Acclimation capacity to global warming of amphibians and freshwater fishes: Drivers, patterns, and data limitations.

Amphibians and fishes play a central role in shaping the structure and function of freshwater environments. These organisms have a limited capacity to disperse across different habitats and the thermal buffer offered by freshwater systems is small. Understanding determinants and patterns of their physiological sensitivity across life history is, therefore, imperative to predicting the impacts of climate change in freshwater systems. Based on a systematic literature review including 345 experiments with 998 estimates on 96 amphibian (Anura/Caudata) and 93 freshwater fish species (Teleostei), we conducted a quantitative synthesis to explore phylogenetic, ontogenetic, and biogeographic (thermal adaptation) patterns in upper thermal tolerance (CTmax) and thermal acclimation capacity (acclimation response ratio, ARR) as well as the influence of the methodology used to assess these thermal traits using a conditional inference tree analysis. We found globally consistent patterns in CTmax and ARR, with phylogeny (taxa/order), experimental methodology, climatic origin, and life stage as significant determinants of thermal traits. The analysis demonstrated that CTmax does not primarily depend on the climatic origin but on experimental acclimation temperature and duration, and life stage. Higher acclimation temperatures and longer acclimation times led to higher CTmax values, whereby Anuran larvae revealed a higher CTmax than older life stages. The ARR of freshwater fishes was more than twice that of amphibians. Differences in ARR between life stages were not significant. In addition to phylogenetic differences, we found that ARR also depended on acclimation duration, ramping rate, and adaptation to local temperature variability. However, the amount of data on early life stages is too small, methodologically inconsistent, and phylogenetically unbalanced to identify potential life cycle bottlenecks in thermal traits. We, therefore, propose methods to improve the robustness and comparability of CTmax/ARR data across species and life stages, which is crucial for the conservation of freshwater biodiversity under climate change.

Contributions of water-borne corticosterone as one non-invasive biomarker in assessing nitrate pollution stress in tadpoles of Rana temporaria.

Among a multitude of stressors to which wildlife is exposed, environmental pollution is a pervasive one that poses a serious threat. The permeable skin of amphibians is likely to increase direct contact of the body with pollutants, making them a group worth studying to access environmental quality. Consequently, finding reliable and complementary biomarkers that will present detectable and predictable changes in response to pollutants is essential to identify pollution sublethal effects on amphibians and to investigate whether these are in part responsible for population declines. The glucocorticoid hormone corticosterone (CORT), involved in many metabolic functions, is often used to measure the physiological stress response to environmental stressors in amphibians. In this study, we evaluated whether water-borne CORT can serve as a non-invasive biomarker for nitrate pollution stress in the European common frog (Rana temporaria) by comparing the effect of nitrate exposure on hormone release rates and on other physiological downstream biomarkers, i.e., ultimate physiological effects of the stressor. Specifically, we investigated the effect of different nitrate concentrations (0, 10, 50, and 100 mg/L) on water-borne CORT release rates, age, size, and body condition. Exposure to nitrate pollution significantly increased age at metamorphosis and water-borne CORT release rates, and led to reduced mass and body condition, but only at higher nitrate concentrations (i.e., 50 and 100 mg/L). Considering this similar sensitivity to other acknowledged biomarkers, water-borne CORT was a reliable biomarker of physiological stress in R. temporaria exposed to nitrate pollution stress in a controlled single-stressor laboratory approach. Thus, water-borne CORT is a promising method to be included in more holistic approaches. We recommend that such approaches keep testing multiple biomarker combinations, as species are exposed to several stressors likely to interact and produce varied outcomes in different biomarkers in their natural habitats.

Timing of parental breeding shapes sensitivity to nitrate pollution in the common frog Rana temporaria.

Climate change is expected to increase mean temperatures and the frequency of extreme weather events, that can lead to earlier/extended breeding seasons in temperate taxa. As a consequence, many organisms that show climate-induced phenological shifts might be exposed to environmental conditions they are not well adapted to while breeding, and their ability to cope with stressful conditions might be influenced. Here, we investigated how parental breeding time shapes the sensitivity to nitrate exposure at three consecutive life stages (embryonic, larval, juvenile) in the European common frog (Rana temporaria). We compared hatching success and life-stage specific survival, growth, standard metabolic rate, body condition, and acute thermal sensitivity of offspring from an earlier-breeding parental cohort (early cohort) vs. a later-breeding parental cohort (late cohort) exposed to a range of environmentally relevant concentrations of nitrate (0-100 mg/L). We also investigated whether nitrate exposure experienced during the embryonic and larval stages affects physiological performance in later life stages (after metamorphosis). Our study reveals that parental breeding time affects the sensitivity to nitrate pollution at three consecutive life stages in Rana temporaria. Breeding later in spring reduced hatching size and survival at high nitrate exposure, but also induced compensatory growth of the offspring. In both early and late cohorts, exposure to nitrate pollution reduced developmental rate and led to larger, but older larvae at the onset of metamorphosis with a greater sensitivity to warmer environmental temperatures. Standard metabolic rate, on the contrary, was neither affected by parental breeding time nor by exposure to nitrate. Exposure to nitrate pollution during embryonic and larval development led to carry-over in juvenile froglets as their sensitivity to temperature was higher. In a world highly impacted by humans, it is thus essential to give stressors a more holistic approach in order to better predict their consequences on species subjected to them.

