Physiological and behavioral responses to novel saline conditions in an invasive treefrog

Salinity can be an environmental stressor for anurans, as their highly permeable skin makes them prone to osmotic stress when exposed to saline conditions. However, certain anuran species have colonized areas near saltwater habitats, suggesting an ability to acclimate to saline conditions. Here, we evaluated physiological and behavioral responses to saline conditions in adult Cuban treefrogs (Osteopilus septentrionalis), an invasive anuran found throughout Florida. To examine their response to salinity, adult frogs were maintained in two treatments simulating a freshwater (0.5 ppt) or brackish (8.0 ppt) environment for 6 weeks. To assess their physiological response to this potential stressor, all frogs were submerged in a brackish solution to quantify individual weight change every 2 weeks. We found that frogs maintained in brackish solution lost more weight at Weeks 2 and 6 when compared to Week 0, suggesting that salinity may be an environmental stressor for Cuban treefrogs. Yet, the weight change at Week 4 was similar to the pre-exposure period, which may indicate that constant exposure to salinity may alter their physiological response to saline conditions. To supplement the physiological analyses, we investigated avoidance behavior toward saline conditions by offering individuals a choice between freshwater or brackish environments. Our results showed that Cuban treefrogs chose freshwater environments more frequently and may thus avoid saline ones. This study reveals that salinity may induce plastic and avoidance responses in Cuban treefrogs, potentially allowing them to expand their range into areas typically stressful for most anurans.

Disentangling direct and indirect effects of extreme events on coastal wetland communities

One of the primary ways in which climate change will impact coastal freshwater wetlands is through changes in the frequency, intensity, timing and distribution of extreme weather events. Disentangling the direct and indirect mechanisms of population- and community-level responses to extreme events is vital to predicting how species composition of coastal wetlands will change under future conditions. We extended static structural equation modelling approaches to incorporate system dynamics in a multi-year multispecies occupancy model to quantify the effects of extreme weather events on a coastal freshwater wetland system. We used data from an 8-year study (2009-2016) on St. Marks National Wildlife Refuge in Florida, USA, to quantify species-specific and community-level changes in amphibian and fish occupancy associated with two flooding events in 2012 and 2013. We examine how physical changes to the landscape, including potential changes in salinity and increased wetland connectivity, may have contributed to or exacerbated the effects of these extreme weather events on the biota of isolated coastal wetlands. We provide evidence that the primary effects of flooding on the amphibian community were through indirect mechanisms via changes in the composition of the sympatric fish community that may have had lethal (i.e. through direct predation) or non-lethal (i.e. through direct or indirect competitive interactions) effects. In addition, we have shown that amphibian species differed in their sensitivity to direct flooding effects and indirect changes in the fish community and wetland-specific conductance, which led to variable responses across the community. These effects led to the overall decline in amphibian species richness from 2009 to 2016, suggesting that wetland-breeding amphibian communities on St. Marks National Wildlife Refuge may not be resilient to predicted changes in coastal disturbance regimes because of climate change. Understanding both direct and indirect effects, as well as species interactions, is important for predicting the effects of a changing climate on individual species, communities and ecosystems.

Increasing salinity stress decreases the thermal tolerance of amphibian tadpoles in coastal areas of Taiwan

Global warming is the main cause for the rise of both global temperatures and sea-level, both major variables threatening biodiversity. Rising temperatures threaten to breach the thermal limits of organisms while rising sea-level threatens the osmotic balance of coastal animals through habitat salinization. However, variations in thermal tolerance under different salinity stresses have not yet been thoroughly studied. In this study, we assessed the critical thermal maxima (CTmax) of amphibian tadpoles in different salinity conditions. We collected tadpoles of Duttaphrynus melanostictus, Fejervarya limnocharis and Microhyla fissipes from coastal areas and housed them in freshwater, low, and high salinity treatments for 7 days of acclimation. The CTmax, survival rate, and development rate of tadpoles in high salinity treatments were significantly lower than that of the two other treatments. Our results indicate that physiological performances and heat tolerances of tadpoles are negatively affected by salinization. Maximum entropy models showed that CTmax and sea-level rise are predicted to negatively affect the distribution of the three focal species. The present results suggest that global warming can lead to negative dual-impacts on coastal animals because of reduced thermal tolerances at elevated salinity. The impacts of global warming on anurans in coastal areas and other habitats impacted by salinization may be more severe than predicted and it is likely to cause similar dual-impacts on other ectotherms.

