Lower Thermal Tolerance Research Articles

Local habitat heterogeneity rivals regional differences in coral thermal tolerance

AbstractVariable temperature regimes that expose corals to sublethal heat stress have been recognized as a mechanism to increase coral thermal tolerance and lessen coral bleaching. However, there is a need to better understand which thermal regimes maximize coral stress hardening. Here, standardized thermal stress assays were used to determine the relative thermal tolerance of three divergent genera of corals (Acropora, Pocillopora, Porites) originating from six reef sites representing an increasing gradient of annual mean diel temperature fluctuations of 1–3 °C day−1. Bleaching severity and dark-acclimated photochemical yield (i.e., Fv/Fm) were quantified following exposure to five temperature treatments ranging from 23.0 to 36.3 °C. The greatest thermal tolerance (i.e., Fv/Fm effective dose 50) was found at the site with intermediate mean diel temperature variability (2.2 °C day−1), suggesting there is an optimal priming exposure that leads to maximal thermal tolerance. Interestingly, Acropora and Pocillopora originating from the least thermally variable regimes (< 1.3 °C day−1) had lower thermal tolerance than corals from the most variable sites (> 2.8 °C day−1), whereas the opposite was true for Porites, suggesting divergent responses across taxa. Remarkably, comparisons across global studies revealed that the range in coral thermal tolerance uncovered in this study across a single reef (< 5 km) were as large as differences observed across vast latitudinal gradients (300–900 km). This finding indicates that local gene flow could improve thermal tolerance between habitats. However, as climate change continues, exposure to intensifying marine heatwaves is already compromising thermal priming as a mechanism to enhance coral thermal tolerance and bleaching resistance.

Diversity in Cyanidiophyceae: Cyanidiofrigus pintoensis gen. et sp. nov. within polyextremophilic red algae (Rhodophyta)

ABSTRACT The polyextremophilic red algal class Cyanidiophyceae inhabits a wide range of extreme habitats (e.g. acidic geothermal areas and dim-light caves), and shows various cell morphologies (e.g., subspherical and club-shape) and trophic strategies (e.g., photoautotrophy, heterotrophy and mixotrophy). Historically, inconsistent observations of morphology and physiology have hindered taxonomic advancements in Cyanidiophyceae despite indications of species diversity from molecular studies. Here we revisited this taxonomic conundrum and explored their cryptic speciation based on multi-locus and morpho-physiological evidence examined from expanded taxa sampling, including 20 cultures and one environmental sample. Our multi-locus phylogeny revealed two well-supported monophyletic lineages: the C-lineage that is generically rich (including Cavernulicola, Cyanidiococcus, Cyanidiofrigus, Cyanidioschyzon, Cyanidium, Gronococcus, and Sciadococcus) and the G-lineage that currently only contains the genus Galdieria. Species delimitation analyses revealed at least 21 species in Cyanidiophyceae. Although our morpho-physiological observations failed to differentiate closely related species apart within a genus, we propose Cyanidiofrigus pintoensis gen. et sp. nov. as this species is in a highly divergent lineage with much lower thermal tolerance than other recognized species. Overall, our study fills the taxonomic knowledge gaps and expands the number of known species in Cyanidiophyceae.

Developmental temperature, more than long-term evolution, defines thermal tolerance in an estuarine copepod.

Climate change is resulting in increasing ocean temperatures and salinity variability, particularly in estuarine environments. Tolerance of temperature and salinity change interact and thus may impact organismal resilience. Populations can respond to multiple stressors in the short-term (i.e., plasticity) or over longer timescales (i.e., adaptation). However, little is known about the short- or long-term effects of elevated temperature on the tolerance of acute temperature and salinity changes. Here, we characterized the response of the near-shore and estuarine copepod, Acartia tonsa, to temperature and salinity stress. Copepods originated from one of two sets of replicated >40 generation-old temperature-adapted lines: ambient (AM, 18°C) and ocean warming (OW, 22°C). Copepods from these lines were subjected to one and three generations at the reciprocal temperature. Copepods from all treatments were then assessed for differences in acute temperature and salinity tolerance. Development (one generation), three generations, and >40 generations of warming increased thermal tolerance compared to Ambient conditions, with development in OW resulting in equal thermal tolerance to three and >40 generations of OW. Strikingly, developmental OW and >40 generations of OW had no effect on low salinity tolerance relative to ambient. By contrast, when environmental salinity was reduced first, copepods had lower thermal tolerances. These results highlight the critical role for plasticity in the copepod climate response and suggest that salinity variability may reduce copepod tolerance to subsequent warming.

