Thermal performance across levels of biological organization

Evolution of Thermal Sensitivity in Changing and Variable Climates

Evolutionary adaptation to temperature and climate depends on both the extent to which organisms experience spatial and temporal environmental variation (exposure) and how responsive they are to the environmental variation (sensitivity). Theoretical models and experiments suggesting substantial potential for thermal adaptation have largely omitted realistic environmental variation. Environmental variation can drive fluctuations in selection that slow adaptive evolution. We review how carefully filtering environmental conditions based on how organisms experience their environment and further considering organismal sensitivity can improve predictions of thermal adaptation. We contrast taxa differing in exposure and sensitivity. Plasticity can increase the rate of evolutionary adaptation in taxa exposed to pronounced environmental variation. However, forms of plasticity that severely limit exposure, such as behavioral thermoregulation and phenological shifts, can hinder thermal adaptation. Despite examples of rapid thermal adaptation, experimental studies often reveal evolutionary constraints. Further investigating these constraints and issues of timescale and thermal history are needed to predict evolutionary adaptation and, consequently, population persistence in changing and variable environments.

Use of complex physiological traits as ecotoxicological biomarkers in tropical freshwater fishes

We review the use of complex physiological traits, of tolerance and performance, as biomarkers of the toxicological effects of contaminants in subtropical and tropical freshwater fishes. Such traits are growing in relevance due to climate change, as exposure to contaminants may influence the capacity of fishes to tolerate and perform in an increasingly stressful environment. We review the evidence that the critical oxygen level, a measure of hypoxia tolerance, provides a valuable biomarker of impacts of diverse classes of contaminants. When coupled with measures of cardiorespiratory variables, it can provide insight into mechanisms of toxicity. The critical thermal maximum, a simple measure of tolerance of acute warming, also provides a valuable biomarker despite a lack of understanding of its mechanistic basis. Its relative ease of application renders it useful in the rapid evaluation of multiple species, and in understanding how the severity of contaminant impacts depends upon prevailing environmental temperature. The critical swimming speed is a measure of exercise performance that is widely used as a biomarker in temperate species but very few studies have been performed on subtropical or tropical fishes. Overall, the review serves to highlight a critical lack of knowledge for subtropical and tropical freshwater fishes. There is a real need to expand the knowledge base and to use physiological biomarkers in support of decision making to manage tropical freshwater fish populations and their habitats, which sustain rich biodiversity but are under relentless anthropogenic pressure.

Variable Environments in an Upwelling System Trigger Differential Thermal Sensitivity in a Low Intertidal Chiton

Environmental variability in coastal oceans associated with upwelling dynamics probably is one of the most pervasive forces affecting the physiological performance of marine life. As the environmental temperature is the abiotic factor with major incidence in the physiology and ecology of marine ectotherms, the abrupt temperature changes in upwelling systems could generate important variations in these organisms’ functional processes. The relationship between ambient temperature and physiological performance can be described through a thermal performance curve (TPC). The parameters of this curve usually show geographic variation usually is in accordance with the predictions of the climate variability hypothesis (CVH), which states that organisms inhabiting more variable environments should have broader ranges of environmental tolerance in order to cope with the fluctuating environmental conditions they experience. Here we study the effect generated by the environmental variability in an active upwelling zone on the physiological performance of the marine ectotherm Achanthopleura echinata. In particular, we compared the parameters of the TPC and the metabolic rate of two populations of A. echinata, one found in high semi-permanent upwelling (Talcaruca), while the other is situated in an adjacent area with seasonal upwelling (Los Molles) and therefore more stable environmental conditions. Our results show that: (1) oxygen consumption increases with body size and this effect is more significant in individuals from the Talcaruca population, (2) optimal temperature, thermal breadth, upper critical limit and maximum performance were higher in the population located in the area of high environmental heterogeneity and (3) individuals from Talcaruca showed greater variance in optimal temperature, thermal breadth, upper critical limit but not in maximum performance. Although it is clear that a variable environment affects the thermal physiology of organisms, expanding their tolerance ranges and generating energy costs in the performance of individuals, it is relevant to note that upwelling systems are multifactorial phenomena where the rise of water masses modifies not only temperature, but also decreases O2, pH, and increases pCO2 which in turn could modify metabolism and TPC.

