Upper thermal limits in terrestrial ectotherms: how constrained are they?

Daily dynamics of ground-dwelling invertebrate communities during and following an extreme high-temperature event in summer 2022, China

The recent increase in the frequency of extreme weather events and declining soil biodiversity in global agricultural ecosystems make it crucial to assess the daily dynamics of soil communities in fields. To elucidate the daily dynamics of ground-dwelling invertebrate communities, their communities were monitored temporally using infrared camera traps in field farmland during and following an extremely high-temperature (EHT) event in summer 2022 in Ningbo City, China. Nine taxa and 1,147 individuals of the ground-dwelling invertebrate community were photographed in the 176,256 images. There were no significant differences in the taxonomic richness and abundance of the total ground-dwelling invertebrate communities during and following the EHT event. The abundance of ants was significantly decreased following the EHT event, whereas the abundance of other taxa was not. Significantly daily dynamics and obvious differences between each day in taxonomic richness, abundance of ground-dwelling invertebrate community, and abundance of each taxon were not observed during and following the EHT event. The results of this study showed that the daily dynamics of richness and abundance of the ground-dwelling invertebrate community and the abundance of each taxon were not significant during and following the EHT event. Overall, this study provides a useful monitoring method to observe the daily dynamics of ground-dwelling invertebrates in field farmlands and suggests that the daily dynamics of soil fauna communities should be further studied when assessing the effects of climate change on soil biodiversity.

Transiently restricting individual amino acids protects Drosophila melanogaster against multiple stressors

Nutrition and resilience are linked, though it is not yet clear how diet confers stress resistance or the breadth of stressors that it can protect against. We have previously shown that transiently restricting an essential amino acid can protect Drosophila melanogaster against nicotine poisoning. Here, we sought to characterize the nature of this dietary-mediated protection and determine whether it was sex, amino acid and/or nicotine specific. When we compared between sexes, we found that isoleucine deprivation increases female, but not male, nicotine resistance. Surprisingly, we found that this protection afforded to females was not replicated by dietary protein restriction and was instead specific to individual amino acid restriction. To understand whether these beneficial effects of diet were specific to nicotine or were generalizable across stressors, we pre-treated flies with amino acid restriction diets and exposed them to other types of stress. We found that some of the diets that protected against nicotine also protected against oxidative and starvation stress, and improved survival following cold shock. Interestingly, we found that a diet lacking isoleucine was the only diet to protect against all these stressors. These data point to isoleucine as a critical determinant of robustness in the face of environmental challenges.

Variability in thermal tolerance of clutches from different mothers indicates adaptation potential to climate warming in sea turtles

The current climate warming is a challenge to biodiversity that could surpass the adaptation capacity of some species. Hence, understanding the means by which populations undergo an increase in their thermal tolerance is critical to assess how they could adapt to climate warming. Specifically, sea turtle populations could respond to increasing temperatures by (1) colonizing new nesting areas, (2) nesting during cooler times of the year, and/or (3) by increasing their thermal tolerance. Differences in thermal tolerance of clutches laid by different females would indicate that populations have the potential to adapt by natural selection. Here, we used exhaustive information on nest temperatures and hatching success of leatherback turtle (Dermochelys coriacea) clutches over 14 years to assess the occurrence of individual variability in thermal tolerance among females. We found an effect of temperature, year, and the interaction between female identity and nest temperature on hatching success, indicating that clutches laid by different females exhibited different levels of vulnerability to high temperatures. If thermal tolerance is a heritable trait, individuals with higher thermal tolerances could have greater chances of passing their genes to following generations, increasing their frequency in the population. However, the high rate of failure of clutches at temperatures above 32°C suggests that leatherback turtles are already experiencing extreme heat stress. A proper understanding of mechanisms of adaptation in populations to counteract changes in climate could greatly contribute to future conservation of endangered populations in a rapidly changing world.

Thermal tolerance plasticity and dynamics of thermal tolerance in Eublepharis macularius: Implications for future climate-driven heat stress

The intensity and duration of heat waves, as well as average global temperatures, are expected to increase due to climate change. Heat waves can cause physiological stress and reduce fitness in animals. Species can reduce overheating risk through phenotypic plasticity, which allows them to raise their thermal tolerance limits over time. This mechanism could be important for ectotherms whose body temperatures are directly influenced by available environmental temperatures. Geckos are a large, diverse group of ectotherms that vary in their thermal habitats and times of daily activity, which could affect how they physiologically adjust to heat waves. Data on thermal physiology are scarce for reptiles, with only one study in geckos. Understanding thermal tolerance and plasticity, and their relationship, is essential for understanding how some species are able to adjust or adapt to changing temperatures. In this study, we estimated thermal tolerance and plasticity, and their interaction, in the crepuscular gecko, Eublepharis macularius, a species that is emerging as a model for reptile biology. After estimating basal thermal tolerance for 28 geckos, thermal tolerance was measured for each individual a second time at several timepoints (3, 6, or 24 h) to determine thermal tolerance plasticity. We found that thermal tolerance plasticity (1) does not depend on the basal thermal tolerance of the organism, (2) was highest after 6 h from initial heat shock, and (3) was negatively influenced by individual body mass. Our findings contribute to the increasing body of work focused on understanding the influence of biological and environmental factors on thermal tolerance plasticity in organisms and provide phenotypic data to further investigate the molecular basis of thermal tolerance plasticity in organisms.

The lack of plasticity and interspecific variability in thermal limits produce a highly heat-tolerant tropical host-parasitoid system

Thermal limits are often used as proxies to assess the vulnerability of ectotherms to environmental change. While meta-analyses point out a relatively low plasticity of heat limits and a large interspecific variability, only few studies have compared the heat tolerance of interacting species. The present study focuses on the thermal limits, and their plasticity (heat hardening), of three species co-occurring in Western Africa: two ectoparasitoid species, Dinarmus basalis (Rondani) (Hymenoptera: Pteromalidae) and Eupelmus vuilleti (Crawford) (Hymenoptera: Eupelmidae), and their common host, Callosobruchus maculatus (F.) (Coleoptera: Bruchidae). The investigation delves into the Critical Thermal Maximum (CTmax), representing the upper tolerance limit, to understand how these species may cope with extreme thermal events. The CTmax of all three species appeared similarly high, hovering around 46.5 °C, exceeding the global mean CTmax observed in insects by 3.5 °C. Short-term exposure to moderate heat stress showed no impact on CTmax, suggesting a potential lack of heat hardening in these species. Therefore, we emphasized the similarity of heat tolerance in these interacting species, potentially stemming from both evolutionary adaptations to high temperatures during development and the stable and similar microclimate experienced by the three species over the years. While the high thermal tolerance should allow these species to endure extreme temperature events, the apparent lack of plasticity raises concerns about their ability to adapt to future climate change scenarios. Overall, this research provides valuable insights into the thermal physiology of these interacting species, providing a basis for understanding their responses to climate change and potential implications for the host-parasitoid system.

Evidence of plasticity, but not evolutionary divergence, in the thermal limits of a highly successful urban butterfly

Despite the generally negative impact of urbanization on insect biodiversity, some insect species persist in urban habitats. Understanding the mechanisms underpinning the ability of insects to tolerate urban habitats is critical given the contribution of land-use change to the global insect decline. Compensatory mechanisms such as phenotypic plasticity and evolutionary change in thermal physiological traits could allow urban populations to persist under the altered thermal regimes of urban habitats. It is important to understand the contributions of plasticity and evolution to trait change along urbanization gradients as the two mechanisms operate under different constraints and timescales. Here, we examine the plastic and evolutionary responses of heat and cold tolerance (critical thermal maximum [CTmax] and critical thermal minimum [CTmin]) to warming among populations of the cabbage white butterfly, Pieris rapae, from urban and non-urban (rural) habitats using a two-temperature common garden experiment. Although we expected populations experiencing urban warming to exhibit greater CTmax and diminished CTmin through plastic and evolutionary mechanisms, our study revealed evidence only for plasticity in the expected direction of both thermal tolerance traits. We found no evidence of evolutionary divergence in either heat or cold tolerance, despite each trait showing evolutionary potential. Our results suggest that thermal tolerance plasticity contributes to urban persistence in this system. However, as the magnitude of the plastic response was low and comparable to other insect species, other compensatory mechanisms likely further underpin this species' success in urban habitats.