Enhancing Sustainability in Building Design: Hybrid Approaches for Evaluating the Impact of Building Orientation on Thermal Comfort in Semi-Arid Climates

The evaluation of human thermal comfort inside buildings plays a pivotal role in reducing energy consumption and enhancing sustainability in the built environment. The estimation of thermal comfort is based on objective (physical factors) and subjective (psychological factors) aspects. This study aimed to find a hybrid way to evaluate more accurate thermal comfort in the buildings as per their orientations. This study assessed the effect of building orientation on thermal comfort conditions in row houses in semi-arid climates, based on a synthesis of the predictive mean vote (PMV) model and the thermal sensation vote (TSV). For this purpose, row houses were selected in the region of this study. This study concluded that the PMV model calculates a lower thermal comfort level than the TSV method, and that the thermal comfort demand within the houses was higher than ASHRAE Standard 55. The occupants inside the houses had a lower thermal tolerance. This implied that the residents of these buildings can consume more energy during the summer, typically the harshest season. This study presented new mathematical models for occupants’ thermal comfort evaluation in the study region, depending on the building’s orientation. In both models, for assessing thermal performance during both the summer and winter seasons, east-facing buildings consistently ranked as the second-best orientation. This suggested that, overall, east-facing buildings can be considered the best choice throughout the entire year in terms of thermal comfort. This study suggested a novel indicator to evaluate the optimum building orientation in the study area in terms of thermal performance.

Climate variability hypothesis is partially supported in thermal limits of juvenile Northwest Atlantic coastal fishes.

As ocean warming continues to impact marine species globally, there is a need to understand the mechanisms underlying shifts in abundance and distribution. There is growing evidence that upper and lower temperature tolerances rather than mean preferences explain range shifts, but the full thermal niche is unknown for many marine species and observational data are often ill-suited to estimate the upper and lower thermal tolerances. We quantified critical thermal maximum (CTmax ) and critical thermal minimum (CTmin ) using standard methods to quantify temperature limits and thermal ranges of 14 economically and ecologically important juvenile fish species on the US Atlantic coast. We then tested the climate variability hypothesis (CVH), which states that higher-latitude species should have a wider temperature tolerance due to higher climatic variability closer to the poles. Our findings generally support the CVH in the juvenile fishes that we evaluated. However, low-latitude species were not uniformly stenothermal. Rather, species with median occurrences across a wide range of latitudes had wide temperature tolerances, but only the tropical species we tested had more narrow ranges. These findings suggest that quantifying temperature tolerances may be used to predict which low-latitude species are most likely to shift in response to warming water and those that may be more sensitive to climate change in this region.

Tropical butterflies use thermal buffering and thermal tolerance as alternative strategies to cope with temperature increase.

Climate change poses a severe threat to many taxa, with increased mean temperatures and frequency of extreme weather events predicted. Insects can respond to high temperatures using behaviour, such as angling their wings away from the sun or seeking cool local microclimates to thermoregulate or through physiological tolerance. In a butterfly community in Panama, we compared the ability of adult butterflies from 54 species to control their body temperature across a range of air temperatures (thermal buffering ability), as well as assessing the critical thermal maxima for a subset of 24 species. Thermal buffering ability and tolerance were influenced by family, wing length, and wing colour, with Pieridae, and butterflies that are large or darker in colour having the strongest thermal buffering ability, but Hesperiidae, small, and darker butterflies tolerating the highest temperatures. We identified an interaction between thermal buffering ability and physiological tolerance, where species with stronger thermal buffering abilities had lower thermal tolerance, and vice versa. This interaction implies that species with more stable body temperatures in the field may be more vulnerable to increases in ambient temperatures, for example heat waves associated with ongoing climate change. Our study demonstrates that tropical species employ diverse thermoregulatory strategies, which is also reflected in their sensitivity to temperature extremes.

