Thermal tolerance and the importance of microhabitats for Andean frogs in the context of land use and climate change

Exposure of protected areas in Central America to extreme weather events

Central America and the Caribbean are regularly battered by megadroughts, heavy rainfall, heat waves, and tropical cyclones. Although 21st-century climate change is expected to increase the frequency, intensity, and duration of these extreme weather events (EWEs), their incidence in regional protected areas (PAs) remains poorly explored. We examined historical and projected EWEs across the region based on 32 metrics that describe distinct dimensions (i.e., intensity, duration, and frequency) of heat waves, cyclones, droughts, and rainfall and compared trends in PAs with trends in unprotected lands. From the early 21st century onward, exposure to EWEs increased across the region, and PAs were predicted to be more exposed to climate extremes than unprotected areas (as shown by autoregressive model coefficients at p < 0.05 significance level). This was particularly true for heat waves, which were projected to have a significantly higher average (tested by Wilcoxon tests at p < 0.01) intensity and duration, and tropical cyclones, which affected PAs more severely in carbon-intensive scenarios. PAs were also predicted to be significantly less exposed to droughts and heavy rainfall than unprotected areas (tested by Wilcoxon tests at p < 0.01). However, droughts that could threaten connectivity between PAs are increasingly common in this region. We estimated that approximately 65% of the study area will experience at least one drought episode that is more intense and longer lasting than previous droughts. Collectively, our results highlight that new conservation strategies adapted to threats associated with EWEs need to be tailored and implemented promptly. Unless urgent action is taken, significant damage may be inflicted on the unique biodiversity of the region.

Thermal tolerance for the tropical clawed frog, Xenopus tropicalis with comments on comparative methods for amphibian studies

Thermal tolerance data are important for identifying the potential range of non-native species following introduction and establishment. Such data are particularly important for understanding invasion risks of tropical species introduced to temperate climates and identifying whether they can survive outside tropical regions. A breeding population of the tropical clawed frog (Xenopus tropicalis) was recently discovered in west-central Florida, U.S.A. This fully aquatic species is native to the rainforest belt of west Africa and has not been documented outside its native range. Because of the lack of invasion history, data are sparse on the thermal limits for this species. We used chronic lethal and critical thermal methodologies to investigate thermal tolerance on adult stages and critical thermal methods on tadpoles. Because of our use of both chronic and critical methodologies, we also examined the literature to reveal common methods used to investigate thermal minimum and maximum temperature in amphibians, which were found to be dominated by the critical maximum. Chronic lethal temperatures for adult X. tropicalis were 9.73 °C and 36.68 °C. Critical temperatures were affected by acclimation temperature and life stage; adults were more tolerant of extreme temperatures. Based on these critical thermal data and the fact that breeding tends to occur when temperatures are suitable for survival, tadpole stages are unlikely to be affected by extreme temperatures. Instead, range expansion in Florida will likely be limited by the adult stages. Our findings indicate that the tropical clawed frog could occupy much of southern Peninsular Florida and other tropical and subtropical regions worldwide.

Impact of rising temperature on physiological and biochemical alterations that affect the viability of blood cells in American bullfrog crossbreeds

The study aimed to examine the impact of increasing environmental temperatures on physiological changes, oxidative stress, nitric oxide production, total antioxidant capacity, and blood cell viability in American bullfrog crossbreeds. Frogs and frog blood cells were exposed to temperature ranges of 25-33 °C and 25-37 °C, respectively. Physiological parameters (body temperature, pulse rate, ventilation rate, and oxygen saturation) and biochemical parameters (total antioxidant power, hydrogen peroxide, malondialdehyde, nitric oxide, and mitochondrial activity) were measured at every 2 °C increment. Results showed that body temperature rose with increased environmental temperature (P < 0.05). Pulse rates at 33 °C were higher than those at 25-31 °C (P < 0.05). Ventilation rates at 31 °C exceeded those at 25 °C and 27 °C (P < 0.05). Oxygen saturation levels remained stable at 25-33 °C (P > 0.05). Total antioxidant power at 25 °C was greater than at 27-37 °C (P < 0.05). Hydrogen peroxide levels at 27 °C were higher compared to 25 °C and 31-37 °C (P < 0.05). Malondialdehyde levels at 25-33 °C were higher than at 35 °C and 37 °C (P < 0.05). Nitric oxide levels at 37 °C were higher than at 25-33 °C (P < 0.05), and at 35 °C were higher than at 25-31 °C (P < 0.05). Blood cell viability at 25-31 °C was higher than at 37 °C (P < 0.05). These results suggest that at an environmental temperature of 33 °C, the frogs' body temperature approached 31 °C or higher, and were likely to be harmful to the frogs. Finally, the environmental temperature that caused frog blood cell death was 37 °C.