Thermal sensitivity of Rhinella arenarum tadpole at low concentrations of dimethoate pesticides

One of the main causes of contamination of aquatic environments, which affects biotic communities, is the use of pesticides in agricultural regions. Amphibians are considered good bio-indicators of aquatic pollution, because they are one of the most susceptible groups to pollution. Several studies suggest that both pollution and climate change produce synergistic effects in amphibians which amplify the toxicity afecting survival, and malformations with an increase in temperature. We studied the sensitivity of sublethal concentrations of dimethoate in Rhinella arenarum tadpoles on two fitness related thermal traits including locomotor swimming performance and thermal tolerance limits (CTmax = critical thermal maximum and CTmin = critical thermal minimum). The locomotor performance of R. arenarum tadpoles decreased with increasing sublethal dimethoate concentrations up to ∼60 % at intermediates dimethoate concentration. The tadpoles showed a tendency to decrease their tolerance to high temperatures (CTmax) with increasing dimethoate concentration around ∼0.5 °C, however no significant differences were found among treatments. Similarly, tadpoles showed decreases in their cold resistance (CTmin) with dimethoate concentrations, around 1 °C the high concentrations of dimethoate. The increase of atypical climatic events, such as heat waves may put R. arenarum tadpoles at greater risk when exposed to dimethoate. Our results show that the sublethal concentrations of the dimethoate pesticide may affect the fitness and survival of the larvae of R. arenarum in natural, and seminatural enviroments.

Global meta-analysis reveals overall higher nocturnal than diurnal activity in insect communities

Insects sustain key ecosystem functions, but how their activity varies across the day-night cycle and the underlying drivers are poorly understood. Although entomologists generally expect that more insects are active at night, this notion has not been tested with empirical data at the global scale. Here, we assemble 331 quantitative comparisons of the abundances of insects between day and night periods from 78 studies worldwide and use multi-level meta-analytical models to show that insect activity is on average 31.4% (CI: -6.3%-84.3%) higher at night than in the day. We reveal diel preferences of major insect taxa, and observe higher nocturnal activity in aquatic taxa than in terrestrial ones, as well as in warmer environments. In a separate analysis of the small subset of studies quantifying diel patterns in taxonomic richness (31 comparisons from 13 studies), we detect preliminary evidence of higher nocturnal richness in tropical than temperate communities. The higher overall (but variable) nocturnal activity in insect communities underscores the need to address threats such as light pollution and climate warming that may disproportionately impact nocturnal insects.

Wolbachia infection negatively impacts Drosophila simulans heat tolerance in a strain- and trait-specific manner

The susceptibility of insects to rising temperatures has largely been measured by their ability to survive thermal extremes. However, the capacity for maternally inherited endosymbionts to influence insect heat tolerance has been overlooked. Further, while some studies have addressed the impact of heat on traits like fertility, which can decline at temperatures below lethal thermal limits, none have considered the impact of endosymbionts. Here, we assess the impact of three Wolbachia strains (wRi, wAu and wNo) on the survival and fertility of Drosophila simulans exposed to heat stress during development or as adults. The effect of Wolbachia infection on heat tolerance was generally small and trait/strain specific. Only the wNo infection significantly reduced the survival of adult males after a heat shock. When exposed to fluctuating heat stress during development, the wRi and wAu strains reduced egg-to-adult survival but only the wNo infection reduced male fertility. Wolbachia densities of all three strains decreased under developmental heat stress, but reductions occurred at temperatures above those that reduced host fertility. These findings emphasize the necessity to account for endosymbionts and their effect on both survival and fertility when investigating insect responses to heat stress.

Thermal tolerance does not explain the altitudinal segregation of lowland and alpine aquatic insects

Elevation gradients provide powerful study systems for examining the influence of environmental filters in shaping species assemblages. High-mountain habitats host specific high-elevation assemblages, often comprising specialist species adapted to endure pronounced abiotic stress, while such harsh conditions prevent lowland species from colonizing or establishing. While thermal tolerance may drive the altitudinal segregation of ectotherms, its role in structuring aquatic insect communities remains poorly explored. This study investigates the role of thermal physiology in shaping the current distribution of high-mountain diving beetles from the Sierra Nevada Iberian mountain range and closely related lowland species. Cold tolerance of five species from each altitudinal zone was measured estimating the supercooling point (SCP), lower lethal temperature (LLT) and tolerance to ice enclosure, while heat tolerance was assessed from the heat coma temperature (HCT). Alpine species exhibited wider fundamental thermal niches than lowland species, likely associated with the broader range of climatic conditions in high-mountain areas. Cold tolerance did not seem to prevent lowland species from colonizing higher elevations, as most studied species were moderately freeze-tolerant. Therefore, fundamental thermal niches seem not to fully explain species segregation along elevation gradients, suggesting that other thermal tolerance traits, environmental factors, and biotic interactions may also play important roles.

Does heat tolerance actually predict animals' geographic thermal limits?

The "climate extremes hypothesis" is a major assumption of geographic studies of heat tolerance and climatic vulnerability. However, this assumption remains vastly untested across taxa, and multiple factors may contribute to uncoupling heat tolerance estimates and geographic limits. Our dataset includes 1000 entries of heat tolerance data and maximum temperatures for each species' known geographic limits (hereafter, Tmax). We gathered this information across major animal taxa, including marine fish, terrestrial arthropods, amphibians, non-avian reptiles, birds, and mammals. We first tested if heat tolerance constrains the Tmax of sites where species could be observed. Secondly, we tested if the strength of such restrictions depends on how high Tmax is relative to heat tolerance. Thirdly, we correlated the different estimates of Tmax among them and across species. Restrictions are strong for amphibians, arthropods, and birds but often weak or inconsistent for reptiles and mammals. Marine fish describe a non-linear relationship that contrasts with terrestrial groups. Traditional heat tolerance measures in thermal vulnerability studies, like panting temperatures and the upper set point of preferred temperatures, do not predict Tmax or are inversely correlated to it, respectively. Heat tolerance restricts the geographic warm edges more strongly for species that reach sites with higher Tmax for their heat tolerance. These emerging patterns underline the importance of reliable species' heat tolerance indexes to identify their thermal vulnerability at their warm range edges. Besides, the tight correlations of Tmax estimates across on-land microhabitats support a view of multiple types of thermal challenges simultaneously shaping ranges' warm edges for on-land species. The heterogeneous correlation of Tmax estimates in the ocean supports the view that fish thermoregulation is generally limited, too. We propose new hypotheses to understand thermal restrictions on animal distribution.

Life tables in entomology: A discussion on tables' parameters and the importance of raw data

Life tables are one of the most common tools to describe the biology of insect species and their response to environmental conditions. Although the benefits of life tables are beyond question, we raise some doubts about the completeness of the information reported in life tables. To substantiate these doubts, we consider a case study (Corcyra cephalonica) for which the raw dataset is available. The data suggest that the Gaussian approximation of the development times which is implied by the average and standard error usually reported in life tables does not describe reliably the actual distribution of the data which can be misleading and hide interesting biological aspects. Furthermore, it can be risky when life table data are used to build models to predict the demographic changes of the population. The present study highlights this aspect by comparing the impulse response generated by the raw data and by its Gaussian approximation based on the mean and the standard error. The conclusions of this paper highlight: i) the importance of adding more information to life tables and, ii) the role of raw data to ensure the completeness of this kind of studies. Given the importance of raw data, we also point out the need for further developments of a standard in the community for sharing and analysing data of life tables experiments.

