Thermal Breadth Research Articles

Elevational and local climate variability predicts thermal breadth of mountain tropical tadpoles

The climate variability hypothesis posits that increased environmental thermal variation should select for thermal generalists, while stable environments should favor thermal specialists. This hypothesis has been tested on large spatial scales, such as latitude and elevation, but less so on smaller scales reflective of the experienced microclimate. Here, we estimated thermal tolerance limits of 75 species of amphibian tadpoles from an aseasonal tropical mountain range of the Ecuadorian Andes, distributed along a 3500 m elevational range, to test the climatic variability hypothesis at a large (elevation) and a small (microhabitat) scale. We show how species from less variable thermal habitats, such as lowlands and those restricted to streams, exhibit narrower thermal tolerance breadths than highland and pond‐dwelling species respectively. Interestingly, while broader thermal tolerance breadths at large scales are driven by higher cold tolerance variation (heat‐invariant hypothesis), at local scales they are driven by higher heat tolerance variation. This contrasting pattern may result from divergent selection on both thermal limits to face environmental thermal extremes at different scales. Specifically, within the same elevational window, exposure to extreme maximum temperatures could be avoided through habitat shifts from temporary ponds to permanent ponds or streams, while minimum peak temperatures remained invariable between habitats but steadily decreased with elevation. Therefore an understanding of the effects of habitat conversion is crucial for future research on resilience to climate change.

Hypoxia Decreases Thermal Sensitivity and Increases Thermal Breadth of Locomotion in the Invasive Freshwater Snail Potamopyrgus antipodarum.

AbstractUnderstanding the physiology of invasive species will contribute to better prediction and prevention measures to avoid the economic and environmental consequences of biological invasions. Predicting the future range of Potamopyrgus antipodarum, a globally invasive aquatic snail, relies on a comprehensive understanding of its physiological tolerances to individual and combined environmental stressors. We conducted a laboratory study to investigate the interacting effects of temperature and dissolved oxygen in shaping the abiotic niche of P. antipodarum. We generated thermal performance curves (7°C-35°C) for resting respiration rate and voluntary locomotor behaviors under normoxia and hypoxia to find the conditions that limited each performance. Extreme high (>30°C) and low (<12°C) temperatures limited respiration and activity, but respiration rate was most oxygen sensitive at low temperatures. Under hypoxic conditions, activity was less thermally sensitive. Increased activity under high temperatures (22°C-28°C) may be fueled by anaerobic metabolism. Relying on anaerobic energy is a time-limited survival strategy, so further warming and deoxygenation of freshwater systems may limit the spread of this very tolerant invasive species.

Thermal stress responses of the antipatharian Stichopathes sp. from the mesophotic reef of Mo'orea, French Polynesia

Antipatharians, also called black corals, are present in almost all oceans of the world, until extreme depths. In several regions, they aggregate in higher densities to form black coral beds that support diverse animal communities and create biodiversity hotspots. These recently discovered ecosystems are currently threatened by fishing activities and illegal harvesting for commercial purposes. Despite this, studies dedicated to the physiology of antipatharians are scarce and their responses to global change stressors have remained hardly explored since recently. Here, we present the first study on the physiological responses of a mesophotic antipatharian Stichopathes sp. (70–90 m) to thermal stress through a 16-d laboratory exposure (from 26 to 30.5 °C). Oxygen consumption measurements allowed identifying the physiological tipping point of Stichopathes sp. (Topt = 28.3 °C; 2.7 °C above mean ambient condition). Our results follow theoretical predictions as performances start to decrease beyond Topt, with lowered oxygen consumption rates, impairment of the healing capacities, increased probability of tissue necrosis and stress responses activated as a function of temperature (i.e. increase in mucocyte density and total antioxidant capacity). Altogether, our work indicates that Stichopathes sp. lives at suboptimal performances during the coldest months of the year, but also that it is likely to have low acclimatization capacity and a narrow thermal breadth.

