Carlos Arturo Navas

As human activity accelerates the spread of pathogens and other invasive species, reconstructing the cryptic early dynamics of undetected invasions has become a central challenge in disease ecology and invasion biology. Here, we leverage museum specimens to reconstruct the invasion of Batrachochytrium dendrobatidis (Bd), a fungal panzootic driving global amphibian biodiversity loss, across historically enigmatic declines in the Brazilian Atlantic Forest. Two hypotheses have been proposed to explain Bd's historical dynamics in this region: long‐term enzootic persistence versus recent epizootic invasion coinciding with declines. Using molecular screening, histopathology, and genotyping of museum and modern specimens, we find no evidence of Bd infection prior to the decline, followed by widespread infection immediately thereafter. Only the global panzootic lineage (Bd‐GPL) was detected during the first year of post‐decline sampling, implicating its invasion in driving regional losses. Infection prevalence varied among species following the decline, likely reflecting species‐specific interactions between baseline susceptibility and ecological exposure risk. Contemporary host populations exhibit more uniform prevalence regardless of decline history, consistent with the evolution of pathogen tolerance under enzootic conditions. Together, these results provide the first site‐specific evidence linking Bd‐GPL invasion to enigmatic amphibian declines in Brazil and demonstrate the power of museum collections for resolving detection lag dynamics in emerging infectious diseases.

Many wildlife pathogens can persist in the environment independently of their hosts, with the habitat itself serving as a reservoir and potential source of transmission. However, reliably detecting and quantifying free‐living pathogen stages across heterogeneous landscapes using environmental DNA methods remains challenging. Understanding how landcover and habitat structure drive pathogen distribution at the environmental and host levels is critical for advancing disease surveillance and ecology. Amphibian chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis ( Bd ), provides a useful model system to address these broader challenges, yet most studies have focused on host–pathogen interactions, with less emphasis on Bd within environmental reservoirs. Using Bd as a model system, we investigated whether pathogen distribution in natural aquatic environments is associated with host infection patterns across gradients of habitat loss. We sampled four tropical amphibian species across eight rainforest landscapes and quantified Bd in paired water and host samples using a high‐capacity water filtration method coupled with digital and real‐time quantitative polymerase chain reaction detection assays. Our results revealed a strong positive correlation between Bd DNA concentrations in water and infection loads on amphibian skin, indicating that environmental Bd DNA is associated with host infection patterns. Forest cover and habitat split (i.e. spatial separation between forests and aquatic breeding sites) were the primary predictors of Bd occurrence, concentration and infection load across environmental and host‐associated forms, and both Bd ‐GPL and Bd ‐Asia‐2/Brazil lineages were detected across our study landscapes. Our study introduces and validates a robust protocol for detecting environmental Bd , providing a scalable approach to quantify pathogen–environment associations across multiple landscapes and environmental gradients. Synthesis and applications : Linking environmental pathogen detection with host infection patterns can improve surveillance of diseases involving environmental transmission, particularly where direct host sampling is logistically challenging. This approach enables the identification of areas of elevated transmission risk and supports targeted monitoring and management under habitat loss and degradation. It also provides a practical tool to guide conservation actions in landscapes undergoing rapid land‐use change.

Understanding pathogen dynamics during environmental life stages is vital for comprehending wildlife diseases, especially for those with a free-living phase. In amphibians affected by the chytrid fungus Batrachochytrium dendrobatidis (Bd), most studies have focused on host-pathogen interactions, with less emphasis on Bd's environmental stage. We tested whether the distribution of Bd in natural aquatic environments can predict host infection patterns. We sampled four tropical amphibian species across eight rainforest landscapes with varying habitat loss, testing whether environmental and host Bd detection varied along gradients of habitat change. Using a high-capacity water filtration method coupled with digital and real-time PCR detection assays, we quantified Bd in water and amphibian samples. Our results revealed a strong positive correlation between Bd DNA concentrations in water samples and infection loads on amphibian skin samples. Forest cover and habitat split were the primary predictors of Bd distribution in both free-living and host-associated forms. We identified Bd-GPL and Bd-Asia-2/Brazil lineages across our study landscapes. Our study introduces and validates a robust protocol for detecting Bd in environmental samples, with the potential to enhance monitoring and inform management strategies. Moreover, our work contributes novel, well-replicated spatial data on Bd associations between hosts and their environments.

