The amphibian fungal disease chytridiomycosis, which affects species across all continents, recently emerged as one of the greatest threats to biodiversity. Yet, many aspects of the basic biology and epidemiology of the pathogen, Batrachochytrium dendrobatidis (Bd), are still unknown, such as when and from where did Bd emerge and what is its true ecological niche? Here, we review the ecology and evolution of Bd in the Americas and highlight controversies that make this disease so enigmatic. We explore factors associated with variance in severity of epizootics focusing on the disease triangle of host susceptibility, pathogen virulence, and environment. Reevaluating the causes of the panzootic is timely given the wealth of data on Bd prevalence across hosts and communities and the recent discoveries suggesting co‐evolutionary potential of hosts and Bd. We generate a new species distribution model for Bd in the Americas based on over 30,000 records and suggest a novel future research agenda. Instead of focusing on pathogen “hot spots,” we need to identify pathogen “cold spots” so that we can better understand what limits the pathogen's distribution. Finally, we introduce the concept of “the Ghost of Epizootics Past” to discuss expected patterns in postepizootic host communities.

Chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis (Bd), is the emerging infectious disease implicated in recent population declines and extinctions of amphibian species worldwide. Bd strains from regions of disease-associated amphibian decline to date have all belonged to a single, hypervirulent clonal genotype (Bd-GPL). However, earlier studies in the Atlantic Forest of southeastern Brazil detected a novel, putatively enzootic lineage (Bd-Brazil), and indicated hybridization between Bd-GPL and Bd-Brazil. Here, we characterize the spatial distribution and population history of these sympatric lineages in the Brazilian Atlantic Forest. To investigate the genetic structure of Bd in this region, we collected and genotyped Bd strains along a 2400-km transect of the Atlantic Forest. Bd-Brazil genotypes were restricted to a narrow geographic range in the southern Atlantic Forest, while Bd-GPL strains were widespread and largely geographically unstructured. Bd population genetics in this region support the hypothesis that the recently discovered Brazilian lineage is enzootic in the Atlantic Forest of Brazil and that Bd-GPL is a more recently expanded invasive. We collected additional hybrid isolates that demonstrate the recurrence of hybridization between panzootic and enzootic lineages, thereby confirming the existence of a hybrid zone in the Serra da Graciosa mountain range of Paraná State. Our field observations suggest that Bd-GPL may be more infective towards native Brazilian amphibians, and potentially more effective at dispersing across a fragmented landscape. We also provide further evidence of pathogen translocations mediated by the Brazilian ranaculture industry with implications for regulations and policies on global amphibian trade.

Exotic predators have driven the extinction of many island species. We examined impacts of feral cats on the abundance and anti-predator behaviours of Aegean wall lizards in the Cyclades (Greece), where cats were introduced thousands of years ago. We compared populations with high and low cat density on Naxos Island and populations on surrounding islets with no cats. Cats reduced wall lizard populations by half. Lizards facing greater risk from cats stayed closer to refuges, were more likely to shed their tails in a standardized assay, and fled at greater distances when approached by either a person in the field or a mounted cat decoy in the laboratory. All populations showed phenotypic plasticity in flight initiation distance, suggesting that this feature is ancient and could have helped wall lizards survive the initial introduction of cats to the region. Lizards from islets sought shelter less frequently and often initially approached the cat decoy. These differences reflect changes since islet isolation and could render islet lizards strongly susceptible to cat predation.

Habitat fragmentation and wildlife disease are two widespread drivers of biodiversity loss, yet few empirical studies have explored their interactions. In this study, we utilized a naturally fragmented island system to examine the impacts of fragmentation on genetic diversity and amphibian infection dynamics. We determined the impacts of fragmentation on genetic diversity at the immunity locus MHC IIB, a hypothesized predictor of disease susceptibility. Contrary to the expectation that MHC diversity would remain high due to balancing selection, island populations lost genetic diversity at this locus while simultaneously experiencing positive selection at MHC IIB. We then used Next-Generation Sequencing to identify a variety of potential eukaryotic parasites from amphibian skin swabs. Island populations exhibited higher potential parasite richness (proportion of eukaryotic microbe operational taxonomic units or OTUs from parasitic taxa) relative to mainland populations. MHC homozygotes hosted a lower diversity of potential parasites, and population-level MHC diversity was negatively associated with parasite richness. Our results show that genetic erosion can occur at the MHC IIB locus following fragmentation, which may contribute to increased susceptibility to parasites.

Global change, including habitat isolation and climate change, has both short‐ and long‐term impacts on wildlife populations. For example, genetic drift and inbreeding result in genetic impoverishment in small, isolated populations, while species undergo range shifts or adaptive phenotypic change in response to shifts in environmental temperatures. In this study, we utilize a model system in which Holocene landscape changes have occurred to examine long‐term effects of population isolation. To examine how isolation may constrain responses to climate change, we characterized ecophysiology across land‐bridge island populations of Erhard's wall lizard Podarcis erhardii. We hypothesized that 1) small, isolated populations that are likely genetically depauperate would exhibit lower phenotypic variability; and 2) populations would be adapted to local microhabitat conditions. We compared a population at a low elevation site on the large island of Naxos with two small populations on nearby islets to determine the effects of population fragmentation. We further compared the low elevation Naxos population with two high elevation sites characterized by disparate microclimates to examine the effects of microclimate. To assess the thermal biology and ecophysiological limits of the study species we measured operative body temperatures (Te), field body temperatures (Tb), preferred temperatures (Tp), thermal tolerances (CTmax and CTmin), and evaporative water loss (EWL). Our results indicate that small, isolated populations did not exhibit thermal biology or evaporative water loss, while EWL and thermoregulatory effort varied according to microhabitat characteristics. This study integrates fine‐scale measurements with environmental data to provide a holistic view of the relationships between ecophysiology, fragmentation, and microclimate. Our methods can be applied to other ectotherms to gain a better understanding of potential impacts of global change on natural populations.

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In Brazil’s Atlantic Forest (AF) biodiversity conservation is of key importance since the fungal pathogen Batrachochytrium dendrobatidis (Bd) has led to the rapid loss of amphibian populations here and worldwide. The impact of Bd on amphibians is determined by the host's immune system, of which the skin microbiome is a critical component. The richness and diversity of such cutaneous bacterial communities are known to be shaped by abiotic factors which thus may indirectly modulate host susceptibility to Bd. This study aimed to contribute to understanding the environment-host–pathogen interaction determining skin bacterial communities in 819 treefrogs (Anura: Hylidae and Phyllomedusidae) from 71 species sampled across the AF. We investigated whether abiotic factors influence the bacterial community richness and structure on the amphibian skin. We further tested for an association between skin bacterial community structure and Bd co-occurrence. Our data revealed that temperature, precipitation, and elevation consistently correlate with richness and diversity of the skin microbiome and also predict Bd infection status. Surprisingly, our data suggest a weak but significant positive correlation of Bd infection intensity and bacterial richness. We highlight the prospect of future experimental studies on the impact of changing environmental conditions associated with global change on environment-host–pathogen interactions in the AF.

The host-associated microbiome has recently captured the attention of wildlife disease researchers seeking to understand and predict disease-associated wildlife declines. In particular, skin microbiome research is burgeoning in the field of amphibian disease. A majority of amphibian disease studies focus on chytridiomycosis caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd). Bd has been linked to severe amphibian declines around the world since at least