Determining the Underlying Mechanisms of Thermal Breadth in Aquatic Insects from Adjacent But Thermally Distinct Streams – a Test of the Climate Variability Hypothesis

Determining the Underlying Mechanisms of Thermal Breadth in Aquatic Insects from Adjacent But Thermally Distinct Streams – a Test of the Climate Variability Hypothesis

Impact of age, sex, and size on the thermal tolerance of the adult black soldier fly (Diptera: Stratiomyidae)

Thermal stresses from both environmental conditions and organismal crowding are common in mass production of the black soldier fly, Hermetia illucens L. (Diptera: Stratiomyidae). In this study, upper and lower critical thermal (CT) limits (i.e. knockdown CTmax and CTmin) for the adult black soldier fly were determined. Impacts of size, age, and sex on these critical temperatures were also assessed. The CTmax ranged from 45.0-51.0 °C with larger and older adults having a ~1 °C higher CTmax than smaller and younger adults. However, no differences in the CTmax were found between sexes, regardless of age or size. The CTmin ranged from 8.0 to 13.0 °C with larger and older females having a ~1 °C higher CTmin than males and smaller or younger females. While reporting the upper and lower critical temperatures, this study also revealed the thermal breadth (i.e. the range of body temperatures over which organisms can locomote) for adult black soldier flies across age, sex, and size. Based on these data, and when recognising not all fly populations are the same, mass-rearing facilities should determine the CTmax and CTmin for their fly population in order to optimise mating and fertile egg production, and ultimately maximise profits and sales. One degree of temperature can be the difference between success and failure in industrialised facilities.

Characterizing environmental stress responses of aposymbiotic Astrangia poculata to divergent thermal challenges

Anthropogenic climate change threatens corals globally and both high and low temperatures are known to induce coral bleaching. However, coral stress responses across wide thermal breadths remain understudied. Disentangling the role of symbiosis on the stress response in obligately symbiotic corals is challenging because this response is inherently coupled with nutritional stress. Here, we leverage aposymbiotic colonies of the facultatively symbiotic coral, Astrangia poculata, which lives naturally with and without its algal symbionts, to examine how broad thermal challenges influence coral hosts in the absence of symbiosis. A. poculata were collected from their northern range limit and thermally challenged in two independent 16-day common garden experiments (heat and cold challenge) and behavioural responses to food stimuli and genome-wide gene expression profiling (TagSeq) were performed. Both thermal challenges elicited significant reductions in polyp extension. However, there were five times as many differentially expressed genes (DEGs) under cold challenge compared to heat challenge. Despite an overall stronger response to cold challenge, there was significant overlap in DEGs between thermal challenges. We contrasted these responses to a previously identified module of genes associated with the environmental stress response (ESR) in tropical reef-building corals. Cold challenged corals exhibited a pattern consistent with more severe stressors while the heat challenge response was consistent with lower intensity stressors. Given that these responses were observed in aposymbiotic colonies, many genes previously implicated in ESRs in tropical symbiotic species may represent the coral host's stress response in or out of symbiosis.

Elevational gradients do not affect thermal tolerance at local scale in populations of livebearing fishes of the genus Limia (Cyprinodontiformes: Poeciliinae)

One of the main assumptions of Janzen’s mountain passes hypothesis is that due the low overlap in temperature regimes between low and high elevations in the tropics, organisms living in high-altitude evolve narrow tolerance for colder temperatures while low-altitude species develop narrow tolerance for warmer temperatures. Some studies have questioned the generality of the assumptions and predictions of this hypothesis suggesting that other factors different to temperature gradients between low and high elevations may explain altitudinal distribution of species in the tropics. In this study we test some predictions of the Janzen’s hypothesis at local scales through the analysis of the individual thermal niche breadth in populations of livebearing fishes of the genus Limia and its relationship with their altitudinal distribution in some islands of the Greater Antilles. We assessed variation in tolerance to extreme temperatures (measured as critical thermal minimum (CTmin) and maximum (CTmax) and compared thermal breadth for populations of eight species of Limia occurring in three Caribbean islands and that occupy different altitudinal distribution. Our results showed that species analyzed had significant differences in thermal limits and ranges. Generally, species distributed in high and low elevations did not differ in thermal limits and showed a wider range of thermal tolerance. However, species living in mid-elevations had narrower range of temperature tolerance. We found no significant effect of phylogeny on CTmin, CTmax and thermal ranges among species. This study did not provide evidence supporting Janzen’s hypothesis at a local scale since thermal tolerance and altitudinal distribution of Limia species were not related to temperature gradients expected in nature. Phylogeny also did not explain the patterns we observed. We suggest that biotic factors such as species interactions, diet specializations, and others should be considered when interpreting current distribution patterns of Limia species.