Competition Increases Risk of Species Extinction during Extreme Warming

AbstractTemperature and interspecific competition are fundamental drivers of community structure in natural systems and can interact to affect many measures of species performance. However, surprisingly little is known about the extent to which competition affects extinction temperatures during extreme warming. This information is important for evaluating future threats to species from extreme high-temperature events and heat waves, which are rising in frequency and severity around the world. Using experimental freshwater communities of rotifers and ciliates, this study shows that interspecific competition can lower the threshold temperature at which local extinction occurs, reducing time to extinction during periods of sustained warming by as much as 2 weeks. Competitors may lower extinction temperatures by altering biochemical characteristics of the natural environment that affect temperature tolerance (e.g., levels of dissolved oxygen, nutrients, and metabolic wastes) or by accelerating population decline through traditional effects of resource depletion on life history parameters that affect population growth rates. The results suggest that changes in community structure in space and time could drive variability in upper thermal limits.

"A comparison of thermal stress response between Drosophila melanogaster and Drosophila pseudoobscura reveals differences between species and sexes"

The environment is changing faster than anticipated due to climate change, making species more vulnerable to its impacts. The level of vulnerability of species is influenced by factors such as the degree and duration of exposure, as well as the physiological sensitivity of organisms to changes in their environments, which has been shown to vary among species, populations, and individuals. Here, we compared physiological changes in fecundity, critical thermalmaximum (CTmax), respiratory quotient (RQ), and DNA damage in ovaries in response to temperature stress in two species of fruit fly, Drosophila melanogaster (25 vs. 29.5 °C) and Drosophila pseudoobscura (20.5 vs. 25 °C). The fecundity of D. melanogaster was more affected by high temperatures when exposed during egg through adult development, while D. pseudoobscura was most significantly affected when exposed to high temperatures exclusively during egg through pupal development. Additionally, D. melanogaster males exhibited a decrease of CTmax under high temperatures, while females showed an increase of CTmax when exposed to high temperatures during egg through adult development. while D. pseudoobscura females and males showed an increased CTmax only when reared at high temperatures during egg through pupae development. Moreover, both species showed an acceleration in oogenesis and an increase in apoptosis due to heat stress. These changes can likely be attributed to key differences in the geographic range, thermal range, development time, and other different factors between these two systems. Through this comparison of variation in physiology and developmental response to thermal stress, we found important differences between species and sexes that suggest future work needs to account for these factors separately in understanding the effects of constant increased temperatures.

Insecticide resistant mosquitoes remain thermal stress resistant, without loss of thermal plasticity

A major component of mosquito's climate change response is their heat tolerance, and any ability to rapidly adjust to extreme environmental conditions through phenotypic plasticity. The excessive use of insecticides for the control of major mosquito species leads to resistant populations, however it is largely unclear if this concurrently impacts thermal stress resistance and their potential to adjust tolerance via phenotypic plasticity. Culex pipiens pipiens, Culex pipiens molestus and Aedes albopictus populations obtained from the same region were subjected for 12 generations to selection trials to larvicides Diflubenzuron (DFB) and Bacillus thuringiensis subsp. israelensis (Bti) to develop insecticide resistance. Adults emerging from the selected populations were acclimated at different temperatures and the upper and lower critical thermal limits (CTmax and CTmin) were estimated using dynamic thermal assays. In addition, the supercooling points (SCPs) of non-acclimated adults of resistant and control populations were determined. Our results revealed marked differences in thermal response among the three species, the different acclimation regimes and sexes. Aedes albopictus was more resistant in high than low temperatures compared to both Culex pipiens biotypes. Culex forms responded similarly to heat but differently to cold stress. In both forms, females responded better than males to all thermal stressors. Acclimation at higher and lower temperatures improves CTmax and CTmin values, respectively in both insecticide resistant and control populations of all three species. Overall, selection to insecticides did not affect the thermal performance of adults. Hence, insecticide-resistant mosquito populations perform similarly to untreated ones and are capable of readily adapting to new environmental changes rising concerns regarding their geographic range expansion and disease transmission globally.

The thermal breadth of temperate and tropical freshwater insects supports the climate variability hypothesis

Climate change involves increases in mean temperature and changes in temperature variability at multiple temporal scales but research rarely considers these temporal scales. The climate variability hypothesis (CVH) provides a conceptual framework for exploring the potential effects of annual scale thermal variability across climatic zones. The CVH predicts ectotherms in temperate regions tolerate a wider range of temperatures than those in tropical regions in response to greater annual variability in temperate regions. However, various other aspects of thermal regimes (e.g. diel variability), organisms' size and taxonomic identity are also hypothesised to influence thermal tolerance. Indeed, high temperatures in the tropics have been proposed as constraining organisms' ability to tolerate a wide range of temperatures, implying that high annual maximum temperatures would be associated with tolerating a narrow range of temperatures. We measured thermal regimes and critical thermal limits (CTmax and CTmin) of freshwater insects in the orders Ephemeroptera (mayflies), Plecoptera (stoneflies) and Trichoptera (caddisflies) along elevation gradients in streams in temperate and tropical regions of eastern Australia and tested the CVH by determining which variables were most correlated with thermal breadth (T br = CTmax - CTmin). Consistent with the CVH, T br tended to increase with increasing annual temperature range. T br also increased with body size and T br was generally wider in Plecoptera than in Ephemeroptera or Trichoptera. We also find some support for a related hypothesis, the climate extreme hypothesis (CEH), particularly for predicting upper thermal limits. We found no evidence that higher annual maximum temperature constrained individuals' abilities to tolerate a wide range of temperatures. The support for the CVH we document suggests that temperate organisms may be able to tolerate wider ranges of temperatures than tropical organisms. There is an urgent need to investigate other aspects of thermal regimes, such as diel temperature cycling and minimum temperature.

Heat-induced hormesis in longevity is linked to heat-stress sensitivity across laboratory populations from diverse altitude of origin in Drosophila buzzatii

Heat-induced hormesis in longevity is the increase in life span resulting from the previous exposure to a mild heat stress early in life. Here we examined heat-induced hormesis of Drosophila buzzatii in five mass-mating populations, which were derived from five wild populations along an elevation gradient from 202 to 1855 m above sea level in North-Western Argentina. Five day old flies were exposed to 37.5 °C for 90 min to induce hormesis and its possible variation across altitudinal populations. This heat treatment strongly extended longevity in lowland-derived flies from the most heat-resistant population only. Both heat-induced effects on longevity and heat-knockdown time (heat-stress sensitivity) were negatively correlated to altitude of population of origin. Hormesis was positively correlated to heat-knockdown time across populations. These results indicate that variation in heat-induced hormesis can not be considered as independent of heat-stress sensitivity (or heat-knockdown time) in populations of insects.

How development and survival combine to determine the thermal sensitivity of insects

Thermal performance curves (TPCs) depict variation in vital rates in response to temperature and have been an important tool to understand ecological and evolutionary constraints on the thermal sensitivity of ectotherms. TPCs allow for the calculation of indicators of thermal tolerance, such as minimum, optimum, and maximum temperatures that allow for a given metabolic function. However, these indicators are computed using only responses from surviving individuals, which can lead to underestimation of deleterious effects of thermal stress, particularly at high temperatures. Here, we advocate for an integrative framework for assessing thermal sensitivity, which combines both vital rates and survival probabilities, and focuses on the temperature interval that allows for population persistence. Using a collated data set of Lepidopteran development rate and survival measured on the same individuals, we show that development rate is generally limiting at low temperatures, while survival is limiting at high temperatures. We also uncover differences between life stages and across latitudes, with extended survival at lower temperatures in temperate regions. Our combined performance metric demonstrates similar thermal breadth in temperate and tropical individuals, an effect that only emerges from integration of both development and survival trends. We discuss the benefits of using this framework in future predictive and management contexts.

