<p>Life-history diversity (portfolios) provides species resistance to severe declines during poor environmental conditions and resilience following favorable conditions. For endangered populations, altered habitat availability and connectivity challenge gene pool management, especially when life-history diversity is difficult to predict or observe, and conservation strategies target one life-history strategy. We developed a life cycle model to quantify potential portfolio benefits for an endangered steelhead Rainbow Trout (<i>Oncorhynchus mykiss</i>) population segment at the southern extent of its range where climate change, water availability, and habitat disconnect are major concerns. The model considers the contributions of three life-history variants (anadromous, resident, and perched) in dynamic environments, supporting variant interaction, specific life strategy adaptations, and climatic variability, including habitat connectivity. Through five diagnostic scenarios, we demonstrate model support for understanding demographic trends and potential management actions under variable climatic conditions, specifically how portfolios influence future extinction risk. We also show how population dynamics can be modeled by allowing life-history variants to interact. Modeled scenarios illustrate how each life-history variant may contribute to others and increase overall population resilience to habitat and climate shifts, highlighting population interactions and dynamics when all three variants are modeled simultaneously. This model underscores life-history diversity value in stabilizing population fluctuations, demonstrating that population dynamics result from composite process effects in freshwater, estuarine, and marine environments. Modeling life-history variants allows the representation of complex population behaviors critical for species resiliency, providing a tool to support population management and recovery and identifying future monitoring needs.</p>