Similar thermal breadth of two temperate coral species from the Mediterranean Sea and two tropical coral species from the Great Barrier Reef

Similar thermal breadth of two temperate coral species from the Mediterranean Sea and two tropical coral species from the Great Barrier Reef

Community-wide seasonal shifts in thermal tolerances of mosquitoes.

The broadening in species' thermal tolerance limits and breadth from tropical to temperate latitudes is proposed to reflect spatial gradients in temperature seasonality, but the importance of seasonal shifts in thermal tolerances within and across locations is much less appreciated. We performed thermal assays to examine the maximum and minimum critical temperatures (CTmax and CTmin , respectively) of a mosquito community across their active seasons. Mosquito CTmin tracked seasonal shifts in temperature, whereas CTmax tracked a countergradient pattern with lowest heat tolerances in summer. Mosquito thermal breadth decreased from spring to summer and then increased from summer to autumn. We show a temporal dichotomy in thermal tolerances with thermal breadths of temperate organisms in summer reflecting those of the tropics ("tropicalization") that is sandwiched between a spring and autumn "temperatization." Therefore, our tolerance patterns at a single temperate latitude recapitulate classical patterns across latitude. These findings highlight the need to understand the temporal and spatial components of thermotolerance variation better, including plasticity and rapid seasonal selection, and the potential for this variation to affect species responses to climate change. With summers becoming longer and increasing winter nighttime temperatures, we expect increasing tropicalization of species thermal tolerances in both space and time.

Phenotypic plasticity is not a cline: Thermal physiology of an intertidal barnacle over 20° of latitude.

Our understanding of the plastic and evolutionary potential of ectothermic organisms and their populational impacts in the face of rapid global change remains limited. Studies attempting on the relationship between the magnitude of thermal variability across latitude and the degree of phenotypic plasticity exhibited by marine ectotherms are inconclusive. We state that the latter arises from the narrow range of thermal variability captured by the limited span of the latitudinal gradients studied to date. Using a mechanistic ecophysiological approach and a satellite-based assessment of the relevant environmental variables (i.e. temperature and food availability), we studied individuals of the intertidal barnacle Jehlius cirratus from seven local populations widely spread along the Humboldt current system that spanning two biogeographic regions. At the same time, we synthesized published information on the local abundance of our study species across a total of 76 sites representing 20° of latitude, and spanning from 18 to 42°S. We examined the effects of latitude and environmental variability on metabolic rate plasticity, thermal tolerance (thermal breadth and thermal safety margins) and their impacts on the abundance of this widespread marine invertebrate. We demonstrate that the phenotypic plasticity of metabolic rate in J. cirratus populations is not related to latitude. In turn, thermal breadth is explained by the temperature variability each population experiences. Furthermore, we found clinal variation with a poleward decrease of the critical thermal minimum, suggesting that episodic extreme low temperatures represent a ubiquitous selective force on the lower thermal limit for ectotherms. Across our study gradient, plasticity patterns indicate that populations at the equatorial extreme are more vulnerable to a warming climate, while populations located in the biogeographic transitional zone (i.e. high environmental heterogeneity), on the centre of the gradient, display higher levels of phenotypic plasticity and may represent a genetic buffer for the effects of ocean warming. Together, our results suggest the existence of a fitness trade-off involving the metabolic cost of plasticity and population density that is evident only across the vast latitudinal gradient examined.

Thermal acclimation has limited effect on the thermal tolerances of summer-collected Arctic and sub-Arctic wolf spiders

High-latitude ectotherms contend with large daily and seasonal temperature variation. Summer-collected wolf spiders (Araneae; Lycosidae) from sub-Arctic and Arctic habitats have been previously documented as having low temperature tolerance insufficient for surviving year-round in their habitat. We tested two competing hypotheses: that they would have broad thermal breadth, or that they would use plasticity to extend the range of their thermal performance. We collected Pardosa moesta and P. lapponica from the Yukon Territory, Canada, P. furcifera, P. groenlandica, and P. hyperborea from southern Greenland, and P. hyperborea from sub-Arctic Norway, and acclimated them to warm (12 or 20 °C) or cool (4 °C) conditions under constant light for one week. We measured critical thermal minimum (CTmin) or supercooling point (SCP) as a measure of lower thermal limit, and critical thermal maximum (CTmax) as a measure of upper thermal limit. We found relatively little impact of acclimation on thermal limits, and some counterintuitive responses; for example, warm acclimation decreased the SCP and/or cool acclimation increased the CTmax in several cases. Together, this meant that acclimation did not appear to modify the thermal breadth, which supports our first hypothesis, but allows us to reject the hypothesis that spiders use plasticity to fine-tune their thermal physiology, at least in the summer. We note that we still cannot explain how these spiders withstand the very cold winters, and speculate that there are acclimatisation cues or processes that we were unable to capture in our study.

