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Quantifying how contaminants change across life cycles of species who undergo metamorphosis is critical to assessing risk to organisms and their consumers. Pond-breeding amphibians can dominate aquatic animal biomass as larvae and are terrestrial prey as metamorphs and adults. Thus, amphibians can be vectors of mercury accumulation in both aquatic and terrestrial food webs. However, it is still unclear how mercury concentrations are affected by exogenous (e.g., habitat or diet) vs. endogenous factors (e.g., catabolism during hibernation) as amphibians undergo large diet shifts and periods of fasting during ontogeny. We measured total mercury (THg), methylmercury (MeHg), and isotopic compositions (?13C, ?15N) in boreal chorus frogs (Pseudacris maculata) across five life stages in two metapopulations in Colorado, USA. We found large differences in MeHg concentrations and percent of THg as MeHg among life stages. Frog MeHg concentrations spiked after metamorphosis and hibernation coinciding with the most energetically demanding stages of their life cycle. Transitions among life stages led to large step changes in mercury concentrations – the endogenous processes of metamorphosis and hibernation biomagnified MeHg, decoupling isotopic compositions and MeHg concentrations. These step changes are not often considered in conventional expectations of how food web processes predict trophic transfer, accumulation, and transport of contaminants. ?

Increasing temperatures and climate-driven disturbances like wildfire are a growing threat to many species,
including cold-water specialists. Montane areas and cold streams are often considered climate refugia that buffer
communities against change. However, climate refugia are often species-specific, and despite growing awareness
that life histories and habitat requirements shape responses to change, small or non-game species are often
under-represented in monitoring and planning programs. A recent study in Montana, USA, revealed much larger
warming-related declines in occupancy for small, non-game slimy sculpin (Cottus cognatus) between 1993 and
1995 and 2011–2013 than for two socially valued salmonid fishes that shape regional conservation efforts. To
broaden insight into climate change vulnerabilities of headwater stream communities, we analyzed data for
Rocky Mountain tailed frogs (Ascaphus montanus) that were collected during those same electrofishing surveys
for fishes from 241 stream reaches. Tailed frogs occupy small, cold streams and have several life-history traits
that make them sensitive to environmental change. We used a Bayesian framework to estimate occupancy,
colonization, and extinction dynamics relative to forest canopy, estimated stream temperature, and wildfire
effects. Tailed frog occupancy decreased by 19 % from 1993 to 1995 to 2011–2013. Changes in occupancy were
linked with increased extinction and reduced colonization where there were fire-driven reductions in canopy
cover, and reduced colonization of stream reaches that warmed on average 0.8 ?C during the study. Our results
highlight extensive extirpations for oft-overlooked species and emphasize the importance of including species
with diverse habitat requirements and life histories in conservation planning.

Invasive species and emerging infectious diseases are two of the greatest
threats to biodiversity. American Bullfrogs (Rana [Lithobates] catesbeiana),
which have been introduced to many parts of the world, are often linked with
declines in native amphibians via predation and the spread of emerging pathogens
such as amphibian chytrid fungus (Batrachochytrium dendrobatidis [Bd])
and ranaviruses. Although many studies have investigated the potential role of
bullfrogs in the decline of native amphibians, analyses that account for shared
habitat affinities and imperfect detection have found limited support for
clear effects. Similarly, the role of bullfrogs in shaping the patch-level distribution
of pathogens is unclear. We used eDNA methods to sample 233 sites in the southwestern USA and Sonora, Mexico (2016–2018) to estimate how
the presence of bullfrogs affects the occurrence of four native amphibians,
Bd, and ranaviruses. Based on two-species, dominant-subordinate occupancy
models fitted in a Bayesian context, federally threatened Chiricahua Leopard
Frogs (Rana chiricahuensis) and Western Tiger Salamanders (Ambystoma
mavortium) were eight times (32% vs. 4%) and two times (36% vs. 18%), respectively,
less likely to occur at sites where bullfrogs occurred. Evidence for the
negative effects of bullfrogs on Lowland Leopard Frogs (Rana yavapaiensis)
and Northern Leopard Frogs (Rana pipiens) was less clear, possibly because of
smaller numbers of sites where these native species still occurred and because
bullfrogs often occur at lower densities in streams, the primary habitat for
Lowland Leopard Frogs. At the community level, Bd was most likely to occur
where bullfrogs co-occurred with native amphibians, which could increase the
risk to native species. Ranaviruses were estimated to occur at 33% of bullfrogonly
sites, 10% of sites where bullfrogs and native amphibians co-occurred,
and only 3% of sites where only native amphibians occurred. Of the 85 sites
where we did not detect any of the five target amphibian species, we also did
not detect Bd or ranaviruses; this suggests other hosts do not drive the distribution
of these pathogens in our study area. Our results provide landscape-scale
evidence that bullfrogs reduce the occurrence of native amphibians and
increase the occurrence of pathogens, information that can clarify risks and
aid the prioritization of conservation actions.

