Determinants of size at metamorphosis in an endangered amphibian and their projected effects on population stability

Managing invasive hybrids with pond hydroperiod manipulation in an endangered salamander system

When invasive and endangered native taxa hybridize, the resulting admixture introduces novel conservation challenges. Across a large region of central California, a hybrid swarm consisting of admixed endangered California tiger salamanders (CTS) (Ambystoma californiense) and introduced barred tiger salamanders (BTS) (Ambystoma mavortium) has replaced native populations, threatening the genetic integrity of CTS and the vernal pool systems they inhabit. We employed a large-scale, genomically informed field experiment to test whether shortening breeding pond hydroperiod would favor native CTS genotypes. We constructed 14 large, seminatural ponds to evaluate the effect of hydroperiod duration on larval survival and mass at metamorphosis. We tracked changes in non-native allele frequencies with a 5237-gene exon capture array and employed a combination of custom Bayesian and generalized linear models to quantify the effect of pond duration on salamander fitness. Earlier work on this system showed hybrid superiority under many conditions and suggested that hybrids are favored in human-modified ponds with artificially long hydroperiods. Consistent with these earlier studies, we found overwhelming evidence for hybrid superiority. Very short hydroperiods substantially reduced the mass (1.1-1.5 fold) and survival probability (10-13 fold) of both native and hybrid larvae, confirming that hydroperiod likely exerts a strong selective pressure in the wild. We identified 86 genes, representing 1.8% of 4723 screened loci, that significantly responded to this hydroperiod-driven selection. In contrast to earlier work, under our more natural experimental conditions, native CTS survival and size at metamorphosis were always less than hybrids, suggesting that hydroperiod management alone will not shift selection to favor native larval genotypes. However, shortening pond hydroperiod may limit productivity of hybrid ponds, complementing other strategies to remove hybrids while maintaining vernal pool ecosystems. This study confirms and expands on previous work that highlights the importance of hydroperiod management to control invasive aquatic species.

Ammonium effects on oxidative stress, telomere length, and locomotion across life stages of an anuran from habitats with contrasting land-use histories

Understanding the mechanistic implications behind wildlife responses to global changes is a central topic in eco-evolutionary research. In particular, anthropic pollution is known to impact wild populations across the globe, which may have even stronger consequences for species with complex life cycles. Among vertebrates, amphibians represent a paradigmatic example of metamorphosis, and their characteristics make them highly vulnerable to pollution. Here, we tested for differences in the redox status, telomere length, and locomotor performance across life stages of green frogs (Pelophylax perezi) from agrosystem and natural habitats, both constitutively and in response to an experimental ammonium exposure (10 mg/L). We found that larvae from the agrosystem constitutively showed an enhanced redox status (better antioxidant balance against H₂O₂, lower lipid peroxidation) but shorter telomeres as compared to larvae from the natural environment. The larval redox response to ammonium was, overall, similar in both habitats. In contrast, after metamorphosis, the redox status of individuals from the natural habitat seemed to cope better with ammonium exposure (denoted by lower lipid peroxidation), and differences between habitats in telomere length were no longer present. Intriguingly, while the swimming performance of larvae did not correlate with individual's physiology, metamorphs with lower glutathione reductase activity and longer telomeres had a better jumping performance. This may suggest that locomotor performance is both traded off with the production of reactive oxygen species and potentiated directly by longer telomeres or indirectly by the mechanisms that buffer their shortening. Overall, our study suggests that contrasting land-use histories can drive divergence in physiological pathways linked to individual health and lifespan. Since this pattern was life-stage dependent, divergent habitat conditions can have contrasting implications across the ontogenetic development of species with complex life cycles.

Applying stochastic and Bayesian integral projection modeling to amphibian population viability analysis

Integral projection models (IPMs) can estimate the population dynamics of species for which both discrete life stages and continuous variables influence demographic rates. Stochastic IPMs for imperiled species, in turn, can facilitate population viability analyses (PVAs) to guide conservation decision-making. Biphasic amphibians are globally distributed, often highly imperiled, and ecologically well suited to the IPM approach. Herein, we present a stochastic size- and stage-structured IPM for a biphasic amphibian, the U.S. federally threatened California tiger salamander (CTS) (Ambystoma californiense). This Bayesian model reveals that CTS population dynamics show greatest elasticity to changes in juvenile and metamorph growth and that populations are likely to experience rapid growth at low density. We integrated this IPM with climatic drivers of CTS demography to develop a PVA and examined CTS extinction risk under the primary threats of habitat loss and climate change. The PVA indicated that long-term viability is possible with surprisingly high (20%-50%) terrestrial mortality but simultaneously identified likely minimum terrestrial buffer requirements of 600-1000 m while accounting for numerous parameter uncertainties through the Bayesian framework. These analyses underscore the value of stochastic and Bayesian IPMs for understanding both climate-dependent taxa and those with cryptic life histories (e.g., biphasic amphibians) in service of ecological discovery and biodiversity conservation. In addition to providing guidance for CTS recovery, the contributed IPM and PVA supply a framework for applying these tools to investigations of ecologically similar species.

