Wildlife diseases are contributing to the current Earth’s sixth mass extinction; one disease, chytridiomycosis