Thermal limits along tropical elevational gradients: Poison frog tadpoles show plasticity but maintain divergence across elevation

Temperature is arguably one of the most critical environmental factors impacting organisms at molecular, organismal, and ecological levels. Temperature variation across elevation may cause divergent selection in physiological critical thermal limits (CTMAX and CTMIN). Generally, high elevation populations are predicted to withstand lower environmental temperatures than low elevation populations. Organisms can also exhibit phenotypic plasticity when temperature varies, although theory and empirical evidence suggest that tropical ectotherms have relatively limited ability to acclimate. To study the effect of temperature variation along elevational transects on thermal limits, we measured CTMAX and CTMIN of 934 tadpoles of a poison frog species, Epipedobates anthonyi, along two elevational gradients (200-1700 m asl) in southwestern Ecuador to investigate their thermal tolerance across elevation. We also tested if tadpoles could plastically shift their critical thermal limits in response to exposure to different temperatures representing the range of temperatures they experience in nature (20 °C, 24 °C, and 28 °C). Overall, we found that CTMAX did not change across elevation. In contrast, CTMIN was lower at higher elevations, suggesting that elevational variation in temperature influences this thermal trait. Moreover, all populations shifted their CTMAX and CTMIN according to treatment temperatures, demonstrating an acclimation response. Overall, trends in CTMIN among high, mid, and low elevation populations were maintained despite plastic responses to treatment temperature. These results demonstrate that, for tadpoles of E. anthonyi across tropical elevational gradients, temperature acts as a selective force for CTMIN, even when populations show acclimation abilities in both, CTMAX and CTMIN. Our findings advance our understanding on how environmental variation affects organisms' evolutionary trajectories and their abilities to persist in a changing climate in a tropical biodiversity hotspot.

Advances in biologging can identify nuanced energetic costs and gains in predators

Foraging is a key driver of animal movement patterns, with specific challenges for predators which must search for mobile prey. These patterns are increasingly impacted by global changes, principally in land use and climate. Understanding the degree of flexibility in predator foraging and social strategies is pertinent to wildlife conservation under global change, including potential top-down effects on wider ecosystems. Here we propose key future research directions to better understand foraging strategies and social flexibility in predators. In particular, rapid continued advances in biologging technology are helping to record and understand dynamic behavioural and movement responses of animals to environmental changes, and their energetic consequences. Data collection can be optimised by calibrating behavioural interpretation methods in captive settings and strategic tagging decisions within and between social groups. Importantly, many species' social systems are increasingly being found to be more flexible than originally described in the literature, which may be more readily detectable through biologging approaches than behavioural observation. Integrating the effects of the physical landscape and biotic interactions will be key to explaining and predicting animal movements and energetic balance in a changing world.

Impacts of anthropogenic climate change on tropical montane forests: an appraisal of the evidence

In spite of their small global area and restricted distributions, tropical montane forests (TMFs) are biodiversity hotspots and important ecosystem services providers, but are also highly vulnerable to climate change. To protect and preserve these ecosystems better, it is crucial to inform the design and implementation of conservation policies with the best available scientific evidence, and to identify knowledge gaps and future research needs. We conducted a systematic review and an appraisal of evidence quality to assess the impacts of climate change on TMFs. We identified several skews and shortcomings. Experimental study designs with controls and long-term (≥10 years) data sets provide the most reliable evidence, but were rare and gave an incomplete understanding of climate change impacts on TMFs. Most studies were based on predictive modelling approaches, short-term (<10 years) and cross-sectional study designs. Although these methods provide moderate to circumstantial evidence, they can advance our understanding on climate change effects. Current evidence suggests that increasing temperatures and rising cloud levels have caused distributional shifts (mainly upslope) of montane biota, leading to alterations in biodiversity and ecological functions. Neotropical TMFs were the best studied, thus the knowledge derived there can serve as a proxy for climate change responses in under-studied regions elsewhere. Most studies focused on vascular plants, birds, amphibians and insects, with other taxonomic groups poorly represented. Most ecological studies were conducted at species or community levels, with a marked paucity of genetic studies, limiting understanding of the adaptive capacity of TMF biota. We thus highlight the long-term need to widen the methodological, thematic and geographical scope of studies on TMFs under climate change to address these uncertainties. In the short term, however, in-depth research in well-studied regions and advances in computer modelling approaches offer the most reliable sources of information for expeditious conservation action for these threatened forests.