Protected areas like national parks are essential elements of conservation because they limit human influence on the landscape, which protects biodiversity and ecosystem function. The role of national parks in conservation, however, often goes far beyond limiting human influence. The U.S. National Park Service and its system of land units contribute substantively to conservation by providing protected lands where researchers can document trends in species distributions and abundances, examine characteristics important for generating these trends, and identify and implement conservation strategies to preserve biodiversity. We reviewed the contribution of U.S. national parks to amphibian research and conservation and highlight important challenges and findings in several key areas. First, U.S. national parks were instrumental in providing strong support that amphibian declines were real and unlikely to be simply a consequence of habitat loss. Second, research in U.S. national parks provided evidence against certain hypothesized causes of decline, like UV-B radiation, and evidence for others, such as introduced species and disease. However, describing declines and identifying causes contributes to conservation only if it leads to management; importantly, U.S. national parks have implemented many conservation strategies and evaluated their effectiveness in recovering robust amphibian populations. Among these, removal of invasive species, especially fishes; conservation translocations; and habitat creation and enhancement stand out as examples of successful conservation strategies with broad applicability. Successful management for amphibians is additionally complicated by competing mandates and stakeholder interests; for example, past emphasis on increasing visitor enjoyment by introducing fish to formerly fishless lakes had devastating consequences for many amphibians. Other potential conflicts with amphibian conservation include increasing development, increased risk of introductions of disease and exotic species with increased visitation, and road mortality. Decision science and leveraging partnerships have proven to be key components of effective conservation under conflicting mandates in national parks. As resource managers grapple with large-scale drivers that are outside local control, public-private partnerships and adaptive strategies are increasing in importance. U.S. national parks have played an important role in many aspects of identifying and ameliorating the amphibian decline crisis and will continue to be essential for the conservation of amphibians in the future.

Most translocation efforts are unsuccessful, often for unknown reasons. We assessed factors linked with population persistence for 25 years of translocations of the federally threatened Chiricahua Leopard Frog. Local features were paramount, including habitat, predators, and restoration history. Timing and life stages stocked affected persistence, but rearing environment did not. Two or more translocations produced an approximate 4-yr increase in predicted population persistence.

To inform responsible energy development, it is important to understand the ecological effects of contamination events. Wastewaters from oil and gas extraction often contain high concentrations of sodium chloride (NaCl) and heavy metals (e.g., strontium and vanadium), but studies of their influence on microbial communities are limited. We sampled water, sediment, and larval amphibian skin (four species) across a gradient of contamination (0.04–17500 mg/L Cl) in a large energy production area of North America. NaCl concentrations affected the similarity among microbiomes of water, sediment, and amphibian skin, but not the diversity or richness of water and skin microbiomes. Strontium concentrations were associated with lower diversity and richness of sediment microbial communities. Amphibian microbiomes were similar to those of water, but not sediment, and sediment microbiomes were similar to those of water. Species identity was the strongest predictor of amphibian microbiomes; frog microbiomes were similar but differed from that of the salamander, whose microbiome had the lowest richness and diversity. Understanding whether effects of wastewaters on microbial communities also influences their ecosystem function will be an important next step. Our study provides novel insight into associations among different wetland microbial communities and effects of wastewaters from energy production.

