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Body size–dependent physiological effects of temperature influence individual growth, reproduction, and survival, which govern animal population responses to global warming. Considerable knowledge has been established on how such effects can affect population growth and size structure, but less is known of their potential role in temperature-driven adaptation in life-history traits. In this study, we 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 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 temperatures increases fitness through survival of small individuals and through the reproduction of larger individuals. In contrast, at high temperatures, 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 the temperature dependence of life-history traits, the DEB–IPM framework is useful for investigating the adaptation of size-structured organisms to warming.

The body size of an animal is probably its most important functional trait. For arthropods, environmental drivers of body size variation are still poorly documented and understood, especially in tropical regions. We use a unique dataset for two species‐rich, phylogenetically independent moth taxa (Lepidoptera: Geometridae; Arctiinae), collected along an extensive tropical elevational gradient in Costa Rica, to investigate the correlates and possible causes of body‐size variation. We studied 15 047 specimens (794 species) of Geometridae and 4167 specimens (308 species) of Arctiinae to test the following hypotheses: 1) body size increases with decreasing ambient temperature, as predicted by the temperature–size rule; 2) body size increases with increasing rainfall and primary productivity, as predicted from considerations of starvation resistance; and 3) body size scales allometrically with wing area, as elevation increases, such that wing loading (the ratio of body size to wing area) decreases with increasing elevation to compensate for lower air density. To test these hypotheses, we examined forewing length as a proxy for body size in relation to ambient temperature, rainfall, vegetation index and elevation as explanatory variables in linear and polynomial spatial regression models. We analysed our data separately for males and females using two principal approaches: mean forewing length of species at each site, and mean forewing length of complete local assemblages, weighted by abundance. Body size consistently increased with elevation in both taxa, both approaches, both sexes, and also within species. Temperature was the best predictor for this pattern (–0.98 < r < –0.74), whereas body size was uncorrelated or weakly correlated with rainfall and enhanced vegetation index. Wing loading increased with elevation. Our results support the temperature–size rule as an important mechanism for body size variation in arthropods along tropical elevational gradients, whereas starvation resistance and optimization of flight mechanics seem to be of minor importance.

Bergmann’s rule is the propensity for species-mean body size to decrease with increasing temperature. Temperature-dependent oxygen limitation has been hypothesized to help drive temperature–size relationships among ectotherms, including Bergmann’s rule, where organisms reduce body size under warm oxygen-limited conditions, thereby maintaining aerobic scope. Temperature-dependent oxygen limitation should be most pronounced among aquatic ectotherms that cannot breathe aerially, as oxygen solubility in water decreases with increasing temperature. We use phylogenetically explicit analyses to show that species-mean adult size of aquatic salamanders with branchial or cutaneous oxygen uptake becomes small in warm environments and large in cool environments, whereas body size of aquatic species with lungs (i.e., that respire aerially), as well as size of semiaquatic and terrestrial species do not decrease with temperature. We argue that oxygen limitation drives the evolution of small size in warm aquatic environments for species with aquatic respiration. More broadly, the stronger decline in size with temperature observed in aquatic versus terrestrial salamander species mirrors the relatively strong plastic declines in size observed previously among aquatic versus terrestrial invertebrates, suggesting that temperature-dependent oxygen availability can help drive patterns of plasticity, micro- and macroevolution.