High sensitivity of tropical forest birds to deforestation at lower altitudes

Habitat conversion is a major driver of tropical biodiversity loss, but its effects are poorly understood in montane environments. While community-level responses to habitat loss display strong elevational dependencies, it is unclear whether these arise via elevational turnover in community composition and interspecific differences in sensitivity or elevational variation in environmental conditions and proximity to thermal thresholds. Here we assess the relative importance of inter- and intraspecific variation across the elevational gradient by quantifying how 243 forest-dependent bird species vary in sensitivity to landscape-scale forest loss across a 3000-m elevational gradient in the Colombian Andes. We find that species that live at lower elevations are strongly affected by loss of forest in the nearby landscape, while those at higher elevations appear relatively unperturbed, an effect that is independent of phylogeny. Conversely, we find limited evidence of intraspecific elevational gradients in sensitivity, with populations displaying similar sensitivities to forest loss, regardless of where they exist in a species' elevational range. Gradients in biodiversity response to habitat loss thus appear to arise via interspecific gradients in sensitivity rather than proximity to climatically limiting conditions.

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.

A comprehensive database of amphibian heat tolerance

Rising temperatures represent a significant threat to the survival of ectothermic animals. As such, upper thermal limits represent an important trait to assess the vulnerability of ectotherms to changing temperatures. For instance, one may use upper thermal limits to estimate current and future thermal safety margins (i.e., the proximity of upper thermal limits to experienced temperatures), use this trait together with other physiological traits in species distribution models, or investigate the plasticity and evolvability of these limits for buffering the impacts of changing temperatures. While datasets on thermal tolerance limits have been previously compiled, they sometimes report single estimates for a given species, do not present measures of data dispersion, and are biased towards certain parts of the globe. To overcome these limitations, we systematically searched the literature in seven languages to produce the most comprehensive dataset to date on amphibian upper thermal limits, spanning 3,095 estimates across 616 species. This resource will represent a useful tool to evaluate the vulnerability of amphibians, and ectotherms more generally, to changing temperatures.

Dispersal abilities favor commensalism in animal-plant interactions under climate change

Scientists still poorly understand how biotic interactions and dispersal limitation jointly interact and affect the ability of species to track suitable habitats under climate change. Here, we examine how animal-plant interactions and dispersal limitations might affect the responses of Brazil nut-dependent frogs facing projected climate change. Using ecological niche modelling and dispersal simulations, we forecast the future distributions of the Brazil nut tree and three commensalist frog species over time (2030, 2050, 2070, and 2090) in the regional rivalry (SSP370) scenario that includes great challenges to mitigation and adaptation. With the exception of one species, projections point to a decrease in suitable habitats of up to 40.6%. For frog species with potential reductions of co-occurrence areas, this is expected to reduce up to 23.8% of suitable areas for binomial animal-plant relationships. Even so, biotic interactions should not be lost over time. Species will depend on their own dispersal abilities to reach analogous climates in the future for maintaining ecological and evolutionary processes associated with commensal taxa. However, ecological and evolutionary processes associated with commensal taxa should be maintained in accordance with their own dispersal ability. When dispersal limitation is included in the models, the suitable range of all three frog species is reduced considerably by the end of the century. This highlights the importance of dispersal limitation inclusion for forecasting future distribution ranges when biotic interactions matter.