Divergence in Thermal Physiology Could Contribute to Vertical Segregation in Intertidal Ecotypes of Littorina saxatilis

AbstractThermal stress is a potentially important selective agent in intertidal marine habitats, but the role that thermal tolerance might play in local adaptation across shore height has been underexplored. Northwest Spain is home to two morphologically distinct ecotypes of the periwinkle Littorina saxatilis, separated by shore height and subject to substantial differences in thermal stress exposure. However, despite other biotic and abiotic drivers of ecotype segregation being well studied, their thermal tolerance has not been previously characterized. We investigated thermal tolerance across multiple life history stages by employing the thermal death time (TDT) approach to determine (i) whether the two ecotypes differ in thermal tolerance and (ii) how any differences vary with life history stage. Adults of the two ecotypes differed in their thermal tolerance in line with their shore position: the upper-shore ecotype, which experiences more extreme temperatures, exhibited greater endurance of thermal stress compared with the lower-shore ecotype. This difference was most pronounced at the highest temperatures tested. The proximate physiological basis for these differences is unknown but likely due to a multifarious interaction of traits affecting different parts of the TDT curve. Differences in tolerance between ecotypes were less pronounced in early life history stages but increased with ontogeny, suggesting partial divergence of this trait during development. Thermal tolerance could potentially play an important role in maintaining population divergence and genetic segregation between the two ecotypes, since the increased thermal sensitivity of the lower-shore ecotype may limit its dispersal onto the upper shore and so restrict gene flow.

How useful are thermal vulnerability indices?

To forecast climate change impacts across habitats or taxa, thermal vulnerability indices (e.g., safety margins and warming tolerances) are growing in popularity. Here, we present their history, context, formulation, and current applications. We highlight discrepancies in terminology and usage, and we draw attention to key assumptions underpinning the main indices and to their ecological and evolutionary relevance. In the process, we flag biases influencing these indices that are not always evaluated. These biases affect both components of index formulations, namely: (i) the characterisation of the thermal environment; and (ii) an organism's physiological and behavioural responses to more frequent and severe warming. Presently, many outstanding questions weaken a thermal vulnerability index approach. We describe ways to validate vulnerability index applications and outline issues to be considered in further developing these indices.

Environment and phenology shape local adaptation in thermal performance

Populations within species often exhibit variation in traits that reflect local adaptation and further shape existing adaptive potential for species to respond to climate change. However, our mechanistic understanding of how the environment shapes trait variation remains poor. Here, we used common garden experiments to quantify thermal performance in eight populations of the marine snail Urosalpinx cinerea across thermal gradients on the Atlantic and the Pacific coasts of North America. We then evaluated the relationship between thermal performance and environmental metrics derived from time-series data. Our results reveal a novel pattern of 'mixed' trait performance adaptation, where thermal optima were positively correlated with spawning temperature (cogradient variation), while maximum trait performance was negatively correlated with season length (countergradient variation). This counterintuitive pattern probably arises because of phenological shifts in the spawning season, whereby 'cold' populations delay spawning until later in the year when temperatures are warmer compared to 'warm' populations that spawn earlier in the year when temperatures are cooler. Our results show that variation in thermal performance can be shaped by multiple facets of the environment and are linked to organismal phenology and natural history. Understanding the impacts of climate change on organisms, therefore, requires the knowledge of how climate change will alter different aspects of the thermal environment.

Climate sensitivity across latitude: scaling physiology to communities

While we know climate change will impact individuals, populations, and communities, we lack a cross-scale synthesis for understanding global variation in climate change impacts and predicting their ecological effects. Studies of latitudinal variation in individuals' thermal responses have developed primarily in isolation from studies of natural populations' warming responses. Further, it is unclear whether latitudinal variation in temperature-dependent population responses will manifest into latitudinal patterns in community stability. Integrating across scales, we discuss the key drivers of latitudinal variation in climate change effects, with the goal of identifying key pieces of information necessary to predict warming effects in natural communities. We propose two experimental approaches synthesizing latitudinal variability in climate change impacts across scales of biological organization.