Interspecific differences in thermal tolerance landscape explain aphid community abundance under climate change

A single critical thermal limit is often used to explain and infer the impact of climate change on geographic range and population abundance. However, it has limited application in describing the temporal dynamic and cumulative impacts of extreme temperatures. Here, we used a thermal tolerance landscape approach to address the impacts of extreme thermal events on the survival of co-existing aphid species (Metopolophium dirhodum, Sitobion avenae and Rhopalosiphum padi). Specifically, we built the thermal death time (TDT) models based on detailed survival datasets of three aphid species with three ages across a broad range of stressful high (34–40 °C) and low (−3∼−11 °C) temperatures to compare the interspecific and developmental stage variations in thermal tolerance. Using these TDT parameters, we performed a thermal risk assessment by calculating the potential daily thermal injury accumulation associated with the regional temperature variations in three wheat-growing sites along a latitude gradient. Results showed that M. dirhodum was the most vulnerable to heat but more tolerant to low temperatures than R. padi and S. avenae. R. padi survived better at high temperatures than Sitobion avenae and M. dirhodum but was sensitive to cold. R. padi was estimated to accumulate higher cold injury than the other two species during winter, while M. dirhodum accrued more heat injury during summer. The warmer site had higher risks of heat injury and the cooler site had higher risks of cold injury along a latitude gradient. These results support recent field observations that the proportion of R. padi increases with the increased frequency of heat waves. We also found that young nymphs generally had a lower thermal tolerance than old nymphs or adults. Our results provide a useful dataset and method for modelling and predicting the consequence of climate change on the population dynamics and community structure of small insects.

Fungal infections lead to shifts in thermal tolerance and voluntary exposure to extreme temperatures in both prey and predator insects

Pathogens can modify many aspects of host behavior or physiology with cascading impacts across trophic levels in terrestrial food webs. These changes include thermal tolerance of hosts, however the effects of fungal infections on thermal tolerances and behavioral responses to extreme temperatures (ET) across trophic levels have rarely been studied. We examined how a fungal pathogen, Beauveria bassiana, affects upper and lower thermal tolerance, and behavior of an herbivorous insect, Acyrthosiphon pisum, and its predator beetle, Hippodamia convergens. We compared changes in thermal tolerance limits (CTMin and CTMax), thermal boldness (voluntary exposure to ET), energetic cost (ATP) posed by each response (thermal tolerance and boldness) between healthy insects and insects infected with two fungal loads. Fungal infection reduced CTMax of both aphids and beetles, as well as CTMin of beetles. Fungal infection modified the tendency, or boldness, of aphids and predator beetles to cross either warm or cold ET zones (ETZ). ATP levels increased with pathogen infection in both insect species, and the highest ATP levels were found in individuals that crossed cold ETZ. Fungal infection narrowed the thermal tolerance range and inhibited thermal boldness behaviors to cross ET. As environmental temperatures rise, response to thermal stress will be asymmetric among members of a food web at different trophic levels, which may have implications for predator–prey interactions, food web structures, and species distributions.

Cold tolerance is similar but heat tolerance is higher in the alien insect Trichocera maculipennis than in the native Parochlus steinenii in Antarctica

Uncontrolled biological invasions have direct and indirect impacts on the structure and functioning of soil invertebrate communities in Antarctica. Among others, invasion success is strongly determined by the ability of species to tolerate broad thermal ranges. Yet, few studies have compared the thermal niches of native and invasive species. Physiological characterizations of upper and lower thermal tolerances are essential to test the extent to which eurythermality can benefit invasive species in a context of changing climates. Here, we compare cold and heat tolerance between adults of the alien winter crane fly Trichocera maculipennis and the native winged midge Parochlus steinenii in Antarctica. Specimens were collected in the field during the 2019/2020 austral summer, and ramping experiments controlling heating and cooling rates were performed to estimate upper and lower critical thermal limits of the two Diptera insect species. Adults of the alien fly remained active between − 5.3 °C and 30.1 °C. In turn, the native midge was active between − 5.0 °C and 28.6 °C. We observed no significant interspecific differences between lower critical thermal limits, but upper thermal limits were significantly higher for the alien species. Hence, the capacity to endure low summer temperatures in most of the Antarctic Peninsula is similar for adults of both species, but the alien crane fly is readily adapted to withstand warming scenarios. Therefore, the broad thermal tolerances exhibited by the alien crane fly can be taken as evidence to predict geographic range expansions, while also warn of high biosecurity risks for all operating research stations in Antarctica.

