In 1989, it dawned on participants at the First World Congress of Herpetology that observed declines in amphibian populations might actually be global in scope and unprecedented in severity. Three decades of research since then has produced an enormous increase in our knowledge of amphibian ecology and appreciation of the complexity of possible causes for amphibian population declines. In September 2019, 30 yr after the First World Congress ended, a day-long, international symposium on amphibian population declines was held at the Redpath Museum of McGill University in Montreal, Canada. Symposium participants drew upon the knowledge gained over three decades of study to look ahead with fresh ideas to address this vital aspect of the global decline of biodiversity. Despite tremendous progress over the past three decades there is still much about amphibian ecology, population biology, and pathology that remains unknown. Amphibian declines have turned out to be more complex than originally expected and the result of multiple possible causes acting across landscapes, among taxa, or between populations in ways that are not at all uniform. The papers in this special issue of Herpetologica, which stem from the symposium, explore much of our current understanding of amphibian declines and their causes.

Early calls for robust long-term time series of amphibian population data, stemming from discussion following the first World Congress of Herpetology, are now being realized after 25 yr of focused research. Inference from individual studies and locations have contributed to a basic consensus on drivers of amphibian declines. Until recently there were no large-scale syntheses of long-term time series data to test hypotheses about the generality of factors driving population dynamics at broad spatial scales. Through the U.S. Geological Survey's Powell Center for Analysis and Synthesis, we brought together a group of scientists to elucidate mechanisms underlying amphibian declines in North America and Europe. We used time series of field data collected across dozens of study areas to make inferences with these combined data using hierarchical and spatial models. We bring together results from four syntheses of these data to summarize our state of knowledge of amphibian declines, identify commonalities that suggest further avenues of study, and suggest a way forward in addressing amphibian declines—by looking beyond specific drivers to how to achieve stability in remaining populations. The common thread of the syntheses is that declines are real but not ubiquitous, and that multiple factors drive declines but the relative importance of each factor varies among species, populations, and regions. We also found that climate is an important driver of amphibian population dynamics. However, the direction and magnitude of sensitivity to change vary among species in ways unlikely to explain overall rates of decline. Thirty years after the initial identification of a major catastrophe for global biodiversity, the scientific community has empirically demonstrated the reality of the problem, identified putative causes, provided evidence of their impacts, invested in broader-scale actions, and attempted meta-analyses to search out global drivers. We suggest an approach that focuses on key demographic rates that may improve amphibian population trends at multiple sites across the landscape.

Amphibian populations are threatened globally, and one of the hypotheses for these declines is climate change. Species distribution models are frequently used to predict changes in suitable habitat as a result of changing climates; however, these projections can be heavily influenced by choice of modeling approach. To evaluate global predictions for amphibians, we conducted a literature review of studies that utilize correlative species distribution models to project changes in climate suitability under various climate change scenarios. We paid particular attention to the use of model selection in choosing among candidate species distribution models (SDMs) so as to control for overparameterization of SDMs. In addition, we conducted a case study with three species of Slender Salamanders (Batrachoseps) to further investigate the impact of differences in modeling decisions on projected amphibian climate suitability. We found 83 studies (including the present case study) in which projections of future climate suitability were made for amphibian species. Of those studies, 36 included estimates of percent change in climate suitability and thus were included in our meta-analysis. These studies included projections for over 1000 species or species complexes, with the majority being Anurans (86%), and encompassed five continents with the most representation in South America, Europe, and North America. Across these studies, average projected change in climate suitability ranged from －70% to 167%, and these projected changes varied with dispersal assumptions representative concentration pathway (RCP) used, the projection year, and taxonomic order. Only three of the 36 studies reported the use of Akaike information criterion (AIC)-based model selection to choose a best-fit SDM. However, our case study demonstrated that predicted change in climate suitability varied whether the best-fit or default SDM was used for projections. Further, this result varied among species (ΔAIC = 0), suggesting that the impact of overparameterization differs across species. Our results illustrate that there is a pressing need to project climate suitability across more species and more geographic regions. In addition, we may need to revisit projections for previously investigated species to evaluate additional climate scenarios and whether overparameterization may have influenced projections. Our ability to accurately model future changes in climate suitability will be essential for successful conservation and management plans for amphibians.