Human impacts on ecosystems have intensified variation in water variability for terrestrial life, thus challenging the maintenance of water balance, or hydroregulation. The accelerated development and accessibility of technologies and computational models over the past decade have enabled researchers to predict changes in animal hydroregulation and environmental water with greater spatial and temporal precision. Focusing on reptiles and amphibians, we discuss current methods, limitations and advances for quantifying ecologically relevant metrics of environmental water stressors and organismal responses to both acute and long-term water stress that are applicable for conservation and management. We also highlight approaches that integrate environmental water data with an organism’s water balance and physiological, behavioural and life history traits to predict the limits of species’ responses and assess their vulnerability to climate change. Finally, we outline promising future directions and opportunities in hydroregulation studies with a conservation focus, including broader inferences about acclimation responses, linking gene expression to functional changes, and exploring inter- and transgenerational plasticity and adaptive evolution. Advances in these fields will facilitate more accurate assessments of species’ capacities and the limits of hydroregulation in response to a more variable and unpredictable future climate.

Synopsis: Janzen's hypothesis (JH) posits that low thermal variation selects for narrow physiological tolerances, and thus small species distributional ranges and high species turnover along tropical elevational gradients. Although this hypothesis has been intensely revisited, it does not explain how many tropical species may exhibit broad distributions, encompassing altitudinal gradients. Moreover, the physiological responses of tropical species remain largely unknown, limiting our understanding on how they respond to climate variation. To fill these knowledge gaps, we tested a major component of JH, the climate variability hypothesis (CVH), which predicts broader thermal tolerance breadth (Tbr = CTmax – CTmin) with broader temperature variation. Specifically, we sampled populations of five amphibian species distributed in two mountain ranges in Brazil's Atlantic Forest to test how CTmin and CTmax vary along elevational gradients. Since both thermal and water balance traits are pivotal to the evolutionary history of amphibians, we also measured rates of dehydration and rehydration and their relations with thermal tolerances. We found that broader temperature variation with increasing altitude did not always lead to broader Tbr, since changes in CTmin and CTmax were species-specific. In addition, we found that water balance did not show consistent variation with altitude, also with low correlations between hydric and thermal traits. While we also found that highland populations are at lower risk of thermal stress than lowland counterparts, both are living far from their upper thermal limits. As a consequence of intraspecific variation in physiological traits and spatial variation in climate along altitude, responses to climate variation in tropical amphibian species were context-dependent and heterogeneous. Together with recent studies showing thermal tolerances of some tropical amphibians comparable to temperate taxa, our findings highlight that several responses to climate variation in tropical species may not conform to predictions made by either the CVH or other important hypotheses concerning physiological variation. This reinforces the need to overcome geographical bias in physiological data to improve predictions of climate change impacts on biodiversity. (Portuguese abstract) Resumo A Hipótese de Janzen (JH) postula que a baixa variação térmica seleciona tolerâncias fisiológicas estreitas e, portanto, amplitudes restritas de distribuição das espécies e alta substituição de espécies ao longo de gradientes altitudinais tropicais. Embora intensamente revisitada, essa hipótese não explica como espécies tropicais podem exibir amplas distribuições geográficas, abrangendo gradientes altitudinais. Além disso, as respostas fisiológicas das espécies tropicais permanecem amplamente desconhecidas, limitando nossa compreensão sobre como elas respondem à variação climática. Para preencher essas lacunas de conhecimento, testamos um componente importante da JH, a Hipótese de Variabilidade Climática (CVH), que prevê uma maior amplitude de tolerância térmica (Tbr = CTmax – CTmin) quando a variação da temperatura ambiental é mais ampla. Especificamente, amostramos populações de cinco espécies de anfíbios distribuídas em duas cadeias montanhosas na Mata Atlântica do Brasil para testar como CTmin e CTmax variam ao longo de gradientes de altitude. Dado que parâmetros térmicos e do balanço hídrico são fundamentais para a história evolutiva dos anfíbios, também medimos as taxas de desidratação e reidratação e suas relações com as tolerâncias térmicas. Encontramos que uma variação de temperatura ambiental mais ampla com o aumento da altitude nem sempre conduz a uma Tbr mais ampla, uma vez que as mudanças em CTmin e CTmax foram espécie-específicas. Além disso, encontramos que o balanço hídrico não apresentou variação consistente com a mudança de altitude, e que as correlações entre parâmetros hídricos e térmicos foram baixas. Embora populações das maiores altitudes apresentaram menor risco de estresse térmico do que populações da mesma espécie em altitudes menores, ambas estão vivendo longe de seus limites térmicos superiores. Em consequência da variação intraespecífica em parâmetros fisiológicos e variação espacial no clima ao longo da altitude, as respostas à variação climática em espécies de anfíbios tropicais foram contexto-dependentes e heterogêneas. Juntamente com estudos recentes indicando tolerâncias térmicas de alguns anfíbios tropicais comparáveis a de táxons temperados, nossas descobertas destacam que várias respostas à variação climática em espécies tropicais podem não estar de acordo com as previsões feitas pela CVH ou outras hipóteses importantes sobre a variação fisiológica. Isso reforça a necessidade de superar o viés geográfico em dados fisiológicos para aperfeiçoar previsões dos impactos das mudanças climáticas sobre a biodiversidade. (Spanish abstract) Resumen La hipótesis de Janzen (JH) postula que la baja variación térmica selecciona tolerancias fisiológicas estrechas y, por lo tanto, rangos de distribución de especies restringidos con alta rotación de especies a lo largo de gradientes de elevación tropicales. Aunque esta hipótesis ha sido intensamente discutida, no explica cómo várias especies tropicales pueden exhibir...