Thermal Responses Differ across Levels of Biological Organization

Temperature is one of the most important environmental factors driving the genome-to-phenome relationship. Metabolic rates and related biological processes are predicted to increase with temperature due to the biophysical laws of chemical reactions. However, selection can also act on these processes across scales of biological organization, from individual enzymes to whole organisms. Although some studies have examined thermal responses across multiple scales, there is no general consensus on how these responses vary depending on the level of organization, or whether rates actually follow predicted theoretical patterns such as Arrhenius-like exponential responses or thermal performance curves (TPCs) that show peak responses. Here, we performed a meta-analysis on studies of ectotherms where biological rates were measured across the same set of temperatures, but at multiple levels of biological organization: enzyme activities, mitochondrial respiration, and/or whole-animal metabolic rates. Our final dataset consisted of 235 pairwise comparisons between levels of organization from 13 publications. Thermal responses differed drastically across levels of biological organization, sometimes showing completely opposite patterns. We developed a new effect size metric, "organizational disagreement" (OD) to quantify the difference in responses among levels of biological organization. Overall, rates at higher levels of biological organization (e.g., whole animal metabolic rates) increased more quickly with temperature than rates at lower levels, contrary to our predictions. Responses may differ across levels due to differing consequences of biochemical laws with increasing organization or due to selection for different responses. However, taxa and tissues examined generally did not affect OD. Theoretical TPCs, where rates increase to a peak value and then drop, were only rarely observed (12%), possibly because a broad range of test temperatures was rarely investigated. Exponential increases following Arrhenius predictions were more common (29%). This result suggests a classic assumption about thermal responses in biological rates is rarely observed in empirical datasets, although our results should be interpreted cautiously due to the lack of complete thermal profiles. We advocate for authors to explicitly address OD in their interpretations and to measure thermal responses across a wider, more incremental range of temperatures. These results further emphasize the complexity of connecting the genome to the phenome when environmental plasticity is incorporated: the impact of the environment on the phenotype can depend on the scale of organization considered.

Understanding how environmental factors influence reproductive aspects of wild myomorphic and hystricomorphic rodents

Myomorphic and hystricomorphic rodents are vital for maintaining various ecosystems around the planet. This review enables a better understanding of how these rodents respond to environmental factors and adapt to climate adversities. Innumerable factors, such as photoperiod, rainfall, and temperature, can impair or contribute to the quality of rodent reproductive parameters. Prolonged animal exposure to high ambient temperatures alters thermoregulation mechanisms and causes testicular and ovarian tissue degeneration and hormonal deregulation. Photoperiod influences the biological circannual rhythm and reproductive cycles of rodents because it strongly regulates melatonin secretion by the pineal gland, which modulates gonadotropic hormone secretion. Rainfall quantity directly regulates the abundance of fruits in an ecosystem, which modulates the reproductive seasonality of species which are most dependent on a seasonal fruit-based diet. Species with a more diversified fruit diet have smaller reproductive seasonality. As such, habitats are chosen by animals for various reasons, including the availability of food, sexual partners, intra-and inter-specific competition, and predation. This knowledge allows us to monitor and establish management plans to aid in conservation strategies for wild rodent species.

Fish heating tolerance scales similarly across individual physiology and populations

Extrapolating patterns from individuals to populations informs climate vulnerability models, yet biological responses to warming are uncertain at both levels. Here we contrast data on the heating tolerances of fishes from laboratory experiments with abundance patterns of wild populations. We find that heating tolerances in terms of individual physiologies in the lab and abundance in the wild decline with increasing temperature at the same rate. However, at a given acclimation temperature or optimum temperature, tropical individuals and populations have broader heating tolerances than temperate ones. These congruent relationships implicate a tight coupling between physiological and demographic processes underpinning macroecological patterns, and identify vulnerability in both temperate and tropical species.