Molecular mechanisms of local adaptation for salt-tolerance in a treefrog

Salinization is a global phenomenon affecting ecosystems and forcing freshwater organisms to deal with increasing levels of ionic stress. However, our understanding of mechanisms that permit salt tolerance in amphibians is limited. This study investigates mechanisms of salt tolerance in locally adapted, coastal populations of a treefrog, Hyla cinerea. Using a common garden experiment, we (i) determine the extent that environment (i.e., embryonic and larval saltwater exposure) or genotype (i.e., coastal vs. inland) affects developmental benchmarks and transcriptome expression, and (ii) identify genes that may underpin differences in saltwater tolerance. Differences in gene expression, survival, and plasma osmolality were most strongly associated with genotype. Population genetic analyses on expressed genes also delineated coastal and inland groups based on genetic similarity. Coastal populations differentially expressed osmoregulatory genes including ion transporters (atp1b1, atp6V1g2, slc26a), cellular adhesion components (cdh26, cldn1, gjb3, ocln), and cytoskeletal components (odc1-a, tgm3). Several of these genes are the same genes expressed by euryhaline fish after exposure to freshwater, which is a novel finding for North American amphibians and suggests that these genes may be associated with local salinity adaptation. Coastal populations also highly expressed glycerol-3-phosphate dehydrogenase 1 (gpd1), which indicates they use glycerol as a compatible osmolyte to reduce water loss - another mechanism of saltwater tolerance previously unknown in frogs. These data signify that Hyla cinerea inhabiting coastal, brackish wetlands have evolved a salt-tolerant ecotype, and highlights novel candidate pathways that can lead to salt tolerance in freshwater organisms facing habitat salinization.

Changes in physiology and microbial diversity in larval ornate chorus frogs are associated with habitat quality

Environmental change associated with anthropogenic disturbance can lower habitat quality, especially for sensitive species such as many amphibians. Variation in environmental quality may affect an organism's physiological health and, ultimately, survival and fitness. Using multiple health measures can aid in identifying populations at increased risk of declines. Our objective was to measure environmental variables at multiple spatial scales and their effect on three indicators of health in ornate chorus frog (Pseudacris ornata) tadpoles to identify potential correlates of population declines. To accomplish this, we measured a glucocorticoid hormone (corticosterone; CORT) profile associated with the stress response, as well as the skin mucosal immune function (combined function of skin secretions and skin bacterial community) and bacterial communities of tadpoles from multiple ponds. We found that water quality characteristics associated with environmental variation, including higher water temperature, conductivity and total dissolved solids, as well as percent developed land nearby, were associated with elevated CORT release rates. However, mucosal immune function, although highly variable, was not significantly associated with water quality or environmental factors. Finally, we examined skin bacterial diversity as it aids in immunity and is affected by environmental variation. We found that skin bacterial diversity differed between ponds and was affected by land cover type, canopy cover and pond proximity. Our results indicate that both local water quality and land cover characteristics are important determinants of population health for ornate chorus frogs. Moreover, using these proactive measures of health over time may aid in early identification of at-risk populations that could prevent further declines and aid in management decisions.

Comparative Analysis of Anuran Amphibian Skin Microbiomes Across Inland and Coastal Wetlands

Amphibians host a community of microbes on their skin that helps resist infectious disease via the dual influence of anti-pathogenic microbial species and emergent community dynamics. Many frogs rely on freshwater habitats, but salinization is rapidly increasing saltwater concentrations in wetlands around the globe, increasing the likelihood that frogs will come into contact with salt-contaminated habitats. Currently, we know little about how increased salt exposure will affect the symbiotic relationship between the skin microbes and frog hosts. To better understand how salt exposure in a natural context affects the frog skin microbiome, we use Hyla cinerea, a North American treefrog species that can inhabit brackish wetlands, to explore three questions. First, we determine the extent that microbial communities in the environment and on frog skin are similar across populations. Second, we assess the microbial species richness and relative abundance on frogs from habitats with different salinity levels to determine how salinity affects the microbiome. Third, we test whether the relative abundances of putatively pathogen-resistant bacterial species differ between frogs from inland and coastal environments. We found that the frog microbiome is more similar among frogs than to the microbial communities found in surface water and soil, but there is overlap between frog skin and the environmental samples. Skin microbial community richness did not differ among populations, but the relative abundances of microbes were different across populations and salinities. We found no differences in the relative abundances of the anti-fungal bacteria Janthinobacterium lividum, the genus Pseudomonas, and Serratia marcescens, suggesting that environmental exposure to saltwater has a limited influence on these putatively beneficial bacterial taxa.

Widespread legacy brine contamination from oil production reduces survival of chorus frog larvae

Advances in drilling techniques have facilitated a rapid increase in hydrocarbon extraction from energy shales, including the Williston Basin in central North America. This area overlaps with the Prairie Pothole Region, a region densely populated with wetlands that provide numerous ecosystem services. Historical (legacy) disposal practices often released saline co-produced waters (brines) with high chloride concentrations, affecting wetland water quality directly or persisting in sediments. Despite the potential threat of brine contamination to aquatic habitats, there has been little research into its ecological effects. We capitalized on a gradient of legacy brine-contaminated wetlands in northeast Montana to conduct laboratory experiments to assess variation in survival of larval Boreal Chorus Frogs (Pseudacris maculata) reared on sediments from 3 local wetlands and a control source. To help provide environmental context for the experiment, we also measured chloride concentrations in 6 brine-contaminated wetlands in our study area, including the 2 contaminated sites used for sediment exposures. Survival of frog larvae during 46- and 55-day experiments differed by up to 88% among sediment sources (Site Model) and was negatively correlated with potential chloride exposure (Chloride Model). Five of the 6 contaminated wetlands exceeded the U.S. EPA acute benchmark for chloride in freshwater (860 mg/L) and all exceeded the chronic benchmark (230 mg/L). However, the Wetland Site model explained more variation in survival than the Chloride Model, suggesting that chloride concentration alone does not fully reflect the threat of contamination to aquatic species. Because the profiles of brine-contaminated sediments are complex, further surveys and experiments are needed across a broad range of conditions, especially where restoration or remediation actions have reduced brine-contamination. Information provided by this study can help quantify potential ecological threats and help land managers prioritize conservation strategies as part of responsible and sustainable energy development.

Adaptive responses to salinity stress across multiple life stages in anuran amphibians

Background

In many regions, freshwater wetlands are increasing in salinity at rates exceeding historic levels. Some freshwater organisms, like amphibians, may be able to adapt and persist in salt-contaminated wetlands by developing salt tolerance. Yet adaptive responses may be more challenging for organisms with complex life histories, because the same environmental stressor can require responses across different ontogenetic stages. Here we investigated responses to salinity in anuran amphibians: a common, freshwater taxon with a complex life cycle. We conducted a meta-analysis to define how the lethality of saltwater exposure changes across multiple life stages, surveyed wetlands in a coastal region experiencing progressive salinization for the presence of anurans, and used common garden experiments to investigate whether chronic salt exposure alters responses in three sequential life stages (reproductive, egg, and tadpole life stages) in Hyla cinerea, a species repeatedly observed in saline wetlands.

Results

Meta-analysis revealed differential vulnerability to salt stress across life stages with the egg stage as the most salt-sensitive. Field surveys revealed that 25% of the species known to occur in the focal region were detected in salt-intruded habitats. Remarkably, Hyla cinerea was found in large abundances in multiple wetlands with salinity concentrations 450% higher than the tadpole-stage LC₅₀. Common garden experiments showed that coastal (chronically salt exposed) populations of H. cinerea lay more eggs, have higher hatching success, and greater tadpole survival in higher salinities compared to inland (salt naïve) populations.

Conclusions

Collectively, our data suggest that some species of anuran amphibians have divergent and adaptive responses to salt exposure across populations and across different life stages. We propose that anuran amphibians may be a novel and amenable natural model system for empirical explorations of adaptive responses to environmental change.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-017-0222-0) contains supplementary material, which is available to authorized users.

Differences in tolerance to anthropogenic stress between invasive and native bivalves

Tolerance towards environmental stress has been frequently considered as one of the key determinants of invasion success. However, empirical evidence supporting the assumption that invasive species can better endure unfavorable conditions compared with native species is limited and has yielded opposing results. In this study, we examined the tolerance to different stress conditions (thermal stress and trace metal zinc pollution stress) in two phylogenetically related and functionally similar freshwater bivalve species, the native Anodonta anatina and the invasive Sinanodonta woodiana. We assessed potential differences in response to stress conditions using several cellular response assays: efficiency of the multixenobiotic resistance mechanism, respiration estimate (INT reduction capacity), and enzymatic biomarkers. Our results demonstrated that the invasive species overall coped much better with unfavorable conditions. The higher tolerance of S. woodiana was evident from (i) significantly decreased Rhodamine B accumulation indicating more efficient multixenobiotic resistance mechanism; (ii) significantly higher INT reduction capacity and (iii) less pronounced alterations in the activity of stress-related enzymes (glutathione-S-transferase, catalase) and of a neurotoxicity biomarker (cholinesterase) in the majority of treatment conditions in both stress trials. Higher tolerance to thermal extremes may provide physiological benefit for further invasion success of S. woodiana in European freshwaters, especially in the context of climate change.

Drought, deluge and declines: the impact of precipitation extremes on amphibians in a changing climate

The Class Amphibia is one of the most severely impacted taxa in an on-going global biodiversity crisis. Because amphibian reproduction is tightly associated with the presence of water, climatic changes that affect water availability pose a particularly menacing threat to both aquatic and terrestrial-breeding amphibians. We explore the impacts that one facet of climate change—that of extreme variation in precipitation—may have on amphibians. This variation is manifested principally as increases in the incidence and severity of both drought and major storm events. We stress the need to consider not only total precipitation amounts but also the pattern and timing of rainfall events. Such rainfall “pulses” are likely to become increasingly more influential on amphibians, especially in relation to seasonal reproduction. Changes in reproductive phenology can strongly influence the outcome of competitive and predatory interactions, thus potentially altering community dynamics in assemblages of co-existing species. We present a conceptual model to illustrate possible landscape and metapopulation consequences of alternative climate change scenarios for pond-breeding amphibians, using the Mole Salamander, Ambystoma talpoideum, as an example. Although amphibians have evolved a variety of life history strategies that enable them to cope with environmental uncertainty, it is unclear whether adaptations can keep pace with the escalating rate of climate change. Climate change, especially in combination with other stressors, is a daunting challenge for the persistence of amphibians and, thus, the conservation of global biodiversity.