Cross-species gut microbiota transplantation predictably affects host heat tolerance

The gut microbiome is known to influence and have regulatory effects in diverse physiological functions of host animals, but only recently has the relationship between host thermal biology and gut microbiota been explored. Here, we examined how early-life manipulations of the gut microbiota in larval amphibians influenced their critical thermal maximum (CTmax) at different acclimation temperatures. We stripped the resident microbiome from egg masses of wild-caught wood frogs (Lithobates sylvaticus) via an antibiotic wash, and then inoculated the eggs with pond water (control), no inoculation, or the intestinal microbiota of another species that has a wider thermal tolerance - green frogs (Lithobates clamitans). We predicted that this cross-species transplant would increase the CTmax of the recipient wood frog larvae relative to the other treatments. In line with this prediction, green frog microbiome-recipient larvae had the highest CTmax while those with no inoculum had the lowest CTmax. Both the microbiome treatment and acclimation temperature significantly influenced the larval gut microbiota communities and α-diversity indices. Green frog microbiome-inoculated larvae were enriched in Rikenellaceae relative to the other treatments, which produce short-chain fatty acids and could contribute to greater energy availability and enhanced heat tolerance. Larvae that received no inoculation had a higher relative abundance of potentially pathogenic Aeromonas spp., which negatively affects host health and performance. Our results are the first to show that cross-species gut microbiota transplants alter heat tolerance in a predictable manner. This finding has repercussions for the conservation of species that are threatened by climate change and demonstrates a need to further explore the mechanisms by which the gut microbiota modulate host thermal tolerance.

Geography and developmental plasticity shape post-larval thermal tolerance in the golden star tunicate, Botryllus schlosseri

Local adaptation and phenotypic plasticity play key roles in mediating organisms' ability to respond to spatiotemporal variation in temperature. These two processes often act together to generate latitudinal or elevational clines in acute temperature tolerance. Phenotypic plasticity is also subject to local adaptation, with the expectation that populations inhabiting more variable environments should exhibit greater phenotypic plasticity of thermal tolerance. Here we examine the potential for local adaptation and developmental plasticity of thermal tolerance in the widespread invasive tunicate Botryllus schlosseri. By comparing five populations across a thermal gradient spanning 4.4° of latitude in the northwest Atlantic, we demonstrate that warmer populations south of the Gulf of Maine exhibit significantly increased (∼0.2 °C) post-larval temperature tolerance relative to the colder populations within it. We also show that B. schlosseri post-larvae possess a high degree of developmental plasticity for this trait, shifting their median temperature of survival (LT50) upwards by as much as 0.18 °C per 1 °C increase in environmental temperature. Lastly, we found that populations vary in their degrees of developmental plasticity, with populations that experience more pronounced short-term temperature variability exhibiting greater developmental plasticity, suggesting the local adaptation of developmental plasticity. By comparing the thermal tolerance of populations across space and through time, we demonstrate how geography and developmental plasticity have shaped thermal tolerance in B. schlosseri. These results help inform our understanding of how species are able to adjust their thermal physiology in new environments, including those encountered during invasion and under increasingly novel climate conditions.

Climate change impacts on insect pests for high value specialty crops in California

California is a global leader in production and supply of walnuts and almonds, and the state is the largest producer of peaches in the U.S. These crops have an important contribution to the California's agricultural economy. Damages to these crops from lepidopteran pests, mainly from Codling moth (Cydia pomonella) (family: Tortricidae), Peach twig borer (Anarsia lineatella) (family: Gelechiidae) and Oriental fruit moth (Grapholita molesta) (family: Tortricidae), are still high, despite the improvement in pest management activities. Given that temperature increase can directly impact the rate of growth and development of these pests, it is important to understand to what extent dynamics of these pests will change in future in California. The objective of this study was to quantify changes in the biofix, lifecycle length, and number of generations for these pests for the entire Central Valley of California. Using a well-established growing-degree days (GDD) model calibrated and validated using observations from orchards of California, and climate change projections from the Coupled Model Intercomparison Project phases 5 and 6 (CMIP5 and CMIP6) General Circulation Models, we found that biofix dates of these pests are expected to shift earlier by up to 28 days, and length of generations is expected to be shortened by up to 19 days, and up to 1.4 extra generations of these pests can be added by the end of the century depending on the scenario. Results from this work would enable industries to prioritize development of practices that are more effective in the long run, such as developing better cultural and biological pest solutions and insect tolerant varieties. Growers and researchers can take proactive actions to minimize future risks associated with these damaging pests. This work can be scalable to other pests and regions to understand regional dynamics of damaging agricultural pests under climate change.

Proteomic differences in seminal fluid of social insects whose sperm differ in heat tolerance

In the coming years, climate change is likely to increase the frequency and intensity of heatwaves. In many organisms, heat stress provokes physiological perturbations and can lead to decreased male fertility. Bumblebees are endo-heterothermic but display interspecific differences in thermotolerance that could have conservation implications. For the species of concern Bombus magnus, exposure to high temperatures can severely reduce sperm quality and, consequently, reproductive success. Such is not the case for B. terrestris, a ubiquitous species. To decipher the mechanisms at play, we characterized the seminal fluid proteomes of the two species. We quantified 1121 proteins, of which 522 were differentially expressed between B. terrestris and B. magnus. Several proteins with protective functions, such as proteases, antioxidant proteins and various heat-shock proteins, were present at higher levels in B. terrestris than in B. magnus under both control and heat-stress conditions. The same was true for proteins involved in cellular homeostasis, immunity, lipid/sugar metabolism and thermotolerance. Furthermore, proteins involved in the capture and elimination of reactive oxygen species also occurred at much high levels in B. terrestris. Overall, these results clearly indicate differences in the seminal proteome of the more thermotolerant B. terrestris versus B. magnus. The differences may contribute to explaining interspecific differences in sperm survival.

Temporal variation of thermal sensitivity to global warming: Acclimatization in the guitarist beetle, Megelenophorus americanus (Coleoptera: Tenebrionidae) from the Monte Desert

Global warming is a major threat to biodiversity, the increase in mean temperature plus the higher rate and intensity of heat waves can severely affect organisms by exposing them to temperatures beyond their tolerance limits. Desert ectotherms are particularly vulnerable due to their dependence on environmental temperatures in an extreme habitat. Thermal tolerance changes depending on environmental conditions, studying these fluctuations provides a better understanding of species susceptibility to global warming. Tenebrionids are successful desert-inhabiting ectotherm taxa because of a series of adaptations for heat tolerance and water loss. We studied the seasonal variation (acclimatization) of thermal tolerance in Megelenophorus americanus, a widely distributed species in the Monte Desert (Argentina). To do this, we measured environmental and operative temperatures: body temperature (Tb), soil temperature (Ts), air temperature (Ta), environmental temperature (Te) and maximum temperature (Tmax), and tolerance proxies volunteer thermal maximum (VTmax), Fluid release (FR) and critical thermal maximum (CTmax) in a population of M. americanus from San Juan province, Argentina from October to March (full activity season). We found that Ts and Ta are accurate predictors of Tb, suggesting thermoconformism. All tolerance proxies showed differences among months, suggesting a natural acclimatization process in situ. Insects were found operating beyond VTmax (thermal stress) but they were far from reaching CTmax under natural conditions. Organisms present different degrees of tolerance plasticity that should be considered when predicting potential impacts of climate change.

Differential Cold Tolerance on Immature Stages of Geographically Divergent Ceratitis capitata Populations

Cold tolerance of adult medflies has been extensively studied but the effect of subfreezing temperatures on the immature stages remains poorly investigated, especially as far as different populations are regarded. In this study, we estimated the acute cold stress response of three geographically divergent Mediterranean fruit fly populations originating from Greece (Crete, Volos) and Croatia (Dubrovnik) by exposing immature stages (eggs, larvae, pupae) to subfreezing temperatures. We first determined the LT50 for each immature stage following one hour of exposure to different temperatures. Then eggs, larvae and pupae of the different populations were exposed to their respective LT50 for one hour (LT50 = -11 °C, LT50 = -4.4 °C, LT50 = -5 °C for eggs, larvae and pupae, respectively). Our results demonstrate that populations responded differently depending on their developmental stage. The population of Dubrovnik was the most cold-susceptible at the egg stage, whereas in that of Crete it was at the larval and pupal stage. The population of Volos was the most cold-tolerant at all developmental stages. The egg stage was the most cold-tolerant, followed by pupae and finally the 3rd instar wandering larvae. This study contributes towards understanding the cold stress response of this serious pest and provides data for important parameters that determine its successful establishment to unfavorable environments with an emphasis on range expansion to the northern, more temperate regions of Europe.