Dam-created reservoirs are common landscape features that can provide habitat for amphibians, but their water level fluctuations and nonnative predators can differ markedly from more natural habitats. We compared fall movement and habitat use by the Oregon Spotted Frog (Rana pretiosa) in the reservoir pool with nearby river and pond habitats at Crane Prairie Reservoir in central Oregon, USA. Movement rate of frogs in the river and ponds declined as water temperature cooled. Reservoir frogs moved further than those in the river or ponds, and their movement rate increased as water temperature cooled. Most frog locations across all site types were in aquatic herbaceous vegetation. We did not find shifts in habitat between early and late fall. Increased movement and the lack of habitat shift in our reservoir frogs deeper into fall contrast with R. pretiosa in non-reservoir sites in this study and others. Consistent use of vegetation by reservoir frogs throughout the fall could indicate cover use in presence of fish predators. Our study provides additional detail on the range of habitats used by R. pretiosa in fall and suggests areas for further work to improve survival in constructed sites with abundant fish predators.

Salinity (sodium chloride, NaCl) from anthropogenic sources is a persistent contaminant that negatively affects freshwater taxa. Amphibians can be susceptible to salinity, but some species are innately or adaptively tolerant. Physiological mechanisms mediating tolerance to salinity are still unclear, but changes in osmoregulatory hormones such as corticosterone (CORT) and aldosterone (ALDO) are prime candidates. We exposed larval barred tiger salamanders (Ambystoma mavortium) to environmentally relevant NaCl treatments (<32–4000 mg·L?1) for 24 days to test effects on growth, survival, and waterborne CORT responses. Of those sampled, we also quantified waterborne ALDO from a subset. Using a glucocorticoid antagonist (RU486), we also experimentally suppressed CORT signaling of some larvae to determine if CORT mediates effects of salinity. There were no strong differences in survival among salinity treatments, but salinity reduced dry mass, snout–vent length, and body condition while increasing water content of larvae. High survival and sublethal effects demonstrated that salamanders were physiologically challenged but could tolerate the experimental concentrations. CORT signaling did not attenuate sublethal effects of salinity. Baseline and stress-induced (after an acute stressor, shaking) CORT were not influenced by salinity. ALDO was correlated with baseline CORT, suggesting it could be difficult to decouple the roles of CORT and ALDO. Future studies comparing ALDO and CORT responses of adaptively tolerant and previously unexposed populations could be beneficial to understand the roles of these hormones in tolerance to salinity. Nevertheless, our study enhances our understanding of the roles of corticosteroid hormones in mediating effects of a prominent anthropogenic stressor.

testIncreased salinity is an emerging contaminant of concern for aquatic taxa. For amphibians exposed to salinity, there is scarce information about the physiological effects and changes in osmoregulatory hormones such as corticosterone (CORT) and aldosterone (ALDO). Recent studies have quantified effects of salinity on CORT physiology of amphibians based on waterborne hormone collection methods, but much less is known about ALDO in iono- and osmoregulation of amphibians. We re-assayed waterborne hormone samples from a previous study to investigate effects of salinity (sodium chloride, NaCl) and a glucocorticoid receptor antagonist (RU486) on ALDO of northern leopard frog (Rana pipiens) larvae. We also investigated relationships between ALDO and CORT. Waterborne ALDO marginally decreased with increasing salinity and was, unexpectedly, positively correlated with baseline and stress-induced waterborne CORT. Importantly, ALDO increased when larvae were exposed to RU486, suggesting that RU486 may also suppress mineralocorticoid receptors or that negative feedback of ALDO is mediated through glucocorticoid receptors. Alternatively, CORT increases with RU486 treatment and might be a substrate for ALDO synthesis, which could account for increases in ALDO with RU486 treatment and the correlation between CORT and ALDO. ALDO was negatively correlated with percent water, such that larvae secreting more ALDO retained less water. Although sample sizes were limited and further validation and studies are warranted, our findings expand our understanding of adrenal steroid responses to salinization in amphibians and proposes new hypotheses regarding the co-regulation of ALDO and CORT.