Human activity is accelerating rates of extinction around the world, and there is therefore an urgent need to understand the potential consequences of species loss on functional diversity and ecosystem functioning. It is frequently assumed that the extinction of threatened species, which are usually rare species, may have limited effects on ecosystem functioning, especially within highly diverse regions, given the low abundance of individuals of each species and the potentially high functional redundancy among them. However, these rare species may contribute unique and irreplaceable functional roles, and therefore their extinction could have disproportionate impacts on ecosystems. We assess the functional importance of highly threatened anurans (frogs and toads) and then explore how the loss of these threatened species would impact spatial patterns of β-diversity in Ecuador, a hotspot of anuran biodiversity and endemism. We found that highly threatened species are, on average, more functionally distinct at an assemblage level than are less threatened anurans. We then show that the potential extinction of these highly endangered species would drive the taxonomic, phylogenetic, and, especially, functional homogenization of anuran assemblages across the Ecuadorian Andes. We suggest this could lead to profound alterations in the stability of local ecosystem functioning. Finally, we highlight the limited scope of the existing network of protected areas in Ecuador to effectively cover anuran assemblages predicted to become increasingly functionally homogenous. Our study underscores the potential losses of functional diversity that accompany species extinctions and the importance of considering the many facets of biodiversity in conservation decision-making.

Understanding the demographic consequences of interactions among pathogens, hosts, and weather conditions is critical in determining how amphibian populations respond to disease and in identifying site-specific conservation actions that can be developed to bolster persistence of amphibian populations. We investigated population dynamics in Boreal Toads (Anaxyrus boreas) relative to abiotic (fall temperatures and snowpack) and biotic (the abundance of another anuran host and disease) characteristics of the local environment in Wyoming, USA. We used capture–recapture data and a multistate model where state was treated as a hidden Markov process to incorporate disease state uncertainty and assess our a priori hypotheses. Our results indicated that snowpack during the coldest week of winter is more influential to toad survival, disease transition probabilities, and the population-level prevalence of the amphibian chytrid fungus (Batrachochytrium dendrobatidis) in the spring, than temperatures in the fall or the presence of another host. As hypothesized, apparent survival at low (i.e., <25 cm) snowpack (0.22; confidence interval [CI] = 0.15–0.31) was lower than apparent survival at high snowpack (90.65; CI = 0.50–0.78). Our findings highlight the potential for local environmental factors, like snowpack, to influence disease and host persistence, and demonstrate the ecological complexity of disease effects on population demography in natural environments. This work further emphasizes the need for improved understanding of how climate change may influence the relationships among pathogens, hosts, and their environment for wild animal populations challenged by disease.

The amphibian response to climate change has been generally described as phenological shifts toward earlier breeding periods, with animals exhibiting smaller body sizes and male frogs producing breeding calls higher in pitch and shorter in duration. However, with >8000 species of amphibians now described, and the effects of climate change intensifying, the amphibian response to variable climates is likely to be broader and more nuanced than scientists have so far observed. For example, our previous work described a dramatic acceleration in the breeding season from spring to the previous fall, associated with warming, in Southern Leopard Frogs (Lithobates sphenocephalus); a correlation between precipitation levels and body mass indices in Crawfish Frogs (Lithobates areolatus); and the expression of a variety of daily activity patterns dependent on seasonal variations in temperature and humidity in Crawfish Frogs. Here, we add to this literature by documenting a shift in wetland breeding habitats from seasonal/semipermanent wetlands to permanent wetlands in response to decadal-long hydrologic cycles by Northern Leopard Frogs (Lithobates pipiens) and Eastern Tiger Salamanders (Ambystoma tigrinum). Three implications follow from this result. First, when challenged by climate variability, conserving amphibian diversity requires preserving wetland diversity. Second, amphibian occupancy of any particular wetland basin shifts over time, complicating conclusions drawn from short-term survey data. Third, the wetland shifts we report in response to natural climate variability might provide amphibians the flexibility to successfully respond to future climate challenges. Our insights herein derive from field studies, and we worry that the well-documented trend toward de-emphasizing fieldwork will limit scientists' ability to accurately assess threats to species from climate challenges. As the legendary writer Jim Harrison retorted when asked why he had never accepted any of the comfortable academic jobs he had been offered, “Somebody's got to stay outside.”

Pathogen-induced population declines and extinction events have been recognized as main threats to amphibian species around the globe. However, the ecological drivers underlying epidemiological patterns are still poorly understood. In an attempt to assess the current knowledge on the ecological drivers of amphibian diseases, we identified 832 peer-reviewed publications on the ecology of amphibian pathogens and diseases published between 2009 and 2019. The vast majority of publications investigated either chytrid or ranavirus infections (79% of the articles), whereas other pathogens such as bacteria and helminths received considerably less attention. Just over half of the studies we reviewed included field research and 40% were experimental in nature, yet only 8% combined field and experimental approaches. More than half of the literature (56%) investigated postmetamorphic stages, whereas premetamorphic stages were considered in 23% of the reviewed studies, and only 13% included both life stages. Susceptibility and mortality have been assessed in almost every study (91%) whereas 37% of them tested for cellular, physiological, or immunological responses. However, other host characteristics such as growth/development, behavior, and specific mucosome/microbiome were considered in only one of four studies. Most research included at least one biotic factor (e.g., host and pathogen identity, species diversity, genetic adaptations), but only one-third considered environmental factors (e.g., temperature, landscape features, inorganic chemicals). Furthermore, there is no general consensus about the factors driving epidemiological patterns of pathogens in amphibian communities, and it is clear that the complexity and specificity of interactions between ecological factors and host–pathogen dynamics make conservation implications difficult and management decisions challenging. To this end, our review identifies some research gaps and proposes future directions to better understand one of the major threats to this class of vertebrates.