The effect of thermal microenvironment in upper thermal tolerance plasticity in tropical tadpoles. Implications for vulnerability to climate warming

Current climate change is generating accelerated increase in extreme heat events and organismal plastic adjustments in upper thermal tolerances, (critical thermal maximum -CT_max ) are recognized as the quicker mitigating mechanisms. However, current research casts doubt on the actual mitigating role of thermal acclimation to face heat impacts, due to its low magnitude and weak environmental signal. Here, we examined these drawbacks by first estimating maximum extent of thermal acclimation by examining known sources of variation affecting CT_max expression, such as daily thermal fluctuation and heating rates. Second, we examined whether the magnitude and pattern of CT_max plasticity is dependent of the thermal environment by comparing the acclimation responses of six species of tropical amphibian tadpoles inhabiting thermally contrasting open and shade habitats and, finally, estimating their warming tolerances (WT = CT_max  - maximum temperatures) as estimator of heating risk. We found that plastic CT_max responses are improved in tadpoles exposed to fluctuating daily regimens. Slow heating rates implying longer duration assays determined a contrasting pattern in CT_max plastic expression, depending on species environment. Shade habitat species suffer a decline in CT_max whereas open habitat tadpoles greatly increase it, suggesting an adaptive differential ability of hot exposed species to quick hardening adjustments. Open habitat tadpoles although overall acclimate more than shade habitat species, cannot capitalize this beneficial increase in CT_max, because the maximum ambient temperatures are very close to their critical limits, and this increase may not be large enough to reduce acute heat stress under the ongoing global warming.

Can tolerances of multiple stressors and calculated safety margins in fiddler crabs predict responses to extreme environmental conditions resulting from climate change?

To comprehend mangrove crab responses to predicted global climate changes, we assessed submersion and desiccation survival durations and salinity tolerances and upper thermal limits in fiddler crabs from Isla del Carmen, Yucatán Peninsula. Based on their tolerances of extreme ambient conditions, we also calculated safety margins using abiotic monitoring data. The two most terrestrial species, Minuca rapax and Leptuca panacea, exhibited submersion tolerances of from 22 to 40 h, and desiccation tolerances of from 30 to 55 h; LC₅₀'s were ≈45‰S and UT₅₀'s were ≈40 °C. The two least terrestrial species, M. vocator and L. speciosa, were less tolerant of all experimental challenges, showing submersion and desiccation tolerances of <6 h, and LC₅₀'s of 36‰S and UT₅₀'s of 38 °C. While these fiddler crabs inhabit niches closer to their salinity and desiccation/submersion tolerances than to their temperature limits, all are clearly vulnerable to the multiple stressors that accompany anticipated global climate change.

Habitat heterogeneity affects the thermal ecology of an endangered lizard

Global climate change is already contributing to the extirpation of numerous species worldwide, and sensitive species will continue to face challenges associated with rising temperatures throughout this century and beyond. It is especially important to evaluate the thermal ecology of endangered ectotherm species now so that mitigation measures can be taken as early as possible. A recent study of the thermal ecology of the federally endangered Blunt-nosed Leopard Lizard (Gambelia sila) suggested that they face major activity restrictions due to thermal constraints in their desert habitat, but that large shade-providing shrubs act as thermal buffers to allow them to maintain surface activity without overheating. We replicated this study and also included a population of G. sila with no access to large shrubs to facilitate comparison of the thermal ecology of G. sila populations in shrubless and shrubbed sites. We found that G. sila without access to shrubs spent more time sheltering inside rodent burrows than lizards with access to shrubs, especially during the hot summer months. Lizards from a shrubbed site had higher midday body temperatures and therefore poorer thermoregulatory accuracy than G. sila from a shrubless site, suggesting that greater surface activity may represent a thermoregulatory trade-off for G. sila. Lizards at both sites are currently constrained from using open, sunny microhabitats for much of the day during their short active seasons, and our projections suggest that climate change will exacerbate these restrictions and force G. sila to use rodent burrows for shelter even more than they do now, especially at sites without access to shrubs. The continued management of shrubs and of burrowing rodents at G. sila sites is therefore essential to the survival of this endangered species.

Environmental heterogeneity shapes physiological traits in tropical direct-developing frogs

