Many species use temperature as a cue to time certain life events. Such phenological events can be particularly sensitive to climate change. In amphibians, seasonal migration events between ponds and winter habitats are crucial to ensure reproduction. Therefore, shifts in phenological events – called phenological shifts – can significantly affect the persistence of amphibian populations, especially when induced by climate change. We used citizen science data collected during the spring migration of the Common Toad (Bufo bufo) from 1981 until 2020 to: (i) estimate the phenological shift, (ii) determine consistency of these changes within the studied region, and (iii) relate its temporal variation with changes in temperature. The results revealed an advancement in the peak migration of 3.1 days per decade. This shift was consistent across all populations in Flanders, indicating a regional effect that is most likely due to climate change. This reasoning is supported by the temperature-based model, which indicates that earlier migration occurs in warmer years. Nonetheless, significant local variations persist in the timing of spring migration. Our results highlight the importance of studying in detail the timing of spring migration and its variability, as it may elucidate mechanisms underlying trends in population decline at both national and international scales.

Amphibians face a multitude of threats and therefore make a prime example of the current biodiversity crisis. Multiple amphibian stressors in anthropogenic landscapes include infectious diseases and agrochemicals. Synergic effects between these stressors may increase the negative impact of infections on amphibian health. In a 56-day trial, we assessed the impact of co-exposure to the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) and the pathogenic fungus Batrachochytrium salamandrivorans (Bsal) on infection parameters (infection intensity and disease severity) and health (body condition and telomere length) in Italian crested newts (Triturus carnifex). Twenty days post-inoculation with Bsal, newts were either exposed to 2,4-D for 12 days or not exposed (control). Most newts developed high infection loads that steadily increased towards the end of the trial. While body condition was negatively correlated with pathogen burden, only one out of 23 animals died. Telomere length remained unaffected by the pesticide and the pathogen. The 2,4-D treatment did not exacerbate Bsal infection. Most newts survived almost two months with significant pathogen loads; thus, even in a pesticide-infested environment, T. carnifex could be an important long-term Bsal reservoir for co-occurring species on the Italian peninsula, a urodele diversity hotspot.

Despite their significance in ecosystems, the decline of common species receives little attention in conservation. Identifying the underlying drivers of common species declines is necessary to mitigate further biodiversity loss. Common Toads ( Bufo bufo ) in Europe are emblematic of a once common species that is presumedly undergoing widespread declines. We aim to quantify population trends of Common Toad in Flanders and assess to which extent population trends depend on general landscape variables (land use identity, structure and change). Using standardised time series obtained from citizen science and spanning over four decades, we used an end‐begin contrast to get a trend value for 234 populations. Next, we developed a testing strategy to find associations between these trend values and the surrounding landscape. Across the study region, 40% of the populations have declined significantly, while only 10% show an increase. Declines were not associated with landscape characteristics in our study area, which is one of the most fragmented regions in Europe. We discuss how the research region, spatial and temporal resolution, as well as the generalist nature of the species, may explain these findings. Our study suggests limited effects of general landscape characteristics on the decline of Common Toad populations, indicating that other major drivers are likely responsible.

Background: Amidst the current biodiversity crisis, amphibians are particularly endangered by the emergence of infectious diseases. The skin disease chytridiomycosis is caused by the fungi Batrachochytrium dendrobatidis (Bd) and B. salamandrivorans (Bsal) , which may interact with bacterial symbionts present on the amphibian epidermis. Extensive research has explored the interactions between the amphibian microbiota and Bd ; yet, little is known about its interactions with Bsal . In this paper, we used the ribbed newt ( Pleurodeles waltl ), a model species displaying pronounced among-individual variation in response to Bsal, to (1) determine whether susceptibility to Bsal and individual microbiota vary between source groups; (2) test whether susceptibility to Bsal can be predicted from skin microbiota before exposure and (3) quantify microbiota volatility over time to determine whether Bsal infection intensity and chytridiomycosis severity correlate with the magnitude of shifts in bacterial communities caused by Bsal exposure. Results: Our results demonstrate that newts of different origin harbor distinct microbiota even under uniform rearing conditions. We show that Bsal infection intensity and disease severity cannot be predicted from the diversity, structure, or composition of the skin microbiota of P. waltl. Instead, a strong relation between newts’ source group and their response to Bsal suggests that other factors might underpin among-individual variation in Bsal susceptibility in this species. Moreover, our results indicate that the intensity of early Bsal infection and longer-term severity of chytridiomycosis do not correlate with the magnitude of microbiota change following Bsal exposure. Conclusion: These results demonstrate a limited involvement of the microbiota in Bsal dynamics in P. waltl, suggesting that other mechanisms contribute to individual Bsal susceptibility. Further research on the relation between chytrid pathogens and their amphibian hosts will be instrumental to improve the conservation of the most endangered vertebrate class on earth. Graphical

The European common toad (Bufo bufo) is undergoing widespread population declines, potentially influenced by multiple environmental stressors, including chemical pollution. This study aimed to link the toad population status and contaminant levels in 20 breeding ponds in Flanders (Belgium). A multi-contaminant analytical approach was applied, using solid-phase extraction (SPE) combined with advanced instrumental methods: ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) for pesticides and mycotoxins, UHPLC-Orbitrap-high resolution MS for pharmaceuticals, inductively coupled plasma-optical emission spectrometry (ICP-OES) and ICP-MS for high and low-concentrated metals, respectively, and ion chromatography (IC) for nutrients. Across ponds, 25 pesticides, 11 mycotoxins, 13 antimicrobial drugs, 5 antiparasitic drugs, and 11 metals were detected, reaching concentrations up to 114 ng/L (pesticides), 88.2 ng/L (mycotoxins), 837 ng/L (antimicrobials), 26.8 ng/L (antiparasitic drugs), and 7382 μg/L (metals). Temporal variability was pronounced, with episodic increases in pesticide and metal levels, declining nitrate and sulphate concentrations, and increasing enniatin levels from March to June. Despite frequent co-contamination, no direct relationship was observed between individual contaminant levels and toad population status. These findings suggest that population declines are unlikely to be driven by a single substance, but rather by combined pressures associated with complex contaminant mixtures and other environmental stressors.

Aim: We aim to quantify population trends of Common Toad in Flanders and assess to which extent population trends depend on general landscape variables (land use identity, structure and change). Location: Flanders, Belgium Methods: Using standardized time series obtained from citizen science and spanning over four decades, we used an end-begin contrast to get a trend value for 234 populations. Next, we developed a testing strategy to find associations between these trend values and the surrounding landscape Results: Across the study region, 40% of the populations have declined significantly, while only 10% show an increase. Declines were not associated with landscape characteristics in our study area, which is one of the most fragmented regions in Europe. Main conclusions: We discuss how the research region, spatial and temporal resolution, as well as the generalist nature of the species, may explain these findings. Our study suggests limited effects of general landscape characteristics on the decline of Common Toad populations, indicating that other major drivers are likely responsible.