Balancing heat, water and nutrients under environmental change: a thermodynamic niche framework

Abstract

Summary Models of the regulatory behaviour of organisms are fundamental to a strong physiologically‐based understanding of species' responses to global environmental change. Biophysical models of heat and water exchange in organisms (biophysical ecology) and nutritionally‐explicit models for understanding feeding behaviour and its fitness consequences (the Geometric Framework of nutrition, GF) are providing such an underpinning. However, temperature, water and nutrition interact in fundamental ways in influencing the responses of the organism to their environment, and a priority is to develop an integrated approach for conceptualising and measuring these interactions. Ideally, such an approach would be based on a thermodynamically‐formalized energy and mass budgeting approach that is sparsely parameterised and sufficiently general to apply across a range of situations and organisms. Here we illustrate how mass‐balance aspects of Dynamic Energy Budget theory can be applied to obtain first‐principles estimates of fluxes of O 2 , CO 2 , H 2 O and nitrogenous waste. Then, using an herbivorous lizard ( Egernia cunninghami ) as a case study, we demonstrate how these estimates can be integrated with heat/water exchange models and environmental data to provide a holistic understanding of how foraging strategy, food availability, habitat and weather interact with heat, water and nutrient/energy budgets across the life‐cycle. The analysis shows the potential importance of the water balance in affecting the energy budgets of ‘ dry skinned’ ectotherms, especially early in ontogeny, and highlights a significant gap in our knowledge of the physiological and behavioural traits that affect water balance when compared with our knowledge of thermal traits. In general, the modelling approach we describe can provide the thermodynamically‐constrained stage on which other evolutionary and ecological interactions play out; the ‘thermodynamic niche’. This in turn provides a solid foundation from which to tackle key questions about organismal responses to environmental change.