Increased salinity (sodium chloride; NaCl) is a prevalent and persistent
contaminant that negatively affects freshwater ecosystems. Although
most studies focus on effects of salinity from roads salts (primarily
NaCl), high-salinity wastewaters from energy extraction (wastewaters)
could be more harmful because they contain NaCl and other toxic
components. Many amphibians are sensitive to salinity and their eggs
are thought to be the most sensitive life history stage. However, there
are few investigations with salinity that include eggs and larvae
sequentially in long-term exposures. We investigated the relative effects
of wastewaters from a large energy reserve, the Williston Basin (USA),
and NaCl on northern leopard (Rana pipiens) and boreal chorus
(Pseudacris maculata) frogs. We exposed eggs to salinity and tracked
responses through larval stages (for 24 days). Wastewaters and NaCl
reduced hatching and larval survival, growth, development, and activity
while also increasing deformities. Chorus frog eggs and larvae were
more sensitive to salinity than leopard frogs suggesting species-specific
responses. Contrary to previous studies, eggs of both species were less
sensitive to salinity than larvae. Our ecologically relevant exposures
suggest that accumulating effects can reduce survival relative to starting
experiments with unexposed larvae. Alternatively, egg casings of some
species may provide some protection against salinity. Notably, effects of
wastewaters on amphibians were predominantly due to NaCl rather than
other components. Therefore, findings from studies with other sources of
increased salinity (e.g., road salts) could guide management of
wastewater-contaminated ecosystems, and vice versa, to mitigate
effects of salinization.

Life-history tradeoffs are common across taxa, but growth-survival tradeoffs—which usually enhance survival at a cost to growth—are less frequently investigated. Increased salinity (NaCl) is a prevalent anthropogenic disturbance that may cause a growth-survival tradeoff for amphibians. Although physiological mechanisms mediating tradeoffs are seldom investigated, hormones are prime candidates. Corticosterone (CORT) is a steroid hormone that independently influences survival and growth and may provide mechanistic insight into growth-survival tradeoffs. We used 24-d trials to test effects of salinity (0 – 4000 mg/L Cl-) on growth, development, survival, CORT responses, and tradeoffs among traits of larval Northern Leopard Frogs (Rana pipiens). We also tested experimentally suppressed CORT signaling to determine whether CORT signaling mediates effects of salinity and life history tradeoffs . Increased salinity reduced survival, growth, and development. Suppressing CORT signaling in conjunction with salinity reduced survival further, but also attenuated negative effects of salinity on growth and development. CORT of control larvae increased or was stable with growth and development, but decreased with growth and development for those exposed to salinity. Therefore, salinity dysregulated CORT physiology. Across all treatments, larvae that survived had higher CORT than larvae that died. By manipulating CORT signaling, we provide strong evidence that CORT physiology mediates the outcome of a growth-survival tradeoff. To our knowledge, this is the first study to concomitantly measure tradeoffs between growth and survival and experimentally link these changes to CORT physiology. Identifying mechanistic links between stressors and fitness-related outcomes is critical to enhance our understanding of tradeoffs.

Wetlands provide ecological functionality by maintaining and promoting regional biodiversity supporting quality habitat for aquatic organisms. Globally, habitat loss, fragmentation and degradation due to increases in agricultural activities and urban development have reduced or altered geographically isolated wetlands, thus reducing biodiversity. The objective of this study was to assess the relative ecological function and integrity of natural ponds, excavated ponds and stormwater basins for amphibian diversity in the New Jersey Pinelands, USA by comparing hydrologic conditions, water quality, pesticide concentrations (water, sediment and tissue) and plant and anuran assemblages. Twenty-four wetlands were selected based on surrounding land-use and sampled for a variety of abiotic and biotic variables. Abiotic and biotic wetland variables were similar between natural and excavated ponds, with notable differences between the ponds and stormwater basins. Natural and excavated ponds displayed characteristic Pinelands water quality (low pH, high organic carbon, and low pesticide concentrations), exhibited high ecological integrity and supported native anuran species. Stormwater basins and degraded ponds surrounded by altered land-use exhibited degraded water quality (high pH, high pesticide concentrations) and were dominated by non-native and introduced plant and anuran species. Results from this study can broadly inform resource conservation strategies for amphibians and other communities with a diverse range of habitat requirements, particularly in areas where conservation and development are competing priorities. To conserve biodiversity in changing landscapes, wetlands with similar functionality and land-use characteristics need to be identified and managed to preserve water quality for species of conservation concern.