Acute mortality in California tiger salamander (Ambystoma californiense) and Santa Cruz long-toed salamander (Ambystoma macrodactylum croceum) caused by Ribeiroia ondatrae (Class: Trematoda)

In early September 2019, a morbidity and mortality event affecting California tiger salamanders (Ambystoma californiense) and Santa Cruz long-toed salamanders (Ambystoma macrodactylum croceum) in late stages of metamorphosis was reported at a National Wildlife Refuge in Santa Cruz County, California, U.S.A. During the postmortem disease investigation, severe integumentary metacercarial (Class: Trematoda) infection, associated with widespread skin lesions, was observed. Planorbid snails collected from the ponds of the refuge within seven days of the mortality event were infected with Ribeiroia ondatrae, a digenetic trematode that can cause malformation and death in some amphibians. We suggest sustained seasonal high-water levels due to active habitat management along with several years of increased rainfall led to increased bird visitation, increased over-wintering of snails, and prolonged salamander metamorphosis, resulting in a confluence of conditions and cascading of host-parasite dynamics to create a hyper-parasitized state.

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Mortality event in wild endangered salamanders in California.

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Infection with Ribeiroia ondatrae caused severe fatal skin lesions.

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Sustaining water levels may increase parasite transmission.

Altered temperature affect body condition and endochondral ossification in Bufo gargarizans tadpoles

Bufo gargarizans is one kind of economic animals with higher medicinal value in China. In this study, B. gargarizans (Bufo gargarizans) tadpoles were reared at three different water temperature (15, 22 and 29 °C) from Gosner stages 28-46. We investigated the effects of temperature on growth, development, survival, metamorphic duration, size and skeletal ossification at Gosner stage 40, 42, and 46, as well as thyroid tissue reached metamorphic climax (Gs42). Besides, we examined the transcription levels of endochondral ossification-related genes in hind limb at metamorphic climax (Gs42). Our results showed that the growth and development of tadpoles conform to the temperature-size rule (TSR). While warm temperature resulted in the decrease in body size and hind limb length, and shorten larval period, cold temperature led to increase in body size and hind limb length but prolonged larval period. Histological examinations revealed that warm and cold temperatures caused damage to thyroid tissue. Also, warm and cold temperatures inhibited the degree of ossification with the double staining methodology. Additionally, the real-time PCR results suggested that warm and cold temperatures significantly up-regulated Runx2, VEGF and VEGFR mRNA levels, and down-regulated TRβ, MMP9, MMP13 and Runx3 mRNA levels. The up-regulation of Dio2 level and down-regulation of Dio3 level were observed in warm temperature. TRα mRNA level was significantly increased in warm temperature, but decreased in cold temperature. Collectively, these observations demonstrated that warm and cold temperatures affected endochondral ossification in B. gargarizans tadpoles, which might influence their capacity to terrestrial locomotion.

Sensitivity of Amphibians to Copper

Amphibian populations are declining globally. Major drivers of these global declines are known. However, the contribution of these major drivers to population declines varies by the presence or absence and the interactive effect of drivers, thus creating local challenges for conservation of populations. Studies have determined that environmental contaminants contribute to amphibian population declines. However, there is a disagreement over the use of amphibians as sentinel species in ecotoxicological testing rather than the traditional taxa used, fish and invertebrates. Reviews of ecotoxicological studies have demonstrated that amphibians are generally less sensitive than fish and invertebrates to different groups of contaminants. Nonetheless, because of the distinct nature and mechanism of toxicity of various contaminants, it is necessary to study contaminants individually to be able to come to any conclusion on the relative sensitivity of amphibians. Copper is one of the most studied environmental contaminants. We conducted a literature review of Cu toxicity to amphibians and the relative sensitivity of amphibians to other aquatic animals. The available data suggest that although amphibians may be tolerant of acute Cu exposure, they are relatively sensitive to chronic exposure (i.e., 100-fold greater sensitivity to chronic compared to acute exposure). In addition, ecologically relevant endpoints specific to amphibians (e.g., duration of metamorphosis and behavior) are shown to provide a better understanding of their sensitivity compared to traditional endpoints (e.g., survival and growth). Our current knowledge on amphibian sensitivity is far from complete. Considering the current status of this globally threatened class of animals, it is necessary to fill the knowledge gaps regarding their sensitivity to individual contaminants, beginning with Cu. Environ Toxicol Chem 2021;40:1810-1821. © 2021 SETAC.

