Summary1. The role of nutrition in linking animals with their environment is increasingly seen as fundamental to explain ecological interactions. 2. The two currently predominant frameworks for exploring questions in nutritional ecologyNutritional Geometry (NG) and Ecological Stoichiometry (ES) -share common features, but also differ in their goals and origins. NG originates from behavioural ecology using terrestrial insects as model organisms in tightly controlled feeding experiments, while ES originates from biogeochemistry focusing on the transfer of key elements across trophic levels, mainly in aquatic environments. 3. Here, we review the history of these two complementary frameworks, emphasizing the key concepts defining their respective aims, methodologies and focal taxa to answer questions at different ecological scales. 4. We identify and explore homeostasis as a shared conceptual cornerstone of each framework that can be used to bridge knowledge gaps and for developing new hypotheses within nutritional ecology. 5. Expanding on the concept of homeostasis, we introduce dynamic energy budget (DEB) models as a general way to address homeostatic regulation at its fundamental level. 6. Specifically, we describe how a two-reserve DEB model can be used to track metabolic pathways of nutrients as well as elements and suggest that multi-reserve DEB models, when integrated and parameterized with NG and ES concepts, can form powerful components of agent-based models to predict how animal nutrition influences individual and trophic interactions in food webs.

Abstract:

Mechanistic forecasts of how species will respond to climate change are highly desired but difficult to achieve. Because processes at different scales are explicit in such models, careful assessments of their predictive abilities can provide valuable insights that will be relevant to functionally similar species. However, there are surprisingly few comprehensive field tests of mechanistic niche models in the literature. We applied a general, thermodynamically grounded modeling framework to determine the fundamental niche of an extremely well‐studied herbivorous ectotherm, the sleepy lizard Tiliqua rugosa. We then compared the model predictions with detailed long‐term field observations that included sub‐hourly data on microclimate, activity levels, home ranges, and body temperatures as well as annual to decadal patterns of body condition and growth. Body temperature predictions inferred from gridded climatic data were within 10% of empirically observed values and explained >70% of observed daytime activity patterns across all lizards. However, some periods of activity restriction were explained by predicted desiccation level rather than by temperature, and metabolically driven activity requirements were much lower than potential activity time. Decadal trajectories of field growth and body condition could also be explained to within 10% of observed values, with the variance in trajectories being attributable to whether individuals had access to permanent water. Continent‐wide applications of the model partly captured the inland distribution limit, but only after accounting for water limitations. Predicted changes in habitat suitability under six climate change scenarios were generally positive within the species’ current range, but varied strongly with predicted rainfall. Temperature is regarded as the major factor that will restrict the distribution and abundance of lizards and other terrestrial ectotherms under climate change. Yet our findings show how water can be more important than temperature in constraining the activity, habitat requirements, and distribution limits of terrestrial ectotherms. Our results demonstrate the feasibility of first‐principles computation of the climatic limits on terrestrial animals from gridded environmental data, providing a coherent picture for how species will respond to climate change at different scales of space and time.

An understanding of the direct links between animals and their environment can offer insights into the drivers and constraints to animal movement. Constraints on movement interact in complex ways with the physiology of the animal (metabolism) and physical environment (food and weather), but can be modelled using physical principles of energy and mass exchange. Here, we describe a general, spatially explicit individual‐based movement model that couples a nutritional energy and mass budget model (dynamic energy budget theory) with a biophysical model of heat exchange. This provides a highly integrated method for constraining an ectothermic animal's movement in response to how food and microclimates vary in space and time. The model uses r to drive a NetLogo individual‐based model together with microclimate and energy‐ and mass‐budget modelling functions from the r package “NicheMapR”. It explicitly incorporates physiological and morphological traits, behavioural thermoregulation, movement strategies and movement costs. From this, the model generates activity budgets of foraging and shade‐seeking, home range behaviour, spatial movement patterns and life history consequences under user‐defined configurations of food and microclimates. To illustrate the model, we run simulations of the Australian sleepy lizard Tiliqua rugosa under different movement strategies (optimising or satisficing) in two contrasting habitats of varying food and shade (sparse and dense). We then compare the results with real, fine‐scale movement data of a wild population throughout the breeding season. Our results show that (1) the extremes of movement behaviour observed in sleepy lizards are consistent with feeding requirements (passive movement) and thermal constraints (active movement), (2) the model realistically captures majority of the distribution of observed home range size, (3) both satisficing and optimising movement strategies appear to exist in the wild population, but home range size more closely approximates an optimising strategy, and (4) satisficing was more energetically efficient than optimising movement, which returned no additional benefit in metabolic fitness outputs. This framework for predicting physical constraints to individual movement can be extended to individual‐level interactions with the same or different species and provides new capabilities for forecasting future responses to novel resource and weather scenarios.

Research on herbivory defence often focuses on leaf chemistry but less on how plant mechanical properties like leaf veins deter herbivores. Herbivores often eat tough, complex plant tissue, yet how mechanical properties affect feeding performance as the consumer grows is unclear. We measured the toughness and strength of five types of leaf tissue -the midrib, the secondary and marginal veins and the lamina inside (inner) and outside (outer) the marginal vein -in mature Eucalyptus viminalis and Eucalyptus ovata leaves with punch tests. Leaf veins were, on average, 6.2 times tougher than lamina. Marginal veins were uniformly strong and tough along the leaf body, while midribs were less strong and secondary veins less tough toward leaf tips. We correlated the force required to puncture leaf tissue with the feeding performance of a chewing insect herbivore (the spiny leaf insect, Extatosoma tiaratum (Phasmida)) across four instar stages to explore the role of tough leaf veins as potential feeding barriers. Larvae more often ate less tough leaf tips and tougher tissue as they grew. However, younger larvae were capable of penetrating the tough marginal vein when starved. We suggest tough leaf veins and consumer position along the leaf body influence insect herbivore feeding performance over their lifetime.