Salinization is harmful to amphibians and waterborne corticosterone could be a useful biomarker. Salinity was only associated with waterborne corticosterone for one of three amphibian species. Ambient corticosterone likely confounded associations and possibly influenced amphibian physiology. We provide suggestions to improve reliability of waterborne corticosterone as a biomarker of salt stress.

Amphibian larvae are commonly used as indicators of aquatic ecosystem health because they are susceptible to contaminants. However, there is limited information on how species characteristics and trophic position influence contaminant loads in larval amphibians. Importantly, there remains a need to understand whether grazers (anurans) and predators (salamanders) provide comparable information on contaminant accumulation or if they are each indicative of unique environmental processes and risks. To better understand the role of trophic position in contaminant accumulation, we analyzed composite tissues for 10 metals from larvae of multiple co-occurring anuran and salamander species from 20 wetlands across the United States. We examined how metal concentrations varied with body size (anurans and salamanders) and developmental stage (anurans) and how the digestive tract (gut) influenced observed metal concentrations. Across all wetlands, metal concentrations were greater in anurans than salamanders for all metals tested except mercury, selenium (Se), and zinc (Zn). Concentrations of individual metals in anurans decreased with increasing weight and developmental stage. In salamanders, which are predatory, metal concentrations were less correlated with weight, indicating diet played a role in contaminant accumulation. Based on batches of similarly sized whole-body larvae compared to larvae with their digestive tracts removed our results indicated that tissue type strongly affected perceived concentrations, especially for anurans (gut represented 50–90% of all metals except Se and Zn). This suggest the reliability of results based on whole body sampling could be biased by metal, larval size, and development. Overall, our data suggest that metal concentrations differs among orders (anuran and salamanders) which suggests that metal accumulation is unique to feeding behavior and potentially trophic position. To truly characterize exposure risk in wetlands, species of different life histories, sizes and developmental stages should be included in biomonitoring efforts.

1. The increasing frequency and severity of drought has the potential to exacerbate existing global amphibian declines. However, interactions between drought and coincident stressors, coupled with high interannual variability in amphibian abundances, can mask the extent and underlying mechanisms of drought-induced declines. The application of dynamic occupancy modeling to longitudinal monitoring data estimates the effect of specific variables on population change, providing key insights into potential management strategies for drought resilience.
2. We synthesized a decade (2009 – 2019) of amphibian survey data from multiple monitoring programs across the California Bay Area and used occupancy modeling to estimate the influence of drought, invasive species, and land use on species’ persistence and colonization probabilities. The geographic and temporal scale of our dataset, consisting of 2574 surveys of seven species in 473 ponds, allowed us to quantify regional trends for an entire community of pond-breeding amphibians.
3. An extreme drought from 2012 – 2015 resulted in losses of breeding sites, with 51% of ponds drying in 2014 compared to <10% in non-drought years. Pond drying reduced persistence rates, and nearly every species exhibited reduced occupancy during the drought, with some species (American bullfrogs and California newts) declining by > 25%. Drought reduced occupancy via additional mechanisms beyond habitat loss; for example, lower spring precipitation (an important cue for breeding) was associated with reduced colonization.
4. During drought, native species’ persistence was higher in permanent relative to temporary ponds, even though these sites were also more likely to contain invasive fish and bullfrogs, which generally reduced native amphibian occupancy. Many of these permanent ponds dried during the worst year of drought, leading to extirpations of invasive species that appeared long-lasting. In contrast, native species rebounded quickly with returning rains and showed evidence of full recovery.
5. Synthesis and applications: Despite experiencing one of most severe droughts in a millennium, native species displayed high resilience. Due to longer recovery times by non-native relative to native species, drought presents a valuable management opportunity to remove invaders from key refugia, and we highlight the value of maintaining hydroperiod diversity to promote the persistence of multiple species.

