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Abstract The body size of aquatic vertebrates is declining across populations and ecosystems worldwide owing to warmer water temperature and changing streamflow. In freshwaters, the effects of stream network position and density‐dependent factors on body size are less understood. We used an extensive dataset spanning 41 stream sites over 7 years to evaluate how density‐dependent and density‐independent factors influence the size of two top predators in small watersheds, Coastal Cutthroat TroutOncorhynchus clarkii clarkiiand Coastal Giant SalamandersDicamptodon tenebrosus. We tested three hypotheses of body‐size variation for trout and salamanders, including intraspecific density dependence, interspecific density dependence, and resource availability, using empirical observations in hierarchical linear mixed models in a model‐selection framework. In our best‐supported models, the strongest predictors of size were conspecific negative density dependence, as expected, suggesting greater intraspecific interactions probably owing to conspecific individuals having similar requirements. We reveal a biogeographic pattern in which body size peaks in middle stream‐network positions and plateaus or declines at lower and upper locations, proposing that stream network position also plays a role in determining body size in small watersheds. Salamander density also has a quadratic effect on adult trout size, with salamanders having a greater overall effect on the body size of both species than trout, suggesting that salamanders might be more dominant than trout in some interactions. Collectively, we found that biotic interactions, mainly conspecific but also interspecific, and stream‐network position affect trout and salamander body sizes in small watersheds.
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This content will become publicly available on July 12, 2026
Dietary niche partitioning between Coastal Giant Salamanders (Dicamptodon tenebrosus) and Coastal Cutthroat Trout (Oncorhynchus clarkii clarkii)
Understanding the mechanisms that enable species coexistence is essential for explaining community structure and biodiversity. We tested the hypothesis that dietary niche partitioning facilitates coexistence between two dominant stream predators in western North America: Coastal Giant Salamanders (Dicamptodon tenebrosus) and Coastal Cutthroat Trout (Oncorhynchus clarkii clarkii). These aquatic predators are important regulators of community dynamics and ecosystem processes in stream networks. We analyzed stomach contents from 81 salamanders and 96 trout collected via electrofishing in a 6-km section of Lookout Creek, Oregon, during low flow conditions in summer. We predicted that salamanders, primarily nocturnal benthic feeders, and trout, visual consumers of both terrestrial and aquatic prey, would exhibit distinct diets reducing direct diet overlap. We identified 4,897 prey items, classifying them into aquatic (50) and terrestrial (77) sources across 127 categories. Salamanders primarily preyed on aquatic invertebrates (Trichoptera, Ephemeroptera, and Plecoptera), while trout consumed a mix of terrestrial and aquatic invertebrates (Diptera, Trichoptera, and Plecoptera). Partial dietary overlap confirmed niche differentiation as a likely mechanism facilitating the coexistence of trout and salamanders. These findings highlight the role of dietary partitioning in structuring predator communities and inform predictions of how environmental changes may impact stream ecosystems.
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- Award ID(s):
- 2025755
- PAR ID:
- 10644587
- Publisher / Repository:
- Springer
- Date Published:
- Journal Name:
- Hydrobiologia
- ISSN:
- 0018-8158
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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