Here we review the knowledge about skin microbiomes in amphibians accumulated over the last two decades and the evidence regarding the protective role of skin bacteria. Amphibians all over the world are declining because of several factors, including chytridiomycosis disease caused by the fungal pathogens Batrachochytrium dendrobatidis and B. salamandrivorans. In this context, the antifungal capacities of many bacteria living symbiotically on amphibian skin, which have been described both in vitro and in vivo, are important in disease prevention. We discuss the major factors influencing amphibian skin bacterial communities, the fungal component of the amphibian skin microbiome, and the potential use of antifungal bacteria as probiotics. The structure of amphibian skin microbial communities is influenced by host-specific microhabitat, biogeographic, and climatic factors, but the functional aspects of these microbiomes and how these nested factors modulate skin microbial functions remains largely unexplored. However, the field has grown considerably, and recent technologies have prompted the exploration of exciting new questions aimed at providing more detailed knowledge about the ecology of amphibian–microbial symbioses and the precise role of the skin microbiome in protecting host amphibians against emerging diseases.

In the approximately 30 years since the recognition of the crisis of global amphibian declines, much has been learned about the likely causes. Among the leading causes are several amphibian diseases including the disease termed chytridiomycosis caused by the chytrid fungi Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal). Here, I briefly review the fundamentals of amphibian immunity, amphibian immune defenses against the chytrid fungi, and the host–pathogen interactions that often favor the pathogen to the detriment of the host. Because amphibians are ectotherms, climate and temperature have a major impact on the amphibian immunity. Thus, I discuss current information about the role that temperature and unpredictable weather events may play in disease and immune responses to the chytrids. Because much research on amphibian declines is directed toward finding management solutions to protect threatened amphibians, I conclude by drawing attention to some of the most promising and novel mitigation strategies that are being proposed.

The emergence of Batrachochytrium dendrobatidis (Bd) and B. salamandrivorans (Bsal), two fungal pathogens responsible for amphibian declines and extinctions worldwide, led to 20 years of intensive research focused on revealing mechanisms underlying infection, disease progression, and host outcome. Genetic and genomic studies in this system have contributed to our understanding of the origin and genetic diversity within these pathogens, their genetic architecture, and mechanisms used for host exploitation and host immune response. Functional genomic studies, specifically differential gene expression studies, have been used to characterize both pathogen and host mechanisms of exploitation and defense as well as host–pathogen interactions and the influence of biotic and abiotic environmental factors on disease outcomes. Here, we summarize the results of functional genomic studies in the amphibian–chytrid host–pathogen system, with a focus on identifying common patterns, and also examine the factors that contribute to the high variability in disease outcomes observed in nature. We found substantial variation in differential gene expression among lineages of Bd and between Bd and Bsal, pointing to some potential virulence factors for these pathogens. On the host side, we found variation in immune response among species with different levels of resistance to chytrid infections. Finally, we found significant effects of temperature and coinfection on host responses, underscoring the important role that abiotic and biotic factors play in modulating immune response.

The fungal pathogen Batrachochytrium dendrobatidis (Bd) is implicated in global declines of amphibian populations and has been documented in African specimens originally collected as far back as the 1930s. Numerous recent surveys focusing on regional pathogen prevalence have greatly increased the number of known occurrences of Bd in African species, but few studies have focused on continental distribution patterns. We analyzed all known positive occurrences of Bd in African amphibians to date, including newly reported data from Cameroon, the Democratic Republic of the Congo, Gabon, Namibia, and the Republic of the Congo. Records from both Namibia and the Republic of the Congo reported herein represent first positive occurrences for these countries. With this most comprehensive sampling of the African continent to date we identified patterns of Bd-positive occurrences associated with (1) location (i.e., biogeographic region, country), (2) taxonomy, (3) life history, and (4) threat of extinction. We used fine-grained (30 arc seconds) environmental niche models (ENMs) to predict the continental distribution of Bd and identify hotspots for the pathogen, including areas not previously modeled to have high suitability for the fungus, and areas of high amphibian biodiversity from which Bd has not yet been documented. Our ENMs predicted that the environmentally suitable range of Bd encompasses vast areas of high amphibian biodiversity, including the Congo Basin and the Albertine Rift. Although our ENMs indicated that West Africa is environmentally suitable for Bd, the fungus has not been reported west of the Dahomey Gap. Likewise, the ENMs also identified regions across the Congo Basin and coastal Angola that are environmentally suitable for the pathogen but from which Bd has not yet been reported, underscoring a need for Bd surveys in these regions. Although amphibian declines in Africa have not been directly attributed to chytridiomycosis, Bd has been detected in over one fifth of the most-threatened African amphibians. Given the presence of the hypervirulent Bd global panzootic lineage (BdGPL) in Africa, we believe that the threat of Bd as a novel pathogen may be underestimated and that focused research is urgently needed to identify which species are susceptible to Bd-driven declines.

