mediated by site context (i.e., local habitat characteristics) and consumer traits (e.g., range, foraging strategy and breeding requirements).Based on these observations as an example, we infer there is not one way for food webs to be reciprocally and spatially linked, but multiple ways that can vary both across a spatial mosaic and through time. Our findings provide empirical evidence suggesting potential relationships between habitat complexity and the maintenance of biodiversity via aquatic‐terrestrial reciprocal fluxes and dynamic interdependence across mosaics.
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- PAR ID:
- 10651524
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Freshwater Biology
- Volume:
- 70
- Issue:
- 9
- ISSN:
- 0046-5070
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Ecosystems that are coupled by reciprocal flows of energy and nutrient subsidies can be viewed as a single “meta‐ecosystem.” Despite these connections, the reciprocal flow of subsidies is greatly asymmetrical and seasonally pulsed. Here, we synthesize existing literature on stream–riparian meta‐ecosystems to quantify global patterns of the amount of subsidy consumption by organisms, known as “allochthony.” These resource flows are important since they can comprise a large portion of consumer diets, but can be disrupted by human modification of streams and riparian zones. Despite asymmetrical subsidy flows, we found stream and riparian consumer allochthony to be equivalent. Although both fish and stream invertebrates rely on seasonally pulsed allochthonous resources, we find allochthony varies seasonally only for fish, being nearly three times greater during the summer and fall than during the winter and spring. We also find that consumer allochthony varies with feeding traits for aquatic invertebrates, fish, and terrestrial arthropods, but not for terrestrial vertebrates. Finally, we find that allochthony varies by climate for aquatic invertebrates, being nearly twice as great in arid climates than in tropical climates, but not for fish. These findings are critical to understanding the consequences of global change, as ecosystem connections are being increasingly disrupted.more » « less
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As terrestrial leaf litter decomposes in rivers, its constituent elements follow multiple pathways. Carbon leached as dissolved organic matter can be quickly taken up by microbes, then respired before it can be transferred to the macroscopic food web. Alternatively, this detrital carbon can be ingested and assimilated by aquatic invertebrates, so it is retained longer in the stream and transferred to higher trophic levels. Microbial growth on litter can affect invertebrates through three pathways, which are not mutually exclusive. First, microbes can facilitate invertebrate feeding, improving food quality by conditioning leaves and making them more palatable for invertebrates. Second, microbes can be prey for invertebrates. Third, microbes can compete with invertebrates for resources bound within litter and may produce compounds that retard carbon and nitrogen fluxes to invertebrates. As litter is broken down into smaller particles, there are many opportunities for its elements to reenter the stream food web. Here, I describe a conceptual framework for evaluating how traits of leaf litter will affect its fate in food webs and ecosystems that is useful for predicting how global change will alter carbon fluxes into and out of streams.more » « less
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Details can be found in the following manuscript. Contact author Brooks for further inquiries. Brooks, J. M., Baxter, C. V., Warren, D. R., & MacNeill, K. L. (2025). Enter the Mosaic: Aquatic‐Terrestrial Reciprocal Fluxes and Food Webs Are Dynamically Interdependent Across Space and Through Time. Freshwater Biology, 70(9), e70094.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1111/fwb.70094