The role of thermal tolerance in determining elevational distributions of four arthropod taxa in mountain ranges of southern Asia

Understanding the role of thermal tolerances in determining species distributions is important for assessing species responses to climate change. Two hypotheses linking physiology with species distributions have been put forward-the climatic variability hypothesis and the climatic extreme hypothesis. The climatic variability hypothesis predicts the selection of individuals with broad thermal tolerance in more variable climatic conditions and the climatic extreme hypothesis predicts the selection of individuals with extreme thermal tolerance values under extreme climatic conditions. However, no study has tested the predictions of these hypotheses simultaneously for several taxonomic groups along elevational gradients. Here, we related experimentally measured critical thermal maxima, critical thermal minima and thermal tolerance breadths for 15,187 individuals belonging to 116 species of ants, beetles, grasshoppers, and spiders from mountain ranges in central and northern Pakistan to the limits and breadths of their geographic and temperature range. Across all species and taxonomic groups, we found strong relationships between thermal traits and elevational distributions both in terms of geography and temperature. The relationships were robust when repeating the analyses for ants, grasshoppers, and spiders but not for beetles. These results indicate a strong role of physiology in determining elevational distributions of arthropods in Southern Asia. Overall, we found strong support for the climatic variability hypothesis and the climatic extreme hypothesis. A close association between species' distributional limits and their thermal tolerances suggest that in case of a failure to adapt or acclimate to novel climatic conditions, species may be under pressure to track their preferred climatic conditions, potentially facing serious consequences under current and future climate change.

Catalase promotes whitefly adaptation to high temperature by eliminating reactive oxygen species

Thermal stress usually leads to excessive production of reactive oxygen species (ROS) in all aerobic organisms. Catalases (CAT) are the key antioxidant enzymes, which act as the first line of defense against ROS in the antioxidant pathway. The highly invasive and widely distributed whitefly Bemisia tabaci MED damages plants by feeding as well as by transmitting many plant viruses. Previous studies have shown that strong adaptability to high temperature helps explain the spread of MED around the world. However, the mechanism underlying high temperature adaptation of this pest is not well understood. In this study, 6 CAT genes were identified from the MED genome and transcriptome dataset, among which BtCAT1, BtCAT2, and BtCAT3 were found to be highly expressed in adults. The expression of BtCAT1, BtCAT2, or BtCAT3 increased with induction temperature and induction time. The MED was exposed with mean high temperature (30 °C or 35 °C) and a short-term extremely high temperature (39 °C or 41 °C) after the silencing of BtCAT1, BtCAT2, or BtCAT3 to significantly increased ROS levels by at least 0.5 times and significantly decreased survival rate and fecundity of MED adults. The ROS level in the treated specimens gradually returned to a normal level after 24 h at 25 °C, but the survival rate still declined significantly. Taken together, our results demonstrate that CAT could help B. tabaci adapt to long-term mean high temperatures and short-term extremely high temperatures by eliminating excessive ROS.

Commonly collected thermal performance data can inform species distributions in a data-limited invader

Predicting potential distributions of species in new areas is challenging. Physiological data can improve interpretation of predicted distributions and can be used in directed distribution models. Nonnative species provide useful case studies. Panther chameleons (Furcifer pardalis) are native to Madagascar and have established populations in Florida, USA, but standard correlative distribution modeling predicts no suitable habitat for F. pardalis there. We evaluated commonly collected thermal traits- thermal performance, tolerance, and preference-of F. pardalis and the acclimatization potential of these traits during exposure to naturally-occurring environmental conditions in North Central Florida. Though we observed temperature-dependent thermal performance, chameleons maintained similar thermal limits, performance, and preferences across seasons, despite long-term exposure to cool temperatures. Using the physiological data collected, we developed distribution models that varied in restriction: time-dependent exposure near and below critical thermal minima, predicted activity windows, and predicted performance thresholds. Our application of commonly collected physiological data improved interpretations on potential distributions of F. pardalis, compared with correlative distribution modeling approaches that predicted no suitable area in Florida. These straightforward approaches can be applied to other species with existing physiological data or after brief experiments on a limited number of individuals, as demonstrated here.

Long-term forecast of thermal mortality with climate warming in riverine amphipods

Forecasting long-term consequences of global warming requires knowledge on thermal mortality and how heat stress interacts with other environmental stressors on different timescales. Here, we describe a flexible analytical framework to forecast mortality risks by combining laboratory measurements on tolerance and field temperature records. Our framework incorporates physiological acclimation effects, temporal scale differences and the ecological reality of fluctuations in temperature, and other factors such as oxygen. As a proof of concept, we investigated the heat tolerance of amphipods Dikerogammarus villosus and Echinogammarus trichiatus in the river Waal, the Netherlands. These organisms were acclimated to different temperatures and oxygen levels. By integrating experimental data with high-resolution field data, we derived the daily heat mortality probabilities for each species under different oxygen levels, considering current temperatures as well as 1 and 2°C warming scenarios. By expressing heat stress as a mortality probability rather than a upper critical temperature, these can be used to calculate cumulative annual mortality, allowing the scaling up from individuals to populations. Our findings indicate a substantial increase in annual mortality over the coming decades, driven by projected increases in summer temperatures. Thermal acclimation and adequate oxygenation improved heat tolerance and their effects were magnified on longer timescales. Consequently, acclimation effects appear to be more effective than previously recognized and crucial for persistence under current temperatures. However, even in the best-case scenario, mortality of D. villosus is expected to approach 100% by 2100, while E. trichiatus appears to be less vulnerable with mortality increasing to 60%. Similarly, mortality risks vary spatially: In southern, warmer rivers, riverine animals will need to shift from the main channel toward the cooler head waters to avoid thermal mortality. Overall, this framework generates high-resolution forecasts on how rising temperatures, in combination with other environmental stressors such as hypoxia, impact ecological communities.

Shifts in survival and reproduction after chronic warming enhance the potential of a marine copepod to persist under extreme heat events

The study of a species' thermal tolerance and vital rates responses provides useful metrics to characterize its vulnerability to ocean warming. Under prolonged thermal stress, plastic and adaptive processes can adjust the physiology of organisms. Yet it is uncertain whether the species can expand their upper thermal limits to cope with rapid and extreme changes in environmental temperature. In this study, we reared the marine copepod Paracartia grani at control (19°C) and warmer conditions (25°C) for >18 generations and assessed their survival and fecundity under short-term exposure to a range of temperatures (11-34°C). After multigenerational warming, the upper tolerance to acute exposure (24 h) increased by 1-1.3°C, although this enhancement decreased to 0.3-0.8°C after longer thermal stress (7 days). Warm-reared copepods were smaller and produced significantly fewer offspring at the optimum temperature. No shift in the thermal breadth of the reproductive response was observed. Yet the fecundity rates of the warm-reared copepods in the upper thermal range were up to 21-fold higher than the control. Our results show that chronic warming improved tolerance to stress temperatures and fecundity of P. grani, therefore, enhancing its chances to persist under extreme heat events.