Altered flow regimes can contribute to dissociation between life history strategies and environmental conditions, leading to reduced persistence reported for many wildlife populations inhabiting regulated rivers. The Oregon spotted frog (Rana pretiosa) is a threatened species occurring in floodplains, ponds, and wetlands in the Pacific Northwest with a core range in Oregon, USA. All life stages of R. pretiosa are reliant on aquatic habitats, and inundation patterns across the phenological timeline can have implications for population success. We conducted capture-mark-recapture (CMR) sampling of adult and subadult R. pretiosa at three sites along the Deschutes River downstream from two dams that regulate flows. We related the seasonal extent of inundated habitat at each site to monthly survival probabilities using a robust design CMR model. We also developed matrix projection models to simulate population dynamics into the future under current river flows. Monthly survival was strongly associated with the extent and variability of inundated habitat, suggesting some within-season fluctuations at higher water levels could be beneficial. Seasonal survival was lowest in the winter for all three sites, owing to limited water availability and the greater number of months within this season relative to other seasons. Population growth for the two river-connected sites was most strongly linked to adult survival, whereas population growth at the river-disconnected site was most strongly tied to survival in juvenile stages. This research identifies population effects of seasonally limited water and highlights conservation potential of enhancing survival of particularly influential life stages.

North America’s protected lands harbor significant biodiversity and provide habitats where species threatened by a variety of stressors in other environments can thrive. Yet disease, climate change, and other threats are not limited by land management boundaries and can interact with conditions within protected landscapes to affect sensitive populations. We examined the population dynamics of a boreal toad (Anaxyrus boreas) metapopulation at a wildlife refuge in northwestern Montana over a 16-year period (2003-2018). We used robust design capture-recapture models to estimate male population size, recruitment, and apparent survival over time and in relation to the amphibian chytrid fungus, Batrachochytrium dendrobatidis. We estimated female population size in years with sufficient captures. Finally, we examined trends in male and female toad body size and condition. We found no evidence of an effect of disease or time on male toad survival but detected a strong negative trend in recruitment of new males to the population. Estimates of male and female abundance decreased dramatically over time. Body size of males and females was inversely related to estimated population size, consistent with reduced recruitment to replace adults, but body condition of adult males was only weakly associated with abundance. Together, these results describe the demography of a near-extinction event, and point to dramatic decreases in the recruitment of new individuals to the breeding population as the cause of this decline. We surmise that processes related to the restoration of historical hydrology within the refuge adversely affected amphibian breeding habitat, and that these changes interacted with disease, life history, and other factors to restrict the recruitment of new individuals to the breeding population over time. Our results point to challenges in understanding and predicting drivers of population change and highlight that current metrics for assessing population status can have limited predictive ability.

Land use alteration such as livestock grazing can affect water quality in habitats of at-risk wildlife species. Data from managed wetlands are needed to understand levels of exposure for aquatic life stages and monitor grazing-related changes afield. We quantified spatial and temporal variation in water quality in wetlands occupied by threatened Oregon spotted frog (Rana pretiosa) at Klamath Marsh National Wildlife Refuge in Oregon, US. We used analyses for censored data to evaluate the importance of habitat type and grazing history in predicting concentrations of nutrients, turbidity, fecal indicator bacteria (FIB; total coliforms, E. coli, and enterococci), and estrogenicity, an indicator of estrogenic activity. Nutrients (orthophosphate and ammonia) and enterococci varied over time and space, while E. coli, total coliforms, turbidity, and estrogenicity were more strongly associated with local livestock grazing metrics. Turbidity was correlated with several grazing-related constituents and may be particularly useful for monitoring water quality in landscapes with livestock use. Concentrations of orthophosphate and estrogenicity were elevated at several sites relative to published health benchmarks, and their potential effects on R. pretiosa warrant further investigation. Our data provided an initial assessment of potential exposure of amphibians to grazing related constituents in western US wetlands.