Rising global temperatures force many species to shift their distribution ranges. However, whether or not (and how fast) such range shifts occur depends on species' dispersal capacities. In most ecological studies, dispersal‐related traits (such as the wing size or wing loading in insects) are treated as fixed, species‐specific characteristics, ignoring the important role of phenotypic plasticity during insect development. We tested the hypothesis that dispersal‐related traits themselves vary in dependence of ambient environmental conditions (temperature regimes, discharge patterns and biotic interactions during individual development). We collected data over 8 years from a natural population of the crane fly Tipula maxima in central Germany. Using linear mixed‐effect models, we analysed how phenotypic traits, phenological characteristics and population densities are affected by environmental conditions during the preceding 3, 6 and 12 months. We found a moderate (5.6%) increase in wing length per 1°C increase in mean annual temperatures during the previous year. At the same time, body weight increased by as much as 17.8% in females and 26.9% in males per 1°C, likely driven by increased habitat productivity, which resulted in a 16.4% (female) and 19.3% (male) increased wing loading. We further found a shorter, more synchronized emergence period (i.e. a narrower time frame for dispersal) with increasing temperatures. Altogether, our results suggest that dispersal abilities of T. maxima were negatively affected by elevated temperatures, and we discuss how similar patterns might affect the persistence of populations of other aquatic insects, especially stenoecious taxa with narrow distribution ranges. Our study calls for integration of information on temperature‐induced phenotypic plasticity of dispersal‐related traits into models forecasting range shifts in the face of climate change. Furthermore, the patterns reported here are likely to affect metapopulation dynamics of aquatic insects under climate change conditions and may contribute to the ongoing decline of insect biomass and diversity. The authors investigated the plastic development of dispersal‐related traits in a natural insect population. Their results suggest that dispersal abilities were negatively affected by elevated temperatures. Similar patterns might affect the persistence of populations of other aquatic insects under climate change conditions, especially stenoecious taxa with narrow distribution ranges.

1. The temperature-size rule (TSR) is a widespread phenomenon, which describes the pheno typic plastic response of species' size to temperature: individuals reared at colder temperatures mature as larger adults than at warmer temperatures. 2. The TSR is driven by an unequal thermal response of growth and development rates. However, we currently lack an understanding of how these rates change through ontogeny and their decoupling. Further, we do not know how this decoupling varies across generations. 3. Using the brine shrimp Artemia franciscana as a model, we examine growth and development rates through ontogeny at different temperatures across two generations. 4. The slopes of natural-logged weight-specific growth rates against temperature are steeper in earlier than later larval stages, indicating their greater temperature dependence, whereas development rates maintain the same temperature dependence across life stages. An inverse TSR is generated in early larval stages; the typical TSR (smaller size at warmer temperatures) is only established later in ontogeny. 5. Phase-specific temperature dependence of growth and development rates is not significantly different across the 1st and 2nd generation, suggesting the TSR is primarily a within-generation outcome. 6. Ontogenetic size responses in Artemia are compared to other crustacean species to identify patterns within this subphylum. Data for a range of crustaceans follow the same ontogenetic pattern: early larval stages show an inverse or no TSR, with TSR being only established in later stages. Adults often, but not always, show the greatest response.

Most ectotherms follow the temperature‐size rule (TSR): in cold environments individuals grow slowly but reach a large asymptotic length. Intraspecific competition can induce plastic changes of growth rate and asymptotic length and competition may itself be modulated by temperature. Our aim was to disentangle the joint effects of temperature and intraspecific competition on growth rate and asymptotic length. We used two distinct clonal lineages of the Collembola Folsomia candida, to describe thermal reaction norms of growth rate, asymptotic length and reproduction over six temperatures between 6 and 29°C. In parallel, we measured the long‐term size structure and dynamics of springtail populations reared under the same temperatures to measure growth rates and asymptotic lengths in populations and to quantify the joint effects of competition and temperature on these traits. We show that intraspecific competition modulates the temperature‐size rule. In dense populations there is a direct negative effect of temperature on asymptotic length, but there is no temperature dependence of the growth rate, the dominant factor regulating growth being competition. The two lineages responded differently to the joint effects of temperature and competition on growth and asymptotic size and these genetic differences have marked effects on population structure along our temperature gradient. Our results reinforce the idea that the TSR of ectotherms can be modulated by biotic and abiotic stressors when studied in non‐optimal laboratory experiments. Untangling complex interactions between the environment and demography will help to understand how growth trajectories respond to environmental change and how climate change may influence population size structure. A free Plain Language Summary can be found within the Supporting Information of this article. Résumé 1La plupart des ectothermes suivent la règle de la taille‐température (temperature‐size rule, TSR): dans les environnements froids, les individus grandissent lentement mais atteignent une grande longueur asymptotique. La compétition intraspécifique peut, elle aussi, affecter les taux de croissance et la longueur asymptotique. Et l'intensité de la compétition peut elle‐même être modulée par la température. Notre objectif ici est de démêler les effets conjoints de la température et de la compétition intraspécifique sur les taux de croissance et la longueur asymptotique. Nous avons utilisé deux lignées clonales du collembole Folsomia candida, pour décrire les normes de réaction thermiques du taux de croissance, de la longueur asymptotique et de la reproduction sur six températures entre 6° et 29°C. En parallèle, nous avons mesuré la structure en taille et la dynamique à long terme des populations de collemboles élevés aux mêmes six températures pour mesurer les taux de croissance, les longueurs asymptotiques et l'intensité de la compétition dans les populations afin de pouvoir quantifier les effets conjoints de la compétition et de la température sur ces deux traits (croissance et taille). Nous montrons que la compétition intraspécifique module la règle température‐taille : dans les populations denses, il y a un effet négatif direct de l'augmentation de température sur la longueur asymptotique, mais il n'y a pas de dépendance directe du taux de croissance à la température, le facteur dominant régulant la croissance étant la compétition. Les deux lignées ont répondu différemment aux effets conjoints de la température et de la compétition sur la croissance et la taille asymptotique et ces différences génétiques ont des effets marqués sur la structure des populations le long de notre gradient de température. Nos résultats renforcent l'idée que la réponse des ectothermes à la température peut être modulée par des facteurs de stress biotiques et abiotiques. Démêler les interactions complexes entre l'environnement (température) et la démographie (compétition) peut permettre de mieux comprendre comment les trajectoires de croissance peuvent répondre à des changements environnementaux et comment le changement climatique peut influencer la structure en taille des populations. A free Plain Language Summary can be found within the Supporting Information of this article.

