Trait-based approaches may improve understanding in ecology by linking environmental variation to fitness-related characteristics of species. Most trait-environment studies focus on assemblage-level relationships; yet intraspecific trait variation is important for community, ecosystem, and evolutionary processes and has substantial implications for these approaches. Assessing population-level trait-environment relationships could test the generality of trait models while assessing intraspecific variation. We evaluated the generality of the trilateral life history model (TLHM of Winemiller and Rose 1992: opportunistic, periodic, and equilibrium endpoints) for fishes - a well-studied trait-environment model at the assemblage level - to populations of three stream fishes in the Midwestern United States in relation to flow regime. The TLHM adequately described major trade-offs in traits among populations in all species. Some TLHM flow-based predictions were confirmed, with periodic traits (high fecundity) favored at sites with greater flow seasonality and lower flow variability in two species, and equilibrium traits (large eggs) in more stable flow conditions in two species. Size at maturity was also inversely related to variability in one species. However, relationships contradicting the TLHM were also found. Coupled with the explanatory power of the TLHM for populations, supporting relationships suggest that synthesizing habitat template models with demographic life history theory could be valuable. Trait-environment models that are well-supported at multiple levels of biological organization could improve understanding of the impacts of environmental change on populations and communities and the valuable ecosystem services that they support.

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