Dynamic and diverse amphibian assemblages: Can we differentiate natural processes from human induced changes?

Amphibians are sensitive to anthropogenic habitat alterations but also respond to natural drivers of assemblage composition at many levels. Additionally, they are usually hard to detect in field inventories. We used a multiscale approach, from microhabitat to the landscape levels, to try to understand the effects of natural changes, and try to distinguish them from the effects of landscape level anthropogenic changes, on dynamic and diverse anuran assemblages, taking imperfect detection into account. We conducted thorough field inventories in 16 streams at Serra do Cipó, in the southern portion of Espinhaço Mountain Range, southeastern Brazil, during two time periods separated by 16 years. We compared species richness and diversity between periods, sampling both tadpoles and adult frogs. We quantified tadpole microhabitat availability, alterations in immediate riparian vegetation, and changes in classes of land cover within buffers around streams (adult habitats) to test for their effects on species composition. We also tested for effects of human occupancy around streams on nestedness and turnover components of species diversity. Microhabitats and riparian vegetation explained some of the changes in species composition (or detection) between time periods. Nestedness seemed to be influenced by the stability of the landscape. Detectabilities were too low to support robust occupancy estimates for most species. Natural changes in local habitats occupied by anurans in montane meadows are likely to influence species distribution. Some species with robust estimates experienced change in their occupancy over the studied 16-year interval, although no anthropogenic causes could be directly associated with such changes. The low detectability of most species, even with thorough sampling effort, makes it very hard to detect amphibian declines and possibly tell them apart from natural population fluctuations. New techniques are needed that improve species detectability in such diverse tropical habitats.

Testing the relationship between human occupancy in the landscape and tadpole developmental stress.

Amphibian population declines are widespread; the main causal factors are human related and include habitat fragmentation due to agriculture, mining, fires, and urban development. Brazil is the richest country in species of amphibians, and the Brazilian regions with the greatest amphibian diversity are experiencing relatively high rates of habitat destruction, but there are presently relatively few reports of amphibian declines. It is thus important to develop research methods that will detect deterioration in population health before severe declines occur. We tested the use of measurements of fluctuating asymmetry (FA) taken on amphibian larvae to detect anthropogenic stress. We hypothesized that greater human occupancy in the landscape might result in more stressful conditions for amphibians. We conducted this study at the Espinhaço mountain range in southeastern Brazil, using as a model an endemic species (Bokermannohyla saxicola, Hylidae). We chose two tadpole denticle rows and eye-nostril distance as traits for FA measurement. We measured percent cover of human-altered habitats in the landscape around tadpole sampling points and measured FA levels in sampled tadpoles. We found FA levels to differ among localities but found no relationship between human modification of the landscape and tadpole FA levels. Levels of FA in the traits we examined may not be strongly affected by environmental conditions, or may be affected by local variables that were not captured by our landscape-scale measures. Alternatively, populations may be genetically differentiated, affecting how FA levels respond to stress and obscuring the effects of anthropogenic disturbance.

Tribute to R. G. Boutilier: skin colour and body temperature changes in basking Bokermannohyla alvarengai (Bokermann 1956).

In amphibians solar basking far from water sources is relatively uncommon since the highly permeable amphibian skin does not represent a significant barrier to the accompanying risk of losing water by evaporation. A South American frog, Bokermannohyla alvarengai (Bokermann 1956), however, spends a significant amount of the day exposed to full sun and relatively high temperatures. The means by which this frog copes with potentially high rates of evaporative water loss and high body temperatures are unknown. Thus, in this study, skin colour changes, body surface temperature, and evaporative water loss rates were examined under a mixture of field and laboratory conditions to ascertain whether changes in skin reflectivity play an important role in this animal's thermal and hydric balance. Field data demonstrated a tight correlation between the lightness of skin colour and frog temperature, with lighter frogs being captured possessing higher body temperatures. Laboratory experiments supported this relationship, revealing that frogs kept in the dark or at lower temperatures (20 degrees C) had darker skin colours, whereas frogs kept in the light or higher temperatures (30 degrees C) had skin colours of a lighter hue. Light exhibited a stronger influence on skin colour than temperature alone, suggesting that colour change is triggered by the increase in incident solar energy and in anticipation of changes in body temperature. This conclusion is corroborated by the observation that cold, darkly coloured frogs placed in the sun rapidly became lighter in colour during the initial warming up period (over the first 5 min), after which they warmed up more slowly and underwent a further, albeit slower, lightening of skin colour. Surprisingly, despite its natural disposition to bask in the sun, this species does not possess a ;waterproof' skin, since its rates of evaporative water loss were not dissimilar from many hylid species that live in arboreal or semi-aquatic environments. The natural history of B. alvarengai is largely unknown and, therefore, it is likely that the herein reported colour change and basking behaviour represent a complex interaction between thermoregulation and water balance with other ecologically relevant functions, such as crypsis.