Does Heat Tolerance Explain Female Polymorphism in Damselflies?

Morphological variation is widespread, and one case is that of female morphs in damselfly insects. Current knowledge indicates that these female morphs are maintained by a fitness balance whereby male-like, clear-colored females (i.e. andromorphs) are more likely to “escape” from male harassment given their male mimicking compared to dark females (i.e. gynomorph). However, males can learn to distinguish the more frequent morph, leading to a frequency-dependent selection. Alternative to frequency-dependent selection, we tested whether temperature plays a role in driving female morph activity and survival. We used Ischnura denticollis damselflies to test for differences between female morphs and males in: (a) daily activity; (b) time to resume activity following a chilling event (10°C); and, (c) survival at 30, 35 and 40°C. There was no difference in activity among groups after a chilling event. However, gynomorph females were more active early in the day (ca. 17°C) and less active by 1600 hrs (after reaching 32°C), and had lower thermal tolerance than andromorph females. Thus, high temperatures may act against gynomorph females. This implies a role for high temperatures in the maintenance of female polymorphism.

Divergence in aerobic scope and thermal tolerance is related to local thermal regime in two populations of introduced Nile perch (Lates niloticus).

We tested whether thermal tolerance and aerobic performance differed between two populations of Nile perch (Lates niloticus) originating from the same source population six decades after their introduction into two lakes in the Lake Victoria basin in East Africa. We used short-term acclimation of juvenile fish to a range of temperatures from ambient to +6°C, and performed critical thermal maximum (CTmax ) and respirometry tests to measure upper thermal tolerance, resting and maximum metabolic rates, and aerobic scope (AS). Across acclimation temperatures, Nile perch from the cooler lake (Lake Nabugabo, Uganda) tended to have lower thermal tolerance (i.e., CTmax ) and lower aerobic performance (i.e., AS) than Nile perch from the warmer waters of Lake Victoria (Bugonga region, Uganda). Effects of temperature acclimation were more pronounced in the Lake Victoria population, with the Lake Nabugabo fish showing less thermal plasticity in most metabolic traits. Our results suggest phenotypic divergence in thermal tolerance between these two introduced populations in a direction consistent with an adaptive response to local thermal regimes.

Thermal Tolerance and Metabolism of Archocentrus nigrofasciatus Acclimated to a Large Daily Temperature Fluctuation Reflect its Success as an Invasive Species

