Optimal energy allocation trade-off driven by size-dependent physiological and demographic responses to warming.

Body size dependent physiological effects of temperature influence individual growth, reproduction and survival, which govern animal population responses to global warming. There is much knowledge of how such effects can affect population growth and size structure, but less of their potential role in temperature-driven adaptation in life history traits. In this study, we therefore ask how warming affects the optimal allocation of energy between growth and reproduction, and disentangle the underlying fitness trade-offs. To this end, we develop a novel dynamic energy budget integral projection model (DEB-IPM), linking individuals' size- and temperature dependent consumption and maintenance via somatic growth, reproduction and size-dependent energy allocation, to emergent population responses. At the population level, we calculate the long-term population growth rate (fitness) and stable size structure, emerging from the demographic processes. Applying the model to an example of pike (Esox lucius), we find that optimal energy allocation to growth decreases with warming. Furthermore, we demonstrate how growth, fecundity and survival contribute to this change in optimal allocation. Higher energy allocation to somatic growth at low temperature increases fitness through survival of small individuals, and through reproduction of larger individuals. In contrast, at high temperature, increased allocation to reproduction is favored because warming induces faster somatic growth of small individuals and increased fecundity, but reduced growth and higher mortality of larger individuals. Reduced optimum allocation to growth leads to further reductions in body size and an increasingly truncated population size structure with warming. Our study demonstrates how, by incorporating general physiological mechanisms driving temperature-dependence of life history traits, the DEB-IPM framework is useful for investigating adaptation of size-structured organisms to warming. This article is protected by copyright. All rights reserved.