Microbial diversity positively influences community resilience of the host microbiome. However, extinction risk factors such as habitat specialization, narrow environmental tolerances, and exposure to anthropogenic disturbance may homogenize host-associated microbial communities critical for stress responses including disease defense. In a dataset containing 43 threatened and 90 non-threatened amphibian species across two biodiversity hotspots (Brazil’s Atlantic Forest and Madagascar), we found that threatened host species carried lower skin bacterial diversity, after accounting for key environmental and host factors. The consistency of our findings across continents suggests the broad scale at which low bacteriome diversity may compromise pathogen defenses in species already burdened with the threat of extinction.

We quantified functional traits (escape strategy, sprint speed and predatory performance) and population density across 10 lizard species representing morphotype stages in the acquisition of burrowing snake-like morphotypes (BSLM), from Brazil. We used phylogenetic mixed models to test if: (a) morphotype and substrate affects flight strategy and speed, (b) BSLM species more effectively access different potential prey types than lacertoid species, when in syntopy, and (c) morphotype is correlated with population abundance and habitat use in a way expected from the output of the previous experiments. BSLM rigidly relied on burrowing as flight strategy, while syntopic lacertoid species changed their strategy according to the substrate. In addition, sand had opposing effects on sprint speed depending on morphotype, making lacertoids run more slowly and BSLM faster. Even though BSLM were overall slower than lacertoids, they were equally effective hunters of challengingly fast prey, and better hunters of underground prey. In their shared habitats, prey is most abundant in the superficial layer of leaf litter, although a large fraction is found beneath this layer, under bushes. Experimental results support the observed higher importance of sand for BSLM’s density and the higher importance of vegetation for lacertoids’ density. Finally, although BSLM species reached the highest population densities among the studied species, a systematic effect of morphological evolution on the abundance of limbless lizards remains elusive.

Significance: Energetic constraints of flight and thermoregulation have long been thought to explain why most bat species are small and live predominantly in warm latitudes. We use physical models to investigate how body size, wing shape, and climate modulate these energetic constraints. Our model predicts that thermoregulatory and flight costs, respectively, impose upper and lower bounds on the wing surface-to-mass ratio, giving rise to an optimum shape, and that variations around this optimum are more constrained in cold regions. A comparative analysis across bat species supports the model’s predictions, suggesting that body shape evolves toward an optimum with stronger selective pressures in cold regions. The model and data together suggest that thermoregulatory and locomotory constraints modulate the evolution of bats’ morphology.