The recognition that invasive alien species (IAS) are among the greatest threats to biodiversity has stimulated a growing interest in their impacts on native amphibians. Here we describe the multifaceted consequences of biological invasions on native amphibians and identify potential mechanisms and strategies that could better enable the long-term persistence of native species. IAS can influence amphibian fitness, population size, and community structure via multiple pathways and can exert major, direct impacts through predation, competition, and hybridization. The consequences of indirect impacts, too, such as habitat alteration and the spread of emerging diseases, can be particularly severe in native populations. Native amphibians may respond to IAS by modulating aspects of their behavior, morphology, or life history. Nevertheless, it is still unclear the extent to which phenotypic plasticity and rapid evolution may help native species withstand the impacts of IAS in invaded communities. Practical management strategies focused on prevention, monitoring, and early control are the most effective approaches to allay the impacts of IAS and should be prioritized in proactive conservation plans. Eradications of IAS and mitigation approaches, should IAS become established, are feasible and can greatly improve the status of native populations.

Although the search for the drivers of amphibian declines continues, there is a need to implement conservation actions. Conservation science usually does not deliver clear answers about which conservation actions are most effective and which ones should be implemented. Furthermore, results often cannot be used directly by conservationists. Given that resources are limited, there is a need to know which conservation actions and management interventions are most likely to succeed. The goal of evidence-based conservation is to assess the effectiveness of conservation actions qualitatively and quantitatively, and comparative effectiveness studies are a powerful tool to evaluate different conservation actions. We use a case study on toad tunnels to discuss the benefits and limitations of comparative effectiveness studies. Although we show that wider tunnels are used by a higher proportion of individuals, the strength of evidence for effects of other characteristics of amphibian tunnels on tunnel use was weak. Despite some equivocal results, our case study illustrates that the approach can readily be used to study the effectiveness of conservation actions and to derive recommendations for conservationists and managers that can be used directly to improve future conservation interventions.

Science-based management strategies are needed to halt or reverse the global decline of amphibians. In many cases, sound management requires reliable models built using monitoring data. Historically, monitoring and statistical modeling efforts have focused on estimating occupancy using detection–nondetection data. Spatial occupancy models are useful for studying colonization–extinction dynamics, but richer insights can be gained from estimating abundance and density-dependent demographic rates. We developed an integrated abundance-based metapopulation model of the processes contributing to spatiotemporal variation in patch population density. We fit our model to a combination of detection–nondetection and count data from a 14-yr study of a reintroduced metapopulation of federally threatened Chiricahua Leopard Frogs (Lithobates chiricahuensis). Pond-specific population growth rate was influenced by pond hydroperiod and frog density, such that permanent and semipermanent ponds with low densities of adult frogs experienced the highest annual population growth rates. Immigration rate declined as the distance among ponds increased. After reintroduction in 2003, metapopulation-level abundance increased and appeared to stabilize around 1300 adult frogs (95% CI = 1192–1471) by year 2015. Further, changes in metapopulation abundance were driven mostly by changes in abundance at a few ponds. These high-density populations, which would not have been identifiable with traditional occupancy-based metapopulation models, are likely especially important for species recovery in the area. Abundance-based metapopulation models can be widely applied to inform conservation efforts, by providing higher quality information needed to prioritize habitat patches for management and can be used to make more accurate predictions of metapopulation extinction risk.

The calamity of amphibian population declines has preoccupied the thoughts of many amphibian biologists for the past 30 yr. Because amphibians provide multiple essential ecosystem services at all life stages, the threats associated with their decline are likely also to be multiple and interconnected. No single cause has yet explained all amphibian declines; one potential threat after another has been investigated in isolation and found to explain only a piece of a larger puzzle. A more holistic, synergistic approach is needed to understand how, and when, normally tolerated environmental influences might exceed the capacities of amphibian populations to cope either phenotypically or evolutionarily. Ultimately, the factors generally implicated in amphibian declines are all secondary to habitat loss and degradation caused by human activity. Because the roots of this problem are cultural and economic, the solutions do not lie solely within the realm of science but instead require political and societal action.