Predicting how rising temperatures will impact different species and communities is imperative and increasingly urgent with ongoing global warming. Here, we describe how thermal-death time curves obtained in the laboratory can be combined with an envelope model to predict the mortality of freshwater fish under field conditions and their distribution limits. We analyze the heat tolerance and distribution of 22 fish species distributed across North America and demonstrate that high temperatures imposed a distribution boundary for eleven of them, employing a null model. Importantly, predicted thermal boundaries closely match the warmest suitable locality of the envelope model. Simulated warming suggests that the distribution of fish species with lower heat tolerances will be disproportionately affected by rising temperatures, and the rate of local extinctions will be higher across fish communities in warmer localities. Ultimately, our analyses illustrate how physiological information can be combined with distribution models to forecast how warming temperatures are expected to impact different species and ecological communities.

Understanding how evolution and phenotypic plasticity contribute to variation in heat tolerance is crucial to predicting responses to warming. Here we analyze 272 thermal death time curves of 53 fish species acclimated to different temperatures and quantify their relative contributions. Analyses show that evolution and plasticity account, respectively, for 80.5 % and 12.4 % of the variation in elevation across curves, whereas their slope remained invariant. Evolutionary and plastic adaptive responses differ in magnitude, with heat tolerance increasing 0.54 ºC between species and 0.32 ºC within species for every 1 ºC increase in environmental temperatures. After successfully predicting critical temperatures under ramping conditions to validate these estimates, we show that fish populations can only partly ameliorate the impact of warming waters via thermal acclimation and this deficit in plasticity could increase as the warming accelerates.

Thermodynamics is a major factor determining rates of biochemical processes, rates of energy expenditure, and ultimately resilience to global warming in ectothermic organisms. Nonetheless, whether ectothermic organisms exhibit general adaptive metabolic responses to cope with different thermal conditions has remained a highly contentious subject over the past several decades. Here we combine a model comparison approach with a global dataset of standard metabolic rates (SMR), which include 1,160 measurements across 788 species of aquatic invertebrates, insects, fishes, amphibians, and reptiles, to investigate the association between metabolic rates and environmental temperatures in their respective habitats. Using Akaike’s information criterion (AICc) to compare different candidate models, our analyses suggest that variation in SMR after removing allometric and thermodynamic effects is best explained by the total temperature range encountered across seasons, which always provided a better fit than the average temperature for the hottest and coldest month, as well as mean annual temperatures. This pattern was consistent across taxonomic groups and robust to sensitivity analyses. Nonetheless, aquatic and terrestrial lineages responded differently to seasonality, with SMR declining –6.8 % ºC–1 of temperature variation in aquatic organisms and increasing 2.8 % ºC– 1 in terrestrial.  These responses may reflect alternative strategies to mitigate the impact of warmer temperatures on energy expenditure, either by means of metabolic reduction in thermally homogeneous water bodies or effective behavioral thermoregulation to exploit temperature heterogeneity on land.

Local adaptation is commonly cited to explain species distribution, but how fitness varies along continuous geographical gradients is not well understood. Here we combine thermal biology and life-history theory to demonstrate that Drosophila populations along a 2,500 km latitudinal cline are adapted to local conditions. We measured how heat tolerance and viability rate across 8 populations vary with temperature in the laboratory, and then simulated their expected cumulative Darwinian fitness employing high-resolution temperature data from their 8 collection sites. Simulations indicate a trade-off between annual survival and cumulative viability, as both mortality and the recruitment of new flies are predicted to increase in warmer regions. Importantly, populations are locally adapted and exhibit the optimal combination of both traits to maximize fitness where they live. In conclusion, our method is able to reconstruct fitness surfaces employing empirical life-history estimates and reconstructs peaks representing locally adapted populations, allowing to study geographic adaptation in silico.