Interannual climate variability improves niche estimates for ectothermic but not endothermic species

Climate is an important limiting factor of species' niches and it is therefore regularly included in ecological applications such as species distribution models (SDMs). Climate predictors are often used in the form of long-term mean values, yet many species experience wide climatic variation over their lifespan and within their geographical range which is unlikely captured by long-term means. Further, depending on their physiology, distinct groups of species cope with climate variability differently. Ectothermic species, which are directly dependent on the thermal environment are expected to show a different response to temporal or spatial variability in temperature than endothermic groups that can decouple their internal temperature from that of their surroundings. Here, we explore the degree to which spatial variability and long-term temporal variability in temperature and precipitation change niche estimates for ectothermic (730 amphibian, 1276 reptile), and endothermic (1961 mammal) species globally. We use three different species distribution modelling (SDM) algorithms to quantify the effect of spatial and temporal climate variability, based on global range maps of all species and climate data from 1979 to 2013. All SDMs were cross-validated and accessed for their performance using the Area under the Curve (AUC) and the True Skill Statistic (TSS). The mean performance of SDMs using only climatic means as predictors was TSS = 0.71 and AUC = 0.90. The inclusion of spatial variability offers a significant gain in SDM performance (mean TSS = 0.74, mean AUC = 0.92), as does the inclusion of temporal variability (mean TSS = 0.80, mean AUC = 0.94). Including both spatial and temporal variability in SDMs shows the highest scores in AUC and TSS. Accounting for temporal rather than spatial variability in climate improved the SDM prediction especially in ectotherm groups such as amphibians and reptiles, while for endothermic mammals no such improvement was observed. These results indicate that including long term climate interannual climate variability into niche estimations matters most for ectothermic species that cannot decouple their physiology from the surrounding environment as endothermic species can.

Humidity - The overlooked variable in the thermal biology of mosquito-borne disease

Vector-borne diseases cause significant financial and human loss, with billions of dollars spent on control. Arthropod vectors experience a complex suite of environmental factors that affect fitness, population growth and species interactions across multiple spatial and temporal scales. Temperature and water availability are two of the most important abiotic variables influencing their distributions and abundances. While extensive research on temperature exists, the influence of humidity on vector and pathogen parameters affecting disease dynamics are less understood. Humidity is often underemphasized, and when considered, is often treated as independent of temperature even though desiccation likely contributes to declines in trait performance at warmer temperatures. This Perspectives explores how humidity shapes the thermal performance of mosquito-borne pathogen transmission. We summarize what is known about its effects and propose a conceptual model for how temperature and humidity interact to shape the range of temperatures across which mosquitoes persist and achieve high transmission potential. We discuss how failing to account for these interactions hinders efforts to forecast transmission dynamics and respond to epidemics of mosquito-borne infections. We outline future research areas that will ground the effects of humidity on the thermal biology of pathogen transmission in a theoretical and empirical framework to improve spatial and temporal prediction of vector-borne pathogen transmission.

Intraspecific variation among Chinook Salmon populations indicates physiological adaptation to local environmental conditions

Understanding interpopulation variation is important to predicting species responses to climate change. Recent research has revealed interpopulation variation among several species of Pacific salmonids; however, the environmental drivers of population differences remain elusive. We tested for local adaptation and countergradient variation by assessing interpopulation variation among six populations of fall-run Chinook Salmon from the western United States. Juvenile fish were reared at three temperatures (11, 16 and 20°C), and five physiological metrics were measured (routine and maximum metabolic rate, aerobic scope, growth rate and critical thermal maximum). We then tested associations between these physiological metrics and 15 environmental characteristics (e.g. rearing temperature, latitude, migration distance, etc.). Statistical associations between the five physiological metrics and 15 environmental characteristics supported our hypotheses of local adaptation. Notably, latitude was a poor predictor of population physiology. Instead, our results demonstrate that populations from warmer habitats exhibit higher thermal tolerance (i.e. critical thermal maxima), faster growth when warm acclimated and greater aerobic capacity at high temperatures. Additionally, populations with longer migrations exhibit higher metabolic capacity. However, overall metabolic capacity declined with warm acclimation, indicating that future climate change may reduce metabolic capacity, negatively affecting long-migrating populations. Linking physiological traits to environmental characteristics enables flexible, population-specific management of disparate populations in response to local conditions.

Glucose Dehydrogenases-Mediated Acclimation of an Important Rice Pest to Global Warming

Global warming is posing a threat to animals. As a large group of widely distributed poikilothermal animals, insects are liable to heat stress. How insects deal with heat stress is worth highlighting. Acclimation may improve the heat tolerance of insects, but the underlying mechanism remains vague. In this study, the high temperature of 39 °C was used to select the third instar larvae of the rice leaf folder Cnaphalocrocis medinalis, an important insect pest of rice, for successive generations to establish the heat-acclimated strain (HA39). The molecular mechanism of heat acclimation was explored using this strain. The HA39 larvae showed stronger tolerance to 43 °C than the unacclimated strain (HA27) persistently reared at 27 °C. The HA39 larvae upregulated a glucose dehydrogenase gene, CmGMC10, to decrease the reactive oxygen species (ROS) level and increase the survival rate under heat stress. The HA39 larvae maintained a higher activity of antioxidases than the HA27 when confronted with an exogenous oxidant. Heat acclimation decreased the H₂O₂ level in larvae under heat stress which was associated with the upregulation of CmGMC10. The rice leaf folder larvae may acclimate to global warming via upregulating CmGMC10 to increase the activity of antioxidases and alleviate the oxidative damage of heat stress.

Linking critical thermal maximum to mortality from thermal stress in a cold-water frog

Estimates of organismal thermal tolerance are frequently used to assess physiological risk from warming, yet the assumption that these estimates are predictive of mortality has been called into question. We tested this assumption in the cold-water-specialist frog, Ascaphus montanus. For seven populations, we used dynamic experimental assays to measure tadpole critical thermal maximum (CTmax) and measured mortality from chronic thermal stress for 3 days at different temperatures. We tested the relationship between previously estimated population CTmax and observed mortality, as well as the strength of CTmax as a predictor of mortality compared to local stream temperatures capturing varying timescales. Populations with higher CTmax experienced significantly less mortality in the warmest temperature treatment (25°C). We also found that population CTmax outperformed stream temperature metrics as the top predictor of observed mortality. These results demonstrate a clear link between CTmax and mortality from thermal stress, contributing evidence that CTmax is a relevant metric for physiological vulnerability assessments.

Still standing: The heat protection delivered by a facultative symbiont to its aphid host is resilient to repeated thermal stress

Insects have evolved diverse strategies to resist extreme high temperatures (EHT). The adaptive value of such strategies has to be evaluated when organisms experience multiple EHT events during their lifetime, as predicted in a changing climate. This is particularly the case for associations with facultative microbial partners involved in insect heat tolerance, the resilience of which to repeated heat stress has never been studied. We compared two artificial lines of the pea aphid (Acyrthosiphon pisum) differing by the absence or presence of the heat-protective facultative bacterium Serratia symbiotica. We exposed insect nymphs to a varying number of EHT events (between 0 and 3), and recorded fitness parameters. Except survival traits, fitness estimates were affected by the interaction between aphid infection status (absence/presence of S. symbiotica) and thermal treatment (number of heat shocks applied). Costs of bacterial infection were detected in the absence of thermal stress: symbiont-hosting aphids incurred longer development, decreased fecundity and body size. However, symbiotic infection turned neutral, and even beneficial for some traits (development and body size), as the number of heat shocks increased, and compared to the aposymbiotic strain. Conversely, symbiotic infection mediated aphid response to heat shock(s): fitness decreased only in the uninfected group. These findings suggest that (i) the facultative symbiont may alternatively act as a pathogen, commensal or mutualist depending on thermal environment, and (ii) the heat protection it delivered to its host persists under frequent EHT. We discuss eco-evolutionary implications and the role of potentially confounding factors (stage-specific effects, genetic polymorphism displayed by the obligate symbiont).

Developmental environments do not affect thermal physiological traits in reptiles: an experimental test and meta-analysis

On a global scale, organisms face significant challenges due to climate change and anthropogenic disturbance. In many ectotherms, developmental and physiological processes are sensitive to changes in temperature and resources. Developmental plasticity in thermal physiology may provide adaptive advantages to environmental extremes if early environmental conditions are predictive of late-life environments. Here, we conducted a laboratory experiment to test how developmental temperature and maternal resource investment influence thermal physiological traits (critical thermal maximum: CTmax and thermal preference: Tpref) in a common skink (Lampropholis delicata). We then compared our experimental findings more broadly across reptiles (snakes, lizards and turtles) using meta-analysis. In both our experimental study and meta-analysis, we did not find evidence that developmental environments influence CTmax or Tpref. Furthermore, the effects of developmental environments on thermal physiology did not vary by age, taxon or climate zone (temperate/tropical). Overall, the magnitude of developmental plasticity on thermal physiology appears to be limited across reptile taxa suggesting that behavioural or evolutionary processes may be more important. However, there is a paucity of information across most reptile taxa, and a broader focus on thermal performance curves themselves will be critical in understanding the impacts of changing thermal conditions on reptiles in the future.