Nicholas Payne et al. use physiological and population-level abundance data from 823 fish species to examine how heating tolerance scales at both the individual and population level. This study shows that heating tolerance declines in the lab and the wild at the same rate, and for a given temperature, individuals and populations from tropical areas have broader heating tolerances than temperate species.

The thermal tolerance of photosynthetic tissues: a global systematic review and agenda for future research

Understanding plant thermal tolerance is fundamental to predicting impacts of extreme temperature events that are increasing in frequency and intensity across the globe. Extremes, not averages, drive species evolution, determine survival and increase crop performance. To better prioritize agricultural and natural systems research, it is crucial to evaluate how researchers are assessing the capacity of plants to tolerate extreme events. We conducted a systematic review to determine how plant thermal tolerance research is distributed across wild and domesticated plants, growth forms and biomes, and to identify crucial knowledge gaps. Our review shows that most thermal tolerance research examines cold tolerance of cultivated species; c. 5% of articles consider both heat and cold tolerance. Plants of extreme environments are understudied, and techniques widely applied in cultivated systems are largely unused in natural systems. Lastly, we find that lack of standardized methods and metrics compromises the potential for mechanistic insight. Our review provides an entry point for those new to the methods used in plant thermal tolerance research and bridges often disparate ecological and agricultural perspectives for the more experienced. We present a considered agenda of thermal tolerance research priorities to stimulate efficient, reliable and repeatable research across the spectrum of plant thermal tolerance.

Shifts in the relative fitness contributions of fecundity and survival in variable and changing environments

Organisms respond to shifts in climate means and variability via distinct mechanisms. Accounting for these differential responses and appropriately aggregating them is central to understanding and predicting responses to climate variability and change. Separately considering fitness components can clarify organismal responses: fecundity is primarily an integrated, additive response to chronic environmental conditions over time via mechanisms such as energy use and acquisition, whereas survival can be strongly influenced by short-term, extreme environmental conditions. In many systems, the relative importance of fecundity and survival constraints changes systematically along climate gradients, with fecundity constraints dominating at high latitudes or altitudes (i.e. leading range edges as climate warms), and survival constraints dominating at trailing range edges. Incorporating these systematic differences in models may improve predictions of responses to recent climate change over models that assume similar processes along environmental gradients. We explore how detecting and predicting shifts in fitness constraints can improve our ability to forecast responses to climate gradients and change.

Climate impacts on organisms, ecosystems and human societies: integrating OCLTT into a wider context

Physiological studies contribute to a cause and effect understanding of ecological patterns under climate change and identify the scope and limits of adaptation. Across most habitats, this requires analyzing organism responses to warming, which can be modified by other drivers such as acidification and oxygen loss in aquatic environments or excess humidity or drought on land. Experimental findings support the hypothesis that the width and temperature range of thermal performance curves relate to biogeographical range. Current warming causes range shifts, hypothesized to include constraints in aerobic power budget which in turn are elicited by limitations in oxygen supply capacity in relation to demand. Different metabolic scopes involved may set the borders of both the fundamental niche (at standard metabolic rate) and the realized niche (at routine rate). Relative scopes for aerobic performance also set the capacity of species to interact with others at the ecosystem level. Niche limits and widths are shifting and probably interdependent across life stages, with young adults being least thermally vulnerable. The principles of thermal tolerance and performance may also apply to endotherms including humans, their habitat and human society. Overall, phylogenetically based comparisons would need to consider the life cycle of species as well as organism functional properties across climate zones and time scales. This Review concludes with a perspective on how mechanism-based understanding allows scrutinizing often simplified modeling approaches projecting future climate impacts and risks for aquatic and terrestrial ecosystems. It also emphasizes the usefulness of a consensus-building process among experimentalists for better recognition in the climate debate.