Tropical ectotherm species tend to have narrower physiological limits than species from temperate areas. As a consequence, tropical species are considered highly vulnerable to climate change since minor temperature increases can push them beyond their physiological thermal tolerance. Differences in physiological tolerances can also be seen at finer evolutionary scales, such as among populations of ectotherm species along elevation gradients, highlighting the physiological sensitivity of such organisms.Here, we analyze the influence of elevation and bioclimatic domains, defined by temperature and precipitation, on thermal sensitivities of a terrestrial direct-developing frog (Craugastor loki) in a tropical gradient. We address the following questions: (a) Does preferred temperature vary with elevation and among bioclimatic domains? (b) Do thermal tolerance limits, that is, critical thermal maximum and critical thermal minimum vary with elevation and bioclimatic domains? and (c) Are populations from high elevations more vulnerable to climate warming?We found that along an elevation gradient body temperature decreases as environmental temperature increases. The preferred temperature tends to moderately increase with elevation within the sampled bioclimatic domains. Our results indicate that the ideal thermal landscape for this species is located at midelevations, where the thermal accuracy (db ) and thermal quality of the environment (de ) are suitable. The critical thermal maximum is variable across elevations and among the bioclimatic domains, decreasing as elevation increases. Conversely, the critical thermal minimum is not as variable as the critical thermal maximum.Populations from the lowlands may be more vulnerable to future increases in temperature. We highlight that the critical thermal maximum is related to high temperatures exhibited across the elevation gradient and within each bioclimatic domain; therefore, it is a response to high environmental temperatures.

Towards more integration of physiology, dispersal and land-use change to understand the responses of species to climate change

The accelerating biodiversity crisis, for which climate change has become an important driver, urges the scientific community for answers to the question of whether and how species are capable of responding successfully to rapidly changing climatic conditions. For a better understanding and more realistic predictions of species' and biodiversity responses, the consideration of extrinsic (i.e. environment-related) and intrinsic (i.e. organism-related) factors is important, among which four appear to be particularly crucial: climate change and land-use change, as extrinsic factors, as well as physiology and dispersal capacity, as intrinsic factors. Here, I argue that these four factors should be considered in an integrative way, but that the scientific community has not yet been very successful in doing so. A quantitative literature review revealed a generally low level of integration within global change biology, with a pronounced gap especially between the field of physiology and other (sub)disciplines. After a discussion of potential reasons for this unfortunate lack of integration, some of which may relate to key deficits e.g. in the reward and incentive systems of academia, I suggest a few ideas that might help to overcome some of the barriers between separated research communities. Furthermore, I list several examples for promising research along the integration frontier, after which I outline some research questions that could become relevant if one is to push the boundary of integration among disciplines, of data and methods, and across scales even further - for a better understanding and more reliable predictions of species and biodiversity in a world of global change.

Hot and bothered: The role of behaviour and microclimates in buffering species from rising temperatures

In Focus: Bladon, A. J., Lewis, M., Bladon, E. K., Buckton, S. J., Corbett, S., Ewing, S. R., … Turner, E. C. (2020). How butterflies keep their cool: Physical and ecological traits influence thermoregulatory ability and population trends. Journal of Animal Ecology. https://doi.org/10.1111/1365-2656.13319 Threatened with rising average temperatures and the new normal of climate extremes, species that cannot keep pace with climate change must adapt where they are, or face extinction. The ranges of many British butterflies have indeed extended northwards as the climate has warmed, but this option is increasingly restricted by the expansion and intensification of urban and agricultural lands. On a day-to-day basis, butterflies can thermoregulate using behaviours such as adjusting their wing positioning or moving into suitable microclimates. The extent to which these two options buffer individuals from free-air temperature, however, is not well known. Nor is the extent to which the different mechanisms are exploited by different species, and whether that has had any bearing on species' population trends over the time-scale of recent climate change. Using a simple and easily replicated approach, Bladon et al. (2020) were able to quantify intra- and interspecific variation in buffering ability, and species' relative reliance on the two thermoregulatory mechanisms of wing adjustment versus microclimate selection. The authors report marked variation in buffering capacity, correlated with wing size, wing colouration and taxonomic family. Species also differed in their thermoregulatory behaviours, with some - such as the Ringlet Aphantopus hyperantus and Large Skipper Ochlodes sylvanus-achieving impressive buffering through wing positioning. Others, like the Brown Argus Aricia agestis and Small Heath Coenonympha pamphilus, were more reliant on microclimate selection, and these were the species most likely to have shown declining population trends over the past 40 years. The study underscores the importance of individual thermoregulatory behaviours for understanding species' vulnerability to climate change. In combination with much improved methods for measuring and modelling climate at biologically relevant scales, the approach of Bladon et al. (2020) can and should be extended to identify the places and species most at risk, and the steps that conservation practitioners can take to maximise resilience to climate change. Much attention has been given to improving habitat connectivity to facilitate range shifts, but we should also consider how microclimate availability can be enhanced to allow species to manage when they cannot move.