Climate Change, Extreme Temperatures and Sex-Related Responses in Spiders

Climatic extremes, such as heat waves, are increasing in frequency, intensity and duration under anthropogenic climate change. These extreme events pose a great threat to many organisms, and especially ectotherms, which are susceptible to high temperatures. In nature, many ectotherms, such as insects, may seek cooler microclimates and 'ride out´ extreme temperatures, especially when these are transient and unpredictable. However, some ectotherms, such as web-building spiders, may be more prone to heat-related mortality than more motile organisms. Adult females in many spider families are sedentary and build webs in micro-habitats where they spend their entire lives. Under extreme heat, they may be limited in their ability to move vertically or horizontally to find cooler microhabitats. Males, on the other hand, are often nomadic, have broader spatial distributions, and thus might be better able to escape exposure to heat. However, life-history traits in spiders such as the relative body size of males and females and spatial ecology also vary across different taxonomic groups based on their phylogeny. This may make different species or families more or less susceptible to heat waves and exposure to very high temperatures. Selection to extreme temperatures may drive adaptive responses in female physiology, morphology or web site selection in species that build small or exposed webs. Male spiders may be better able to avoid heat-related stress than females by seeking refuge under objects such as bark or rocks with cooler microclimates. Here, we discuss these aspects in detail and propose research focusing on male and female spider behavior and reproduction across different taxa exposed to temperature extremes.

Climate drives global functional trait variation in lizards

A major challenge in ecology and evolution is to disentangle the mechanisms driving broad-scale variation in biological traits such as body size, colour, thermal physiology traits and behaviour. Climate has long been thought to drive trait evolution and abiotic filtering of trait variation in ectotherms because their thermal performance and fitness are closely related to environmental conditions. However, previous studies investigating climatic variables associated with trait variation have lacked a mechanistic description of the underpinning processes. Here, we use a mechanistic model to predict how climate affects thermal performance of ectotherms and thereby the direction and strength of the effect of selection on different functional traits. We show that climate drives macro-evolutionary patterns in body size, cold tolerance and preferred body temperatures among lizards, and that trait variation is more constrained in regions where selection is predicted to be stronger. These findings provide a mechanistic explanation for observations on how climate drives trait variation in ectotherms through its effect on thermal performance. By connecting physical, physiological and macro-evolutionary principles, the model and results provide an integrative, mechanistic framework for predicting organismal responses to present climates and climate change.

Infection burdens and virulence under heat stress: ecological and evolutionary considerations

As a result of global change, hosts and parasites (including pathogens) are experiencing shifts in their thermal environment. Despite the importance of heat stress tolerance for host population persistence, infection by parasites can impair a host's ability to cope with heat. Host-parasite eco-evolutionary dynamics will be affected if infection reduces host performance during heating. Theory predicts that within-host parasite burden (replication rate or number of infecting parasites per host), a key component of parasite fitness, should correlate positively with virulence-the harm caused to hosts during infection. Surprisingly, however, the relationship between within-host parasite burden and virulence during heating is often weak. Here, we describe the current evidence for the link between within-host parasite burden and host heat stress tolerance. We consider the biology of host-parasite systems that may explain the weak or absent link between these two important host and parasite traits during hot conditions. The processes that mediate the relationship between parasite burden and host fitness will be fundamental in ecological and evolutionary responses of host and parasites in a warming world. This article is part of the theme issue 'Infectious disease ecology and evolution in a changing world'.

Vulnerability to climate change increases with trophic level in terrestrial organisms

The resilience of ecosystem function under global climate change is governed by individual species vulnerabilities and the functional groups they contribute to (e.g. decomposition, primary production, pollination, primary, secondary and tertiary consumption). Yet it remains unclear whether species that contribute to different functional groups, which underpin ecosystem function, differ in their vulnerability to climate change. We used existing upper thermal limit data across a range of terrestrial species (N = 1701) to calculate species warming margins (degrees distance between a species upper thermal limit and the maximum environmental temperature they inhabit), as a metric of climate change vulnerability. We examined whether species that comprise different functional groups exhibit differential vulnerability to climate change, and if vulnerability trends change across geographic space while considering evolutionary history. Primary producers had the broadest warming margins across the globe (μ = 18.72 °C) and tertiary consumers had the narrowest warming margins (μ = 9.64 °C), where vulnerability tended to increase with trophic level. Warming margins had a nonlinear relationship (second-degree polynomial) with absolute latitude, where warming margins were narrowest at about 33°, and were broader at lower and higher absolute latitudes. Evolutionary history explained significant variation in species warming margins, as did the methodology used to estimate species upper thermal limits. We investigated if variation in body mass across the trophic levels could explain why higher trophic level organisms had narrower warming margins than lower trophic level organisms, however, we did not find support for this hypothesis. This study provides a critical first step in linking individual species vulnerabilities with whole ecosystem responses to climate change.

Comparative transcriptome analysis of Callosobruchus chinensis (L.) (Coleoptera: Chrysomelidae-Bruchinae) after heat and cold stress exposure

Callosobruchus chinensis is regarded as one of the cosmopolitan pests of legume crops and can cause tremendous losses to a variety of beans. This study focused on comparative transcriptome analyses of C. chinensis exposed to 45 °C (heat stress), 27 °C (ambient temperature) and -3 °C (cold stress) for 3 h to investigate the gene differences and underlying molecular mechanisms. There were 402 and 111 differentially expressed genes (DEGs) identified in the heat and cold stress treatments, respectively. "cell process", "cell" and "binding" were the main enriched functions and biological processes revealed by gene ontology (GO) analysis. The clusters of orthologous genes (COG) showed that DEGs were assigned to the categories: "posttranslational modification, protein turnover, chaperones", "lipid transport and metabolism", and "general function prediction only". With respect to the Kyoto Encyclopedia of Genes and Genomes (KEGG), the "longevity regulating pathway-multiple species", "carbon metabolism", "peroxisome", "protein processing in endoplasmic", "glyoxylate and dicarboxylate metabolism" pathways were significantly enriched. The annotation and enrichment analysis revealed that genes encoding heat shock proteins (Hsps) and cuticular proteins were significantly upregulated under high and low-temperature stresses, respectively. In addition, some DEGs encoding "Protein lethal essential for life", "Reverse transcriptase", "DnaJ domain", "Cytochrome" and "Zinc finger protein" were also upregulated to varying degrees. Transcriptomic data were validated using qRT‒PCR, which confirmed that they were consistent. In this paper, the temperature tolerance of C. chinensis adults was evaluated and the results showed that female adults were more sensitive to heat and cold stress than males, and the upregulation of heat shock protein and epidermal protein was the largest in DEGs after heat and cold stress, respectively. These findings provide a reference for further understanding the biological characteristics of C. chinensis adults and the molecular mechanisms underlying the response to low and high temperatures.

Experimental temperatures shape host microbiome diversity and composition

Global climate change has led to more extreme thermal events. Plants and animals harbour diverse microbial communities, which may be vital for their physiological performance and help them survive stressful climatic conditions. The extent to which microbiome communities change in response to warming or cooling may be important for predicting host performance under global change. Using a meta-analysis of 1377 microbiomes from 43 terrestrial and aquatic species, we found a decrease in the amplicon sequence variant-level microbiome phylogenetic diversity and alteration of microbiome composition under both experimental warming and cooling. Microbiome beta dispersion was not affected by temperature changes. We showed that the host habitat and experimental factors affected microbiome diversity and composition more than host biological traits. In particular, aquatic organisms-especially in marine habitats-experienced a greater depletion in microbiome diversity under cold conditions, compared to terrestrial hosts. Exposure involving a sudden long and static temperature shift was associated with microbiome diversity loss, but this reduction was attenuated by prior-experimental lab acclimation or when a ramped regime (i.e., warming) was used. Microbial differential abundance and co-occurrence network analyses revealed several potential indicator bacterial classes for hosts in heated environments and on different biome levels. Overall, our findings improve our understanding on the impact of global temperature changes on animal and plant microbiome structures across a diverse range of habitats. The next step is to link these changes to measures of host fitness, as well as microbial community functions, to determine whether microbiomes can buffer some species against a more thermally variable and extreme world.