The role of mechanistic physiology in investigating impacts of global warming on fishes

Warming of aquatic environments as a result of climate change is already having measurable impacts on fishes, manifested as changes in phenology, range shifts and reductions in body size. Understanding the physiological mechanisms underlying these seemingly universal patterns is crucial if we are to reliably predict the fate of fish populations with future warming. This includes an understanding of mechanisms for acute thermal tolerance, as extreme heatwaves may be a major driver of observed effects. The hypothesis of gill oxygen limitation (GOL) is claimed to explain asymptotic fish growth, and why some fish species are decreasing in size with warming; but its underlying assumptions conflict with established knowledge and direct mechanistic evidence is lacking. The hypothesis of oxygen- and capacity-limited thermal tolerance (OCLTT) has stimulated a wave of research into the role of oxygen supply capacity and thermal performance curves for aerobic scope, but results vary greatly between species, indicating that it is unlikely to be a universal mechanism. As thermal performance curves remain important for incorporating physiological tolerance into models, we discuss potentially fruitful alternatives to aerobic scope, notably specific dynamic action and growth rate. We consider the limitations of estimating acute thermal tolerance by a single rapid measure whose mechanism of action is not known. We emphasise the continued importance of experimental physiology, particularly in advancing our understanding of underlying mechanisms, but also the challenge of making this knowledge relevant to the more complex reality.

Thermal performance of the Chagas disease vector, Triatoma infestans, under thermal variability

Vector-borne diseases (VBD) are particularly susceptible to climate change because most of the diseases' vectors are ectotherms, which themselves are susceptible to thermal changes. The Chagas disease is one neglected tropical disease caused by the protozoan parasite, Trypanosoma cruzi. One of the main vectors of the Chagas disease in South America is Triatoma infestans, a species traditionally considered to be restricted to domestic or peridomestic habitats, but sylvatic foci have also been described along its distribution. The infestation of wild individuals, together with the projections of environmental changes due to global warming, urge the need to understand the relationship between temperature and the vector's performance. Here, we evaluated the impact of temperature variability on the thermal response of T. infestans. We acclimated individuals to six thermal treatments for five weeks to then estimate their thermal performance curves (TPCs) by measuring the walking speed of the individuals. We found that the TPCs varied with thermal acclimation and body mass. Individuals acclimated to a low and variable ambient temperature (18°C ± 5°C) exhibited lower performances than those individuals acclimated to an optimal temperature (27°C ± 0°C); while those individuals acclimated to a low but constant temperature (18°C ± 0°C) did not differ in their maximal performance from those at an optimal temperature. Additionally, thermal variability (i.e., ± 5°C) at a high temperature (30°C) increased performance. These results evidenced the plastic response of T. infestans to thermal acclimation. This plastic response and the non-linear effect of thermal variability on the performance of T. infestans posit challenges when predicting changes in the vector's distribution range under climate change.

Comparative characterization of the hemocyanin-derived phenol oxidase activity from spiders inhabiting different thermal habitats

Enzymes adapted to cold temperatures are commonly characterized for having higher Michaelis-Menten constants (K_M) values and lower optimum and denaturation temperature, when compared to other meso or thermophilic enzymes. Phenoloxidase (PO) enzymes are ubiquitous in nature, however, they have not been reported in spiders. It is the oxygen carrier protein hemocyanin (Hc), found at high concentrations in their hemolymph, which displays an inducible PO activity. Hence, we hypothesize that Hc-derived PO activity could show features of cold adaptation in alpine species. We analyzed the Hc from two species of Theraphosidae from different thermal environments: Euathlus condorito (2400 m a.s.l.) and Grammostola rosea (500 m a.s.l.). Hc was purified from the hemolymph of both spiders and was characterized by identifying subunit composition and measuring the sodium dodecyl sulfate (SDS)-induced PO activity. The high-altitude spider Hc showed higher PO activity under all conditions and higher apparent Michaelis-Menten constant. Moreover, the optimum temperature for PO activity was lower for E. condorito Hc. These findings suggest a potential adaptation at the level of Hc-derived PO activity in Euathlus condorito, giving insights on possible mechanisms used by this mygalomorph spider to occupy extremes and variable thermal environments.