The U.S. Fish and Wildlife Service listed Anaxyrus canorus, the Yosemite toad, as federally threatened in 2014 based upon reported population declines and vulnerability to global-change factors. A. canorus lives only in California’s central Sierra Nevada at medium to sub-alpine elevations. Lands throughout its range are protected from development, but climate and other global-change factors potentially can limit populations. A. canorus reproduces in ultra-shallow wetlands that typically hydrate seasonally via melting of the winter snowpack. Lesser snowpacks in drier years can render wetland water volumes and hydroperiods insufficient to allow for successful breeding and reproduction. Additionally, breeding and embryogenesis occur very soon after wetlands thaw when overnight temperatures can be below freezing. Diseases, such as chytridiomycosis, which recently decimated regional populations of ranid species, also might cause declines of A. canorus populations. However, reported studies focused on whether climate interacts with any pathogens to affect fitness in A. canorus have been scarce. We investigated effects of these factors on A. canorus near Tioga Pass from 1996 to 2001. We found breeding subpopulations were distributed widely but inconsistently among potentially suitable wetlands and frequently consisted of small numbers of adults. We occasionally observed small but not alarming numbers of dead adults at breeding sites. In contrast, embryo mortality often was notably high, with the majority of embryos dead in some egg masses while mortality among coincidental Pseudacris regilla (Pacific treefrog) embryos in deeper water was lower. After sampling and experimentation, we concluded that freezing killed A. canorus embryos, especially near the tops of egg masses, which enabled Saprolegnia diclina (a water mold [Oomycota]) to infect and then spread through egg masses and kill more embryos, often in conjunction with predatory flatworms (Turbellaria spp.). We also concluded exposure to ultraviolet-B radiation did not play a role. Based upon our assessments of daily minimum temperatures recorded around snow-off during years before and after our field study, the freezing potential we observed at field sites during embryogenesis might have been commonplace beyond the years of our field study. However, interactions among snow quantity, the timing of snow-off, and coincidental air temperatures that determine such freezing potential make projections of future conditions highly uncertain, despite overall warming trends. Our results describe important effects from ongoing threats to the fitness and abundance of A. canorus via reduced reproduction success and demonstrate how climate conditions can exacerbate effects from pathogens to threaten the persistence of amphibian populations.

ABSTRACT: We conducted a national-scale assessment of mercury (Hg) bioaccumulation in aquatic ecosystems using dragonfly
larvae as biosentinels by developing a citizen-science network to facilitate biological sampling. Implementing a carefully designed
sampling methodology for citizen scientists, we developed an effective framework for a landscape-level inquiry that might otherwise
be resource limited. We assessed the variation in dragonfly Hg concentrations across &gt;450 sites spanning 100 United States National
Park Service units and examined intrinsic and extrinsic factors associated with the variation in Hg concentrations. Mercury
concentrations ranged between 10.4 and 1411 ng/g of dry weight across sites and varied among habitat types. Dragonfly total Hg
(THg) concentrations were up to 1.8-fold higher in lotic habitats than in lentic habitats and 37% higher in waterbodies, with
abundant wetlands along their margins than those without wetlands. Mercury concentrations in dragonflies differed among families
but were correlated (R2 &gt; 0.80) with each other, enabling adjustment to a consistent family to facilitate spatial comparisons among
sampling units. Dragonfly THg concentrations were positively correlated with THg in both fish and amphibians from the same
locations, indicating that dragonfly larvae are effective indicators of Hg bioavailability in aquatic food webs. Collectively, this
continental-scale study demonstrates the utility of dragonfly larvae for estimating the potential mercury risk to fish and wildlife in
aquatic ecosystems and provides a framework for engaging citizen science as a component of landscape Hg monitoring programs.