Fish exhibit thermal tolerance and metabolism correlated to recent thermal history, and more studies should examine species’ responses to large fluctuations in daily temperatures. We compared thermal tolerance and metabolic responses between fish acclimated to constant temperatures and a large, daily thermal cycle of 10°C. Black Convict cichlid (Archocentrus nigrofasciatus) is a freshwater teleost native to Central America, with non‐native populations established in numerous locations. Black Convict cichlid were acclimated for two weeks to constant 20°C or 30°C, or a daily cycle of 20–30°C (“cycling group”, ~0900–1500 hrs). Following acclimation, routine metabolism (oxygen consumption, Mo2) and Critical thermal maxima/minima (CTmax/CTmin) were measured. Critical thermal limits were compared via ANOVA, with the cycling group sampled from both the daily minimum at 20°C (0900 hrs) and daily maximum at 30°C (1600 hrs). CTmax (0.3°C per degree of acclimation) and CTmin (0.4 °C per degree of acclimation) increased with acclimation. CTmax for the cycling group were 0.7°C higher when sampled from the maximum 30°C (39.3°C) of the daily cycle compared to 20°C (38.6°C). CTmin values for the cycling group were 2.2°C higher when sampled from the maximum 30°C (14.6°C) of the daily cycle compared to 20°C (12.4°C). Mo2 was measured acutely at 20°C and 30°C for each acclimation group, with the cycling group sampled from the daily minimum at 20°C (0900 hrs) and daily maximum at 30°C (1600 hrs). ANOVA showed that mass‐specific Mo2 decreased with increasing acclimation temperature for fish measured acutely at 30°C, where fish acclimated to 20°C (Mo2 = 0.329 mg/h/g) demonstrated an 84% higher Mo2 than fish acclimated to 30°C (Mo2 = 0.178 mg/h/g). Mass‐specific Mo2 also decreased with increasing acclimation temperature for fish measured acutely at 20°C, where fish acclimated to 20°C (Mo2 = 0.170 mg/h/g) demonstrated a 139% higher Mo2 than fish acclimated to 30°C (Mo2 = 0.071 mg/h/g). Mo2 was measured continually across the 24h daily thermal acclimation cycle of 20 – 30°C for all groups. Total oxygen consumption across the 24h cycle was highest for the constant 30°C group naively exposed to the cycle and lowest for the cycling group. Black Convict cichlid retains more upper thermal tolerance than lower thermal tolerance with altered thermal acclimation, and this hardy species shows metabolic compensation following high temperature acclimation (30°C) and large, daily cycling temperature acclimation (20 – 30°C). These aspects of fish thermal physiology phenotypes are reflective of a successful invasive fish species that may respond more robustly to impacts from anthropogenic climate change.Support or Funding InformationFunding was provided through startup funds to JE from CSUSM.

Upper thermal tolerance of early juveniles of six terrestrial hermit crab species

Abstract Because temperature strongly influences biological processes of ectotherms, they must adapt to the thermal conditions of their habitats. We evaluated the upper thermal tolerance of early juveniles of six terrestrial hermit crab species in the family Coenobitidae (genera Birgus and Coenobita), B. latro, C. brevimanus, C. cavipes, C. purpureus, C. rugosus, and C. violascens, that occur in the northwestern Pacific region of Japan. A total of 30 laboratory-raised juveniles (approximately 1 mm in shield length) carrying gastropod shells were individually stocked in small plastic cups with sandy bottoms in temperature-controlled incubation chambers at ∼27°C. The temperature was increased by 1°C every 48 h, and the juveniles were observed until all the crabs had died. The median upper lethal temperature was estimated as the temperature at which 50% of the test juveniles had died. The median upper lethal temperature estimates significantly varied among the species. Coenobita violascens, which mainly occurs in mangrove estuaries with lower thermal conditions, had the lowest median upper lethal temperature values. The median upper lethal temperature values estimated for B. latro were slightly lower than those for the other Coenobita species, probably reflecting its cryptic nature in natural habitats during the juvenile stage. The most northerly distributed species, C. purpureus, had the highest median upper lethal temperature values, suggesting the existence of common physiological mechanisms that regulate both upper and lower thermal tolerance abilities.

Predicting thermal sensitivity of three Amazon fishes exposed to climate change scenarios

Increased temperature and CO2 levels are predicts for the next decades. Tropical species are expected to be especially vulnerable to these alterations once many of them appear to have a narrower thermal tolerance range compared to subtropical and temperate species; and live closer to their thermal limits. Herein, we experimentally investigated the effects of climate change scenario on metabolic and oxidative stress in three ornamental fishes of Amazon forest streams. Apistogramma agassizii, Pyrhullina brevis and Hyphessobrychon melazonatus were exposed to current and extreme climate scenarios (4.5 °C and 900 ppm CO2 above current levels) and had respiratory profile, antioxidant enzymes and neurotransmitter responses evaluated. The integrated biomarkers response index (IBR) was calculated to examine specie’s acclimation abilities. After 30 days of exposure, we observed distinct physiological mechanisms to cope with climate change. Overall, Amazonian fish species are susceptible to climate change since they showed increase in metabolic rate and oxidative stress. Yet, sedentary ones A. agassizii and P. brevis, appeared to be less impacted by climate change than H. melazonatus, once they presented high survival rates, thermal tolerance and low IBR values. In contrast, H. melazonatus, an athletic species, low survival rates, lipid peroxidation, lower thermal tolerance and high IBR. This study provides evidence that future climate changes will affect energy supply and promote species-specific damages in metabolic pathways, with consequent physiological impairments, which may have detrimental effects at population and ecosystem levels.