Neotropical stingless bees display a strong response in cold tolerance with changes in elevation

Tropical pollinators are expected to experience substantial effects due to climate change, but aspects of their thermal biology remain largely unknown. We investigated the thermal tolerance of stingless honey-making bees, the most ecologically, economically and culturally important group of tropical pollinators. We assessed changes in the lower (CT_Min) and upper (CT_Max) critical thermal limits of 17 species (12 genera) at two elevations (200 and 1500 m) in the Colombian Andes. In addition, we examined the influence of body size (intertegular distance, ITD), hairiness (thoracic hair length) and coloration (lightness value) on bees' thermal tolerance. Because stingless beekeepers often relocate their colonies across the altitudinal gradient, as an initial attempt to explore potential social responses to climatic variability, we also tracked for several weeks brood temperature and humidity in nests of three species at both elevations. We found that CT_Min decreased with elevation while CT_Max was similar between elevations. CT_Min and CT_Max increased (low cold tolerance and high heat tolerance) with increasing ITD, hair length and lightness value, but these relationships were weak and explained at most 10% of the variance. Neither CT_Min nor CT_Max displayed significant phylogenetic signal. Brood nest temperature tracked ambient diel variations more closely in the low-elevation site, but it was constant and higher at the high-elevation site. In contrast, brood nest humidity was uniform throughout the day regardless of elevation. The stronger response in CT_Min, and a similar CT_Max between elevations, follows a pattern of variation documented across a wide range of taxa that is commonly known as the Brett's heat-invariant hypothesis. Our results indicate differential thermal sensitivities and potential thermal adaptations to local climate, which support ongoing conservation policies to restrict the long-distance relocations of colonies. They also shed light on how malleable nest thermoregulation can be across elevations.

High thermal tolerance in high-elevation species and laboratory-reared colonies of tropical bumble bees

Bumble bees are key pollinators with some species reared in captivity at a commercial scale, but with significant evidence of population declines and with alarming predictions of substantial impacts under climate change scenarios. While studies on the thermal biology of temperate bumble bees are still limited, they are entirely absent from the tropics where the effects of climate change are expected to be greater. Herein, we test whether bees' thermal tolerance decreases with elevation and whether the stable optimal conditions used in laboratory-reared colonies reduces their thermal tolerance. We assessed changes in the lower (CT_Min) and upper (CT_Max) critical thermal limits of four species at two elevations (2600 and 3600 m) in the Colombian Andes, examined the effect of body size, and evaluated the thermal tolerance of wild-caught and laboratory-reared individuals of Bombus pauloensis. We also compiled information on bumble bees' thermal limits and assessed potential predictors for broadscale patterns of variation. We found that CT_Min decreased with increasing elevation, while CT_Max was similar between elevations. CT_Max was slightly higher (0.84°C) in laboratory-reared than in wild-caught bees while CT_Min was similar, and CT_Min decreased with increasing body size while CT_Max did not. Latitude is a good predictor for CT_Min while annual mean temperature, maximum and minimum temperatures of the warmest and coldest months are good predictors for both CT_Min and CT_Max. The stronger response in CT_Min with increasing elevation, and similar CT_Max, supports Brett's heat-invariant hypothesis, which has been documented in other taxa. Andean bumble bees appear to be about as heat tolerant as those from temperate areas, suggesting that other aspects besides temperature (e.g., water balance) might be more determinant environmental factors for these species. Laboratory-reared colonies are adequate surrogates for addressing questions on thermal tolerance and global warming impacts.

Acute, diel, and annual temperature variability and the thermal biology of ectotherms

Global warming is increasing mean temperatures and altering temperature variability at multiple temporal scales. To better understand the consequences of changes in thermal variability for ectotherms it is necessary to consider thermal variation at different time scales (i.e., acute, diel, and annual) and the responses of organisms within and across generations. Thermodynamics constrain acute responses to temperature, but within these constraints and over longer time periods, organisms have the scope to adaptively acclimate or evolve. Yet, hypotheses and predictions about responses to future warming tend not to explicitly consider the temporal scale at which temperature varies. Here, focusing on multicellular ectothermic animals, we argue that consideration of multiple processes and constraints associated with various timescales is necessary to better understand how altered thermal variability because of climate change will affect ectotherms.

Tolerance of high temperature and associated effects on reproduction in euedaphic Collembola

Studies show that tropical and mid-latitude terrestrial ectotherms are more vulnerable to global warming than species from high latitudes. However, thermal tolerance studies from these regions still lack soil invertebrates. In the present study, we investigated six euedaphic species of Collembola (of the genera Onychiurus and Protaphorura) sampled across latitudes ranging from 31° N to 64° N and determined their upper thermal limit (UTL) by static assays. In another experiment, we submitted springtails to high temperatures for exposure times, causing 5% to 30% mortality within each species. Survivors from this series of increasing heat injuries were used to determine the time-to-first-oviposition and the number of eggs produced following heat exposure. Two hypotheses are tested in this study: 1) heat tolerance of species correlates positively with the environmental temperature of their habitat; 2) the most heat-tolerant species require less time to regain reproduction and produce more eggs than the least heat-tolerant species. Results showed that the UTL positively correlates to the soil temperature of the sampling site. The sequence of UTL₆₀ (the temperature causing 50% mortality after 60 min of exposure) from highest to least was O. yodai > P. fimata > P. armata ≈ P. tricampata > P. macfadyeni > P. pseudovanderdrifti. Heat stress inflicted on springtails can delay reproduction in all species, and two species showed a reduced egg production rate after heat exposure. For heat stress causing up to 30% mortality, the most heat-tolerant species did not have advantages over the least heat-tolerant species for what concerns the recovery of reproduction. The relation between UTL and recovery from heat stress is not linear. Our study provides evidence for a potential long-term effect of high-temperature exposure on euedaphic species of Collembola and highlights the need for further studies on the effects of global warming on soil-living organisms.

There and back again: A meta-analytical approach on the influence of acclimation and altitude in the upper thermal tolerance of amphibians and reptiles

Realistic predictions about the impacts of climate change onbiodiversity requires gathering ecophysiological data and the critical thermal maxima (CTMax) is the most frequently used index to assess the thermal vulnerability of species. In the present study, we performed a systematic review to understand how acclimation and altitude affect CTMax estimates for amphibian and non-avian reptile species. We retrieved CTMax data for anurans, salamanders, lizards, snakes, and turtles/terrapins. Data allowed to perform a multilevel random effects meta-analysis to answer how acclimation temperature affect CTMax of Anura, Caudata, and Squamata and also meta-regressions to assess the influence of altitude on CTMax of frogs and lizards. Acclimation temperature influenced CTMax estimates of tadpoles, adult anurans, salamanders, and lizards, but not of froglets. In general, the increase in acclimation temperature led to higher CTMax values. Altitudinal bioclimatic gradient had an inverse effect for estimating the CTMax of lizards and anuran amphibians. For lizards, CTMax was positively influenced by the mean temperature of the wettest quarter. For anurans, the relationship is inverse; we recover a trend of decreasing CTMax when max temperature of warmest month and precipitation seasonality increase. There is an urgent need for studies to investigate the thermal tolerance of subsampled groups or even for which we do not have any information such as Gymnophiona, Serpentes, Amphisbaena, and Testudines. Broader phylogenetic coverage is mandatory for more accurate analyses of macroecological and evolutionary patterns for thermal tolerance indices as CTMax.