Thermal tolerance, safety margins and vulnerability of coastal species: Projected impact of climate change induced cold water variability in a temperate African region

Anthropogenic induced climate change is predicted to increase the thermal variability in coastal waters, which can have strong physiological effects on individuals and populations of marine ectotherms. The magnitude and direction of these thermal effects varies depending on species, life stage, biogeography, habitat and season. This study aimed to compare the thermal tolerance of a range of juvenile fish and adult macro-invertebrates from intertidal and estuarine habitats in a warm-temperate, thermally variable region on the south-east coast of South Africa. Seasonal variability in thermal tolerance was compared between species, taxonomic groups, biogeographical distribution and habitat affinity and related to existing and projected water temperature data to gauge the local vulnerability of each species. Critical thermal maximum (CTmax), critical thermal minimum (CTmin), thermal breadths and scopes, and the thermal safety margins of each species were quantified. The greatest differences in thermal tolerance patterns were based on taxonomy, with macro-invertebrates having broader thermal tolerance compared to fish, with the exception of the Cape sea urchin, in both summer and winter. Relatively narrow lower breadths in tolerance and safety margin values for transient juvenile sub-tropical and temperate fish species from the intertidal rocky low-shore habitat were observed in both summer and winter. This indicates that these fish species and the Cape sea urchin may be more vulnerable to projected increases in cold temperature (upwelling in summer) than warm temperature variability in this warm-temperate region if they are unable to seek thermal habitat refuge.

Photosystem II heat tolerances characterize thermal generalists and the upper limit of carbon assimilation.

The heat tolerance of photosystem II (PSII) may promote carbon assimilation at higher temperatures and help explain plant responses to climate change. Higher PSII heat tolerance could lead to (a) increases in the high-temperature compensation point (Tmax ); (b) increases in the thermal breadth of photosynthesis (i.e. the photosynthetic parameter Ω) to promote a thermal generalist strategy of carbon assimilation; (c) increases in the optimum rate of carbon assimilation Popt and faster carbon assimilation and/or (d) increases in the optimum temperature for photosynthesis (Topt ). To address these hypotheses, we tested if the Tcrit , T50 and T95 PSII heat tolerances were correlated with carbon assimilation parameters for 21 plant species. Our results did not support Hypothesis 1, but we observed that T50 may be used to estimate the upper thermal limit for Tmax at the species level, and that community mean Tcrit may be useful for approximating Tmax . The T50 and T95 heat tolerance metrics were positively correlated with Ω in support of Hypothesis 2. We found no support for Hypotheses 3 or 4. Our study shows that high PSII heat tolerance is unlikely to improve carbon assimilation at higher temperatures but may characterize thermal generalists with slow resource acquisition strategies.

Thermal preferences of two spider species: an orb-web weaver and a synanthropic funnel-web weaver

Temperature, an important factor in the ecological niche, determines the individual fitness, distribution and abundance of many spider species. Typically, spiders from different habitats have different thermal preferences, which vary depending on the season, time of day, and reproductive status. Selected temperatures may also depend on thermal acclimation of animals. The study aimed at determining preferred temperatures of females of two spider species, synanthropic Eratigena atrica and garden Araneus diadematus, exposed to low (14°C) and high (23°C) acclimation temperatures. The mean preferred temperature observed in a thermal gradient varied between the species: E. atrica selected 13.7 ± 1.0°C, while A. diadematus selected 22.5 ± 1.5°C, irrespective of the acclimation temperature. Accordingly, thermal niche overlaps between the species were significantly lower than predicted by the random distribution. E. atrica changed their thermal environment on average 12.8 ± 1.9 times during the test, which made them more mobile than A. diadematus, changing their thermal environment 2.7 ± 0.5 times. On the other hand, the thermal niche breadth of A. diadematus acclimated to warm temperature was wider than that of E. atrica. The female behaviour may be related to the selection of optimal conditions with regard to food availability (location for web construction), shelter and/or, in the case of E. atrica, embryonic development.