Complex body size differences in thermal tolerance among army ant workers (Eciton burchellii parvispinum)

In social insects, group members can differ in thermal physiology, and these differences may affect colony function. Upper thermal tolerance limits (CTmax) generally increase with body size among and within ant species, but size effects on lower thermal tolerances (CTmin) are poorly known. To test whether CTmin co-variation with body size matched patterns for CTmax, we measured CTmax and CTmin in workers of four size-based worker subcastes in the army ant Eciton burchellii parvispinum. CTmax increased with worker body size as expected. CTmin showed a more complex relationship with size: the two intermediate-size subcastes (media and porters) tolerated lower temperatures than the smallest (minims) and the largest (soldiers) worker subcastes. Body-size effects on CTmax were not predictive of body-size effects on CTmin. These patterns held for colonies collected across elevations that spanned approximately 8 °C in mean annual temperature, even though high-elevation colonies had significantly lower CTmin overall. We predict Eciton army ant subcastes will be differentially affected by directional changes in high and low temperature extremes. Worker subcastes perform distinct but complementary roles in colony function, and differential temperature effects among subcastes could impair colony performance and negatively impact colony fitness.

Comparison of thermal tolerance and standard metabolic rate of two Great Lakes invasive fish species

Abstract Round goby (Neogobius melanostomus) and western tubenose goby (Proterorhinus semilunaris) invaded the Laurentian Great Lakes at approximately the same time and area yet have shown substantial differences in their post-invasion success with more rapid establishment and development of much larger abundances of round goby populations throughout the invaded habitat. In this study, we compared differences in physiological performance (thermal tolerance and standard metabolic rate) between round and tubenose goby collected from the Huron-Erie corridor. Tubenose goby were observed to have lower thermal tolerance but exhibited similar standard metabolic rate across environmental temperatures compared to round goby. At temperatures exceeding 31 °C, tubenose goby demonstrated significantly higher mortalities and shorter times to death relative to round goby. The observed differences in thermal tolerance were consistent with differences in the native geographic ranges observed for each species at their southern ranges. The observed differences in physiological performance combined with species differences in other life history traits such body size, reproduction, feeding ecology and habitat affiliation may also explain differences in the invasiveness experienced by these two Great Lakes invasive fish including a greater ability of round gobies to occupy extreme habitats with large water temperature fluctuations.

Thermal tolerance of the zoea I stage of four Neotropical crab species (Crustacea: Decapoda)

. Although larval stages are often considered particularly vulnerable to stressors, for many marine invertebrates studies of thermal tolerance have focused on adults. Here we determined the upper thermal limit (LT50) of the zoea I of four Caribbean crab species (Macrocoelomatrispinosum, Aratuspisonii, Armasesricordi, and Minucarapax) and compared their thermal tolerance over time and among species. The zoea from the subtidal species M.trispinosum and tree climbing mangrove species A.pisonii had a lower thermal tolerance, 35 and 38.5 °C respectively, than did the semiterrestrial A.ricordi and M.rapax. In all four species tested, the estimates of thermal tolerance depend on the duration of exposure to elevated temperatures. Longer exposures to thermal stress produce lower estimates of LT50, which decreased by ~1 °C from a two- to a six-hour exposure. Crab embryos develop on the abdomen of the mother until the larvae are ready to hatch. Therefore, the thermal tolerances of the embryos which need to coincide with the environmental conditions experienced by the adult stage, may carry over into the early zoea stage. Our results suggest that semiterrestrial species, in which embryos may need to withstand higher temperatures than embryos of subtidal species also produce larvae with higher thermal tolerances. Over the short term, the larvae of these tropical crab species can withstand significantly higher temperatures than those experienced in their marine habitat. Longer term rearing studies are necessary to determine the temperature at which chronic exposure has a negative impact on embryonic and larval survival.