Understanding how species adapt to different temperatures is crucial to predict their response to global warming, and thermal performance curves (TPCs) have been employed recurrently to study this topic. Nevertheless, fundamental questions regarding how thermodynamic constraints and evolution interact to shape TPCs in lineages inhabiting different environments remain unanswered. Here, we study Drosophila simulans along a latitudinal gradient spanning 3,000 km to test opposing hypotheses based on thermodynamic constraints (‘hotter-is-better’) versus biochemical adaptation (‘jack-of-all-temperatures’) as primary determinants of TPCs variation across populations. We compare thermal responses in metabolic rate and the egg-to-adult survival as descriptors of organismal performance and fitness, respectively, and show that different descriptors of TPCs vary in tandem with mean environmental temperatures, providing strong support to hotter-is-better. Thermodynamic constraints also resulted in a strong negative association between maximum performance and thermal breadth. Lastly, we show that descriptors of TPCs for metabolism and egg-to-adult survival are highly correlated, providing evidence of coadaptation and that curves for egg-to-adult survival are systematically narrower and displaced towards lower temperatures. Taken together, results support the pervasive role of thermodynamics constraining thermal responses in Drosophila populations along a latitudinal gradient, that are only partly compensated by evolutionary adaptation.

Antarctic marine animals face one of the most extreme thermal environments, characterized by a stable and narrow range of low seawater temperatures. At the same time, the Antarctic marine ecosystems are threatened by accelerated global warming. Determining the upper thermal limits (CTmax) is crucial to project the persistence and distribution areas of the Antarctic marine species. Using thermal death time curves (TDT), we estimated CTmax at different temporal scales from 1 minute to daily and seasonal, the predict vulnerability to the current thermal variation and two potential heatwave scenarios. Our results revealed that CTmax at 1 min are far from the temperature present in the marine intertidal area where our study species, showing Echinoderm species higher CTmax than the Chordata and Arthropods species. Simulations indicated that seasonal thermal variation from the intertidal zone contributed to basal mortality, which increased after considering moderate scenarios of heatwaves (+2 °C) in the Shetland Archipelago intertidal zone. Our finding highlighted the relevance of including exposure time explicitly on the CTmax estimates, which deliver closer and more realistic parameters according to the species that may be experiencing in the field.

Here we combined controlled experiments and field surveys to determine if estimates of heat tolerance predict distributional ranges and phenology of different Drosophila species in southern South America.  We contrasted thermal death time curves, which consider both magnitude and duration of the challenge to estimate heat tolerance, against the thermal range where populations are viable based on field surveys in an 8-yr longitudinal study.  We observed a strong correspondence of the physiological limits, the thermal niche for population growth, and the geographic ranges across studied species, which suggests that the thermal biology of different species provides a common currency to understand how species will respond to warming temperatures both at a local level and throughout their distribution range.  Our approach represents a novel analytical toolbox to anticipate how natural communities of ectothermic organisms will respond to global warming.

Phenotypic plasticity may increase performance and fitness and allow organisms to cope with variable environmental conditions. We studied within-generation plasticity and transgenerational effects of thermal conditions on temperature tolerance and demographic parameters in Drosophila melanogaster. We employed a fully factorial design, in which both parental (P) and offspring generations (F1) were reared in a constant or a variable thermal environment. Thermal variability during ontogeny increased heat tolerance in P, but with demographic cost as this treatment resulted in substantially lower survival, fecundity and net reproductive rate. The adverse effects of thermal variability (V) on demographic parameters were less drastic in flies from the F1, which exhibited higher net reproductive rates than their parents. These compensatory responses could not totally overcome the challenges of the thermally variable regime, contrasting with the offspring of flies raised in a constant temperature (C) that showed no reduction in fitness with thermal variation. Thus, the parental thermal environment had effects on thermal tolerance and demographic parameters in fruit-fly. These results demonstrate how transgenerational effects of environmental conditions on heat tolerance, as well as their potential costs on other fitness components, can have a major impact on populations’ resilience to warming temperatures and more frequent thermal extremes.