Countergradient variation concealed adaptive responses to temperature increase in Daphnia from heated lakes

AbstractTo test the general assumption that global warming will induce body size reduction in aquatic organisms, we used a system of lakes continually heated for six decades by warm water discharge from power plants. Their temperature elevation of 3–4°C corresponds with climate change forecasts for the end of the 21st century. We compared body size and reproduction of Daphnia longispina complex communities inhabiting heated and non‐heated (control) lakes nearby. No difference in body size was found, but Daphnia communities from heated lakes had a wider thermal breadth for reproduction. The two lake groups varied in the taxonomic composition of Daphnia communities. Thus, to disentangle inter‐ and intraspecific sources of variation, and to examine evolution vs. phenotypic plasticity of investigated traits, we performed two life history experiments: (1) a between‐species experiment compared D. galeata inhabiting heated lakes with D. longispina typical of nearby control lakes, under three temperature regimes; (2) a within‐species experiment compared D. galeata from heated lakes with conspecifics from high latitude (cold control) and low latitude (warm control) lakes, under two temperature regimes. The experiments revealed countergradient variation: environmental constraints on body size in situ concealed evolution of larger potential body size in Daphnia from heated lakes. In turn, evolution of increased body size plasticity resulted in an efficient resource allocation trade‐off: more effective reproduction at high temperature, at the cost of size reduction. We suggest that large size is adaptive during active overwintering, while plastic size reduction is a coping strategy for high temperatures.

A classification system for predicting invasiveness using climatic niche traits and global distribution models: application to alien plant species in Chile

Functional traits that predict plant invasiveness are a central issue in invasion ecology. However, in many cases they are difficult to determine, especially for a large set of species. Climatic niche traits can overcome this problem due to the ease of acquiring them for a large number of species. This effort is critical given that knowledge of species invasiveness is necessary (although not sufficient) to anticipate/manage invasive species. In this study, we examined thermal and hydric niche traits to predict plant invasiveness. We used a set of 49 alien plant species, representative of the alien flora of Chile. Niche traits were obtained using environmental information (WorldClim) and global occurrences. Invasiveness was estimated using global niche models and projection of the potential distribution in Chile. As a final step, we reviewed the literature for a subset of species, documenting their impacts on a) biodiversity, b) crop agriculture and c) livestock. Thermal niche breadth and thermal niche position were the most important niche traits to predict potential distribution (a proxy of invasiveness). Using thermal niche breadth and niche position traits, we constructed a graphical model that classifies alien species as highly invasive (wide thermal niche breadth and low niche position) or low potential to be invasive (narrow niche breadth and high niche position). We also found no association between our invasiveness classification and the documented impact of alien species.

Implications of increasing temperature stress for predatory biocontrol of vector mosquitoes

BackgroundPredators play a critical role in regulating larval mosquito prey populations in aquatic habitats. Understanding predator-prey responses to climate change-induced environmental perturbations may foster optimal efficacy in vector reduction. However, organisms may differentially respond to heterogeneous thermal environments, potentially destabilizing predator-prey trophic systems.MethodsHere, we explored the critical thermal limits of activity (CTLs; critical thermal-maxima [CTmax] and minima [CTmin]) of key predator-prey species. We concurrently examined CTL asynchrony of two notonectid predators (Anisops sardea and Enithares chinai) and one copepod predator (Lovenula falcifera) as well as larvae of three vector mosquito species, Aedes aegypti, Anopheles quadriannulatus and Culex pipiens, across instar stages (early, 1st; intermediate, 2nd/3rd; late, 4th).ResultsOverall, predators and prey differed significantly in CTmax and CTmin. Predators generally had lower CTLs than mosquito prey, dependent on prey instar stage and species, with first instars having the lowest CTmax (lowest warm tolerance), but also the lowest CTmin (highest cold tolerance). For predators, L. falcifera exhibited the narrowest CTLs overall, with E. chinai having the widest and A. sardea intermediate CTLs, respectively. Among prey species, the global invader Ae. aegypti consistently exhibited the highest CTmax, whilst differences among CTmin were inconsistent among prey species according to instar stage.ConclusionThese results point to significant predator-prey mismatches under environmental change, potentially adversely affecting natural mosquito biocontrol given projected shifts in temperature fluctuations in the study region. The overall narrower thermal breadth of native predators relative to larval mosquito prey may reduce natural biotic resistance to pests and harmful mosquito species, with implications for population success and potentially vector capacity under global change.Graphical