Organisms of gigantic proportions inhabited the world at a time of a hyperoxic prehistoric atmosphere (Palaeozoic gigantism). Extant giants are found in cold polar waters, with large quantities of dissolved oxygen (polar gigantism). Oxygen is usually deemed central to explain such gigantism. Examples of one category of gigantism are often cited in support of the other, but novel insights into the bioavailability of oxygen imply that they cannot be taken as equivalent manifestations of the effect of oxygen on body size. Recently, the availability of oxygen has been shown to be lower in cold waters, despite greater oxygen solubility. Consequently, gigantism in cold, oxygenated waters and gigantism in an oxygen‐pressurized world are fundamentally different: Palaeozoic gigantism likely arose because of greater oxygen availability, while polar gigantism arises in spite of lower oxygen availability. The traditional view of respiration focuses on meeting the challenge of extracting sufficient amounts of oxygen, which essentially is a toxic gas. We present a broader perspective, which specifically includes risks of oxygen poisoning. We discuss how challenges pertaining to balancing oxygen uptake capacity and risks of oxygen poisoning are very different for animals breathing either air or water. We propose a novel explanation for polar gigantism in aquatic ectotherms, arguing that their larger body size represents a respiratory advantage that helps to overcome the larger viscous forces in water. Being large helps organisms to balance the opposing risks of asphyxiation and poisoning, especially in colder, more viscous, water. This results in a selection for larger sizes, with polar gigantism as the extreme manifestation. Hence, a larger size provides respiratory benefits to water‐breathing ectotherms, but not terrestrial ectotherms. This can explain why clines in body size across temperature and latitude are stronger in aquatic ectotherms.

Ecological rules such as Bergmann’s rule and the temperature–size rule state that body-size decline is a universal response to warm temperatures in both homeotherms and poikilotherms. In the present study, we investigated the biological responses of Nannophya koreana, an endangered dragonfly species in Korea, by comparing body size in two habitats with large differences in water temperature, Mungyong-si (MG, terraced paddy fields) and Muui-do (MU, a mountainous wetland). To conserve the dragonfly populations, the collected larvae were photographed and released, and their head widths and body lengths were measured. There was no difference in the annual mean air temperature and precipitation between the two sites; however, the annual mean water temperature was substantially lower in MU than in MG. There was little difference in larval head width between the two sites; however, body length in the MU population was smaller than that in the MG population. Larval growth rate per 100-degree-days was 0.75 mm for MG and 1.16 for MU. The relationship between temperature and body size of N. koreana larvae showed opposite trends to Bergmann’s rule and the temperature–size rule. Since the larval growth period during a year in MU was shorter than that in MG, the MU population potentially exhibits a higher growth rate as a mechanism of compensating for the low water temperature. Our study established the relationship between temperature and body size of N. koreana in two wetlands that had an obvious difference in water temperature despite being geographically close. The results highlight the importance of considering detailed factors such as habitat type when studying the temperature–size responses of organisms.

