The pathogenic chytrid fungi Batrachochytrium dendrobatidis (Bd) and B. salamandrivorans (Bsal) cause infections that have become primary drivers of amphibian biodiversity loss. While globally widespread, the distribution margins of Bd and Bsal have not been determined, and the presence of these pathogens has probably gone unnoticed in many areas, especially in northern Eurasia. To better understand the presence and distribution of both pathogens in the northern temperate and boreal forest biomes, 243 individuals were sampled from 8 native amphibian species across Estonia. Additionally, 68 amphibians were sampled from captive collections in Estonia and Latvia. Pathogen infection was assessed using metabarcoding of the ITS2 marker. No positive matches for Bsal infection were found. Bd was detected in 13 specimens, 3 of which were sampled at the Riga Zoo (with a prevalence of 5.2%) and 10 in natural environments in Estonia (3.3%). The infected wild individuals belonged to 6 amphibian species and were detected throughout the mainland of Estonia, but not on islands. Prevalence of infection with Bd ranged between 3.1 and 12.5% among native species. In addition, we found molecular evidence for a potentially new sister species to Bd in nature. Although outbreaks of chytridiomycosis have never been observed in Estonia, it cannot be excluded that the dynamics of local amphibian populations are affected by Bd infections. Therefore, further work, including capture-mark-recapture studies and long-term monitoring, are required to clarify the impact of Bd on amphibians in Northern Europe.

We explored the breeding behaviour of a threatened amphibian, the natterjack toad, at its northern range limit in Estonia, to determine the extent to which reproduction is affected by harsh and unstable climatic conditions. Using photo identification of specimens, we found that in optimal weather conditions males formed three breeding cohorts, while in adverse conditions only a single cohort occurred and under extreme conditions reproduction was skipped entirely. During the extended breeding season, larger males participated in reproduction throughout the breeding period, while smaller males appeared in later cohorts. Breeding success was related to the calling effort of a male, where larger males had greater mating success than smaller ones. We found that the natterjack toad males exhibit significant plasticity in reproductive behaviour at the northern range limit, which, given the energetic cost of reproduction and the increased risk of predation, allows them to increase their fitness at high latitudes.