Thermal adaptations to extreme freeze–thaw cycles in the high tropical Andes

AbstractTemperature plays a key role in the biology of ectotherms, including anurans, which are found at higher elevations in the tropics than anywhere in the temperate zone. High elevation tropical environments are characterized by extreme daily thermal fluctuation including high daily maxima and nightly freezing. Our study investigated the contrasting operative temperatures of the anurans Telmatobius marmoratus and Pleurodema marmoratum in different environmental contexts at the same elevation and biome above 5,200 m. Telmatobius marmoratus avoids extremes of daily temperature fluctuation by utilizing thermally buffered aquatic habitat at all life stages, with minimal operative temperature variation (range: 4.6–8.0°C). Pleurodema marmoratum, in contrast, experienced operative temperatures from −3.5 to 44°C and has one of the widest thermal breadths reported for any tropical frog, from &gt;32°C (critical thermal maximum) to surviving freezing periods of 1 and 6 hr down to −3.0°C. Our findings expand experimental evidence of frost tolerance in amphibians to the widespread Neotropical family Leptodactylidae, the first such evidence of frost tolerance in a tropical amphibian. Our study identifies three strategies (wide thermal tolerance breadth, use of buffered microhabitats, and behavioral thermoregulation), which allow these tropical frogs to withstand the current wide daily thermal fluctuation above 5,000 m.a.s.l. and which may help them adapt to future climatic changes.Abstract in Spanish is available with online material

Loss of heat acclimation capacity could leave subterranean specialists highly sensitive to climate change

AbstractPhysiological traits are key in determining the vulnerability of narrow range, highly specialized animals to climate change. It is generally predicted that species from more stable environments possess lower thermal tolerance breadths and thermal plasticity than those from more variable habitats – the so‐called ‘climatic variability hypothesis’. However, evolutionary trade‐offs between thermal breadth and its plasticity are also seen in some taxa, and the evolution of thermal physiology remains poorly understood. Subterranean environments are excellent systems for exploring these issues, being characterized by stable climatic conditions, with environmental variability increasing predictably from deep to shallow habitats. Acclimation capacity will be fundamental in determining the sensitivity of subterranean species to climate change, since they have poor dispersal capacity and limited possibility to exploit thermally different microhabitats in the uniform cave environment. We assessed critical thermal maximum (CTmax) and short‐term heat acclimation capacity in three related beetles (Leiodidae: Leptodirini) with differing degrees of specialization to the subterranean environment (deep, shallow and facultatively subterranean, respectively) and therefore exposed to contrasting thermal variability in nature. Only the facultative subterranean species showed any acclimatory capacity, also having the highest CTmax across the taxa studied. However, this species might experience the highest thermal stress in its habitat under climate change. The studied subterranean specialists will be poorly able to cope physiologically with temperature increase, but in contrast exposed to lower magnitude and rate of warming. Our results fit the climatic variability hypothesis, suggesting that adaptation to cave conditions has selected against the retention of acclimation mechanisms. We show that the pathways that determine vulnerability of subterranean species to climate change depend on their degree of specialization to deep subterranean environments. This information, combined with evaluation of exposure to climatic changes at their present locations, is fundamental in identifying species or populations at greatest risk.

Transmission of West Nile and five other temperate mosquito-borne viruses peaks at temperatures between 23°C and 26°C.

The temperature-dependence of many important mosquito-borne diseases has never been quantified. These relationships are critical for understanding current distributions and predicting future shifts from climate change. We used trait-based models to characterize temperature-dependent transmission of 10 vector-pathogen pairs of mosquitoes (Culex pipiens, Cx. quinquefascsiatus, Cx. tarsalis, and others) and viruses (West Nile, Eastern and Western Equine Encephalitis, St. Louis Encephalitis, Sindbis, and Rift Valley Fever viruses), most with substantial transmission in temperate regions. Transmission is optimized at intermediate temperatures (23-26°C) and often has wider thermal breadths (due to cooler lower thermal limits) compared to pathogens with predominately tropical distributions (in previous studies). The incidence of human West Nile virus cases across US counties responded unimodally to average summer temperature and peaked at 24°C, matching model-predicted optima (24-25°C). Climate warming will likely shift transmission of these diseases, increasing it in cooler locations while decreasing it in warmer locations.