Trait‐based studies are needed to understand the plastic and genetic responses of organisms to warming. A neglected organismal trait is elemental composition, despite its potential to cascade into effects on the ecosystem level. Warming is predicted to shape elemental composition through shifts in storage molecules associated with responses in growth, body size and metabolic rate. Our goals were to quantify thermal response patterns in body composition and to obtain insights into their underlying drivers and their evolution across latitudes. We reconstructed the thermal response curves (TRCs) for body elemental composition [C (carbon), N (nitrogen) and the C:N ratio] of damselfly larvae from high‐ and low‐latitude populations. Additionally, we quantified the TRCs for survival, growth and development rates and body size to assess local thermal adaptation, as well as the TRCs for metabolic rate and key macromolecules (proteins, fat, sugars and cuticular melanin and chitin) as these may underlie the elemental TRCs. All larvae died at 36°C. Up to 32°C, low‐latitude larvae increased growth and development rates and did not suffer increased mortality. Instead, growth and development rates of high‐latitude larvae were lower and levelled off at 24°C, and mortality increased at 32°C. This latitude‐associated thermal adaptation pattern matched the ‘hotter‐is‐better’ hypothesis. With increasing temperatures, low‐latitude larvae decreased C:N, while high‐latitude larvae increased C:N. These patterns were driven by associated changes in N contents, while C contents did not respond to temperature. Consistent with the temperature‐size rule and the thermal melanism hypothesis, body size and melanin levels decreased with warming. While all traits and associated macromolecules (except for metabolic rate that showed thermal compensation) assumed to underlie thermal responses in elemental composition showed thermal plasticity, these were largely independent and none could explain the stoichiometric TRCs. Our results highlight that thermal responses in elemental composition cannot be explained by traditionally assumed drivers, asking for a broader perspective including the thermal dependence of elemental fluxes. Another key implication is that thermal evolution can reverse the plastic stoichiometric thermal responses and hence reverse how warming may shape food web dynamics through changes in body composition at different latitudes. Although warming is predicted to shape body stoichiometry through changes in growth, body size and metabolic rate mediated by changes in macromolecules, these predictions have never been comprehensively assessed. These results highlight the need for a broader perspective to explain thermal responses in body stoichiometry.

The observation that ectotherm size decreases with increasing temperature (temperature‐size rule; TSR) has been widely supported. This phenomenon intrigues researchers because neither its adaptive role nor the conditions under which it is realized are well defined. In light of recent theoretical and empirical studies, oxygen availability is an important candidate for understanding the adaptive role behind TSR. However, this hypothesis is still undervalued in TSR studies at the geographical level. We reanalyzed previously published data about the TSR pattern in diatoms sampled from Icelandic geothermal streams, which concluded that diatoms were an exception to the TSR. Our goal was to incorporate oxygen as a factor in the analysis and to examine whether this approach would change the results. Specifically, we expected that the strength of size response to cold temperatures would be different than the strength of response to hot temperatures, where the oxygen limitation is strongest. By conducting a regression analysis for size response at the community level, we found that diatoms from cold, well‐oxygenated streams showed no size‐to‐temperature response, those from intermediate temperature and oxygen conditions showed reverse TSR, and diatoms from warm, poorly oxygenated streams showed significant TSR. We also distinguished the roles of oxygen and nutrition in TSR. Oxygen is a driving factor, while nutrition is an important factor that should be controlled for. Our results show that if the geographical or global patterns of TSR are to be understood, oxygen should be included in the studies. This argument is important especially for predicting the size response of ectotherms facing climate warming. We reanalyzed previously published data about the TSR pattern in diatoms sampled from Icelandic geothermal streams. By applying the different approach of including the oxygen availability into data analysis, we found out, in contrast to the original authors' conclusion, that diatoms are not an exception from the temperature‐size rule. We suggest that including the oxygen into any size‐to‐temperature response studies would give a more valuable and complete pattern