Functional perspectives on the dynamics of desert lizard assemblages

Ecology has long sought whether there are general assembly rules that underlie patterns of community organization. However, new perspectives of community disassembly and reassembly have emerged as environments depart from baseline conditions due to agents of global change. A trait-based approach can provide insights as to how changes affect performance of a species and their subsequent loss, gain, or persistence in an assemblage. We quantified the spatiotemporal dynamics in species and functional diversity from systematic surveys of local lizard assemblages distributed along an elevation gradient from three surveys in a 50-year period (1950s–2000s) in Big Bend National Park (BBNP), USA. BBNP was subjected to and then released from intensive grazing, which has resulted in changes to vegetative composition and cover. We examine changes in local assemblages in the context lizard functional traits, including thermal niche breadth. We hypothesized that temperature specialists were more likely to colonize or be lost from local sites. Species richness was lowest in the 1950s (N = 3 species), and subsequent surveys revealed that species richness increased in both the 1960s and 2000s (N = 6 and N = 8 species, respectively), but not all sites responded uniformly. We found that functional diversity in this system was not affected by this drastic increase species richness, indicative of functional redundancy in species traits. Lizards that were added to sites tended to be smaller bodied with lower thermal tolerances. Lizards with high preferred body temperatures were locally extirpated from high elevation sites that experienced high shrub encroachment, likely due to a reduction in the preferred thermal gradients these species. These results reveal that long-term desert lizard functional diversity was maintained, but the consequences of changes to functional diversity at the local scale need to be examined further.

The Janus of macrophysiology: stronger effects of evolutionary history, but weaker effects of climate on upper thermal limits are reversed for lower thermal limits in ants.

Species may exhibit similar traits via different mechanisms: environmental filtering and local adaptation (geography) and shared evolutionary history (phylogeny) can each contribute to the resemblance of traits among species. Parsing trait variation into geographic and phylogenetic sources is important, as each suggests different constraints on trait evolution. Here, we explore how phylogenetic distance, geographic distance, and geographic variation in climate shape physiological tolerance of high and low temperatures using a global dataset of ant thermal tolerances. We found generally strong roles for evolutionary history and geographic variation in temperature, but essentially no detectable effects of spatial proximity per se on either upper or lower thermal tolerance. When we compared the relative importance of the factors shaping upper and lower tolerances, we found a much stronger role for evolutionary history in shaping upper versus lower tolerance, and a moderately weaker role for geographic variation in temperature in shaping upper tolerance when compared with lower tolerance. Our results demonstrate how geographic variation in climate and evolutionary history may have differential effects on the upper and lower endpoints of physiological tolerance. This Janus effect, where the relative contributions of geographic variation in climate and evolutionary history are reversed for lower versus upper physiological tolerances, has gained some support in the literature, and our results for ant physiological tolerances provide further evidence of this pattern. As the climate continues to change, the high phylogenetic conservatism of upper tolerance may suggest potential constraints on the evolution of tolerance of high temperatures.

Lower thermal tolerance in nocturnal than in diurnal ants: a challenge for nocturnal ectotherms facing global warming

1. The thermal adaptation hypothesis proposes that because thermoregulation involves a high metabolic cost, thermal limits of organisms must be locally adapted to temperatures experienced in their environments. There is evidence that tolerance to high temperatures decreases in insects inhabiting colder habitats and microclimates. However, it is not clear if thermal limits of ectotherms with contrasting temporal regimes, such as diurnal and nocturnal insects, are also adapted to temperatures associated with their circadian activities.2. This study explores differences in heat tolerance among diurnal and nocturnal ant species in four ecosystems in Mexico: tropical montane, tropical rainforest, subtropical dry forests, and high‐elevation semi‐desert.3. The critical thermal maximum (CTmax), i.e. the temperature at which ants lost motor control, was estimated for diurnal and nocturnal species. CTmax for 19 diurnal and 12 nocturnal ant species distributed among 45 populations was also estimated.4. Semi‐desert and subtropical dry forest ants displayed higher tolerances to high temperatures than did ants in tropical rainforest. The lowest tolerance to high temperatures was recorded in tropical montane forest ants. In general, among all habitats, the CTmax of nocturnal ants was lower than that of diurnal ants.5. An increase in nocturnal temperatures, combined with lower tolerance to high temperatures, may represent a substantial challenge for nocturnal ectotherms in a warming world.