Thermal performance curves reveal shifts in optima, limits and breadth in early life

Understanding thermal performance at life stages that limit persistence is necessary to predict responses to climate change, especially for ectotherms whose fitness (survival and reproduction) depends on environmental temperature. Ectotherms often undergo stage-specific changes in size, complexity and duration that are predicted to modify thermal performance. Yet performance is mostly explored for adults, while performance at earlier stages that typically limit persistence remains poorly understood. Here, we experimentally isolate thermal performance curves at fertilization, embryo development and larval development stages in an aquatic ectotherm whose early planktonic stages (gametes, embryos and larvae) govern adult abundances and dynamics. Unlike previous studies based on short-term exposures, responses with unclear links to fitness or proxies in lieu of explicit curve descriptors (thermal optima, limits and breadth), we measured performance as successful completion of each stage after exposure throughout, and at temperatures that explicitly capture curve descriptors at all stages. Formal comparisons of descriptors using a combination of generalized linear mixed modelling and parametric bootstrapping reveal important differences among life stages. Thermal performance differs significantly from fertilization to embryo development (with thermal optimum declining by ∼2°C, thermal limits shifting inwards by ∼8-10°C and thermal breadth narrowing by ∼10°C), while performance declines independently of temperature thereafter. Our comparisons show that thermal performance at one life stage can misrepresent performance at others, and point to gains in complexity during embryogenesis, rather than subsequent gains in size or duration of exposure, as a key driver of thermal sensitivity in early life.

Thermal effects vary predictably across levels of organization: empirical results and theoretical basis

Thermal performance curves have provided a common framework to study the impact of temperature in biological systems. However, few generalities have emerged to date. Here, we combine an experimental approach with theoretical analyses to demonstrate that performance curves are expected to vary predictably with the levels of biological organization. We measured rates of enzymatic reactions, organismal performance and population viability in Drosophila acclimated to different thermal conditions and show that performance curves become narrower with thermal optima shifting towards lower temperatures at higher levels or organization. We then explain these results on theoretical grounds, showing that this pattern reflects the cumulative impact of asymmetric thermal effects that piles up with complexity. These results and the proposed framework are important to understand how organisms, populations and ecological communities might respond to changing thermal conditions.

Effects of temperature on feeding and digestive processes in fish

As most fish are ectotherms, their physiology is strongly affected by temperature. Temperature affects their metabolic rate and thus their energy balance and behavior, including locomotor and feeding behavior. Temperature influences the ability/desire of the fish to obtain food, and how they process food through digestion, absorb nutrients within the gastrointestinal tract, and store excess energy. As fish display a large variability in habitats, feeding habits, and anatomical and physiological features, the effects of temperature are complex and species-specific. The effects of temperature depend on the timing, intensity, and duration of exposure as well as the speed at which temperature changes occur. Whereas acute short-term variations of temperature might have drastic, often detrimental, effects on fish physiology, long-term gradual variations might lead to acclimation, e.g. variations in metabolic and digestive enzyme profiles. The goal of this review is to summarize our current knowledge on the effects of temperature on energy homeostasis, with specific focus on metabolism, feeding, digestion, and how fish are often able to "adapt" to changing environments through phenotypic and physiological changes.

Physiological diversity, biodiversity patterns and global climate change: testing key hypotheses involving temperature and oxygen

Documenting and explaining global patterns of biodiversity in time and space have fascinated and occupied biologists for centuries. Investigation of the importance of these patterns, and their underpinning mechanisms, has gained renewed vigour and importance, perhaps becoming pre-eminent, as we attempt to predict the biological impacts of global climate change. Understanding the physiological features that determine, or constrain, a species' geographical range and how they respond to a rapidly changing environment is critical. While the ecological patterns are crystallizing, explaining the role of physiology has just begun. The papers in this volume are the primary output from a Satellite Meeting of the Society of Experimental Biology Annual Meeting, held in Florence in July 2018. The involvement of two key environmental factors, temperature and oxygen, was explored through the testing of key hypotheses. The aim of the meeting was to improve our knowledge of large-scale geographical differences in physiology, e.g. metabolism, growth, size and subsequently our understanding of the role and vulnerability of those physiologies to global climate warming. While such an aim is of heuristic interest, in the midst of our current biodiversity crisis, it has an urgency that is difficult to overstate. This article is part of the theme issue 'Physiological diversity, biodiversity patterns and global climate change: testing key hypotheses involving temperature and oxygen'.