Shifts in sensitivity of amphibian metamorphosis to endocrine disruption: the common frog (Rana temporaria) as a case study

Effective conservation actions require knowledge on the sensitivity of species to pollution and other anthropogenic stressors. Many of these stressors are endocrine disruptors (EDs) that can impair the hypothalamus-pituitary-thyroid axis and thus alter thyroid hormone (TH) levels with physiological consequences to wildlife. Due to their specific habitat requirements, amphibians are often sentinels of environmental degradation. We investigated how altered TH levels affected the bioenergetics of growth and development (i.e. age, size, metabolism, cardiac function and energy stores) before, during and after metamorphosis in the European common frog (Rana temporaria). We also determined how ontogenetic stage affected susceptibility to endocrine disruption and estimated juvenile performance. TH levels significantly affected growth and energetics at all developmental stages. Tadpoles and froglets exposed to high TH levels were significantly younger, smaller and lighter at all stages compared to those in control and low TH groups, indicating increased developmental and reduced growth rates. Across all ontogenetic stages tested, physiological consequences were rapidly observed after exposure to EDs. High TH increased heart rate by an average of 86% and reduced energy stores (fat content) by 33% compared to controls. Effects of exposure were smallest after the completion of metamorphosis. Our results demonstrate that both morphological and physiological traits of the European common frog are strongly impacted by endocrine disruption and that ontogenetic stage modulates the sensitivity of this species to endocrine disruption. Since endocrine disruption during metamorphosis can impair the physiological stress response in later life stages, long-term studies examining carry-over effects will be an important contribution to the conservation physiology of amphibians.

Altered thyroid hormone levels affect the capacity for temperature-induced developmental plasticity in larvae of Rana temporaria and Xenopus laevis

Anuran larvae show phenotypic plasticity in age and size at metamorphosis as a response to temperature variation. The capacity for temperature-induced developmental plasticity is determined by the thermal adaptation of a population. Multiple factors such as physiological responses to changing environmental conditions, however, might influence this capacity as well. In anuran larvae, thyroid hormone (TH) levels control growth and developmental rate and changes in TH status are a well-known stress response to sub-optimal environmental conditions. We investigated how chemically altered TH levels affect the capacity to exhibit temperature-induced developmental plasticity in larvae of the African clawed frog (Xenopus laevis) and the common frog (Rana temporaria). In both species, TH level influenced growth and developmental rate and modified the capacity for temperature-induced developmental plasticity. High TH levels reduced thermal sensitivity of metamorphic traits up to 57% (R. temporaria) and 36% (X. laevis). Rates of growth and development were more plastic in response to temperature in X. laevis (+30%) than in R. temporaria (+6%). Plasticity in rates of growth and development is beneficial to larvae in heterogeneous habitats as it allows a more rapid transition into the juvenile stage where rates of mortality are lower. Therefore, environmental stressors that increase endogenous TH levels and reduce temperature-dependent plasticity may increase risks and the vulnerability of anuran larvae. As TH status also influences metabolism, future studies should investigate whether reductions in physiological plasticity also increases the vulnerability of tadpoles to global change.

Positive correlation between dispersal and body size in Green Frogs (Rana clamitans) naturally colonizing an experimental landscape

Dispersers are often assumed to have the mean phenotype observed across the entire metapopulation, despite growing evidence of dispersal–phenotype correlations. We examined three dispersal–phenotype correlations in Green Frogs (Rana clamitans Latreille, 1801 = Lithobates clamitans (Latreille, 1801)). Two were in traits that have been previously tied to fitness (body size and body condition), while a third (relative hindlimb length) has been linked to movement performance. We constructed a spatially dispersed array of experimental ponds in close proximity to source ponds known to support Green Frog breeding populations. Over the course of two breeding seasons (four sampling periods), we measured phenotypes of all Green Frogs that had colonized the experimental ponds and a sample of individuals from the source ponds. After only 1 month, a positive correlation was detected between dispersal and body size within the population of dispersers occupying the experimental ponds. After a 2nd month, this positive dispersal – body size correlation was also present when comparing the population of dispersers to the population of nondispersers remaining at the source ponds. Even if generated solely by plasticity, a positive correlation between dispersal and body size (a trait tightly linked to fitness) has the ability to alter metapopulation capacity and thus the probability of regional species persistence.

Thyroid hormone levels and temperature during development alter thermal tolerance and energetics of Xenopus laevis larvae

Environmental variation induced by natural and anthropogenic processes including climate change may threaten species by causing environmental stress. Anuran larvae experiencing environmental stress may display altered thyroid hormone (TH) status with potential implications for physiological traits. Therefore, any capacity to adapt to environmental changes through plastic responses provides a key to determining species vulnerability to environmental variation. We investigated whether developmental temperature (T dev), altered TH levels and whether the interactive effect of both affect standard metabolic rate (SMR), body condition (BC), survival and thermal tolerance in larvae of the African clawed frog (Xenopus laevis) reared at five temperatures with experimentally altered TH levels. At metamorphosis, SMR, BC and survival were significantly affected by T dev, TH status and their interaction with the latter often intensified impacts. Larvae developing at warmer temperatures exhibited significantly higher SMRs and BC was reduced at warm T dev and high TH levels suggesting decreased ability to acclimate to variation in temperature. Accordingly, tadpoles that developed at warm temperatures had higher maximum thermal limits but more narrow thermal tolerance windows. High and low TH levels decreased and increased upper thermal limits, respectively. Thus, when experiencing both warmer temperatures and environmental stress, larvae may be less able to compensate for changes in T dev. Our results demonstrate that physiological traits in larvae of X. laevis are strongly affected by increased TH levels and warmer temperatures. Altered TH levels and increasing T dev due to global change may result in a reduced capacity for physiological plasticity. This has far reaching consequences since the energetic requirement at the onset of metamorphosis is known to determine metamorphic success and thus, is indirectly linked to individual fitness in later life stages.

Delayed effects and complex life cycles: How the larval aquatic environment influences terrestrial performance and survival

Species with complex life cycles are susceptible to environmental stressors across life stages, but the carryover and latent effects between stages remain understudied. For species with biphasic life histories, such as pond-breeding amphibians, delayed effects of aquatic conditions can influence terrestrial juveniles and adults directly or indirectly, usually mediated through fitness correlates such as body size. We collected adult southern toads (Anaxyrus terrestris) from 2 source populations-a natural reference wetland and a metal-contaminated industrial wetland-and exposed their offspring to 2 aquatic stressors (a metal contaminant, copper [Cu], and a dragonfly predator cue) in outdoor mesocosms (n = 24). We then reared metamorphs in terraria for 5 mo to examine delayed effects of early life stage environmental conditions on juvenile performance, growth, and survival. Larval exposure to Cu, as well as having parents from a contaminated wetland, resulted in smaller size at metamorphosis-a response later negated by compensatory growth. Although Cu exposure and parental source did not affect larval survival, we observed latent effects of these stressors on juvenile survival, with elevated Cu conditions and metal-contaminated parents reducing postmetamorphic survival. Parental source and larval Cu exposure affected performance at metamorphosis through carryover effects on body size but, 1 mo later, latent effects of parental source and larval predator exposure directly (i.e., not via body size) influenced performance. The carryover and latent effects of parental source population and aquatic Cu level on postmetamorphic survival and juvenile performance highlight the importance of conducting studies across life stages and generations. Environ Toxicol Chem 2018;37:2660-2669. © 2018 SETAC.

Ecological equivalency as a tool for endangered species management

The use of taxon substitutes for extinct or endangered species is a controversial conservation measure. We use the example of the endangered California tiger salamander (Ambystoma californiense; CTS), which is being replaced by hybrids with the invasive barred tiger salamander (Ambystoma mavortium), to illustrate a strategy for evaluating taxon substitutes based on their position in a multivariate community space. Approximately one-quarter of CTS's range is currently occupied by "full hybrids" with 70% nonnative genes, while another one-quarter is occupied by "superinvasives" where a specific set of 3/68 genes comprising 4% of the surveyed genome is nonnative. Based on previous surveys of natural CTS breeding ponds, we stocked experimental mesocosms with field-verified, realistic densities of tiger salamander larvae and their prey, and used these mesocosms to evaluate ecological equivalency between pure CTS, full hybrids, and superinvasives in experimental pond communities. We also included a fourth treatment with no salamanders present to evaluate the community effects of eliminating Ambystoma larvae altogether. We found that pure CTS and superinvasive larvae were ecologically equivalent, because their positions in the multivariate community space were statistically indistinguishable and they did not differ significantly along any univariate community axes. Full hybrids were ecologically similar, but not equivalent, to the other two genotypes, and the no-Ambystoma treatment was by far the most divergent. We conclude that, at least for the larval stage, superinvasives are adequate taxon substitutes for pure CTS and should probably be afforded protection under the Endangered Species Act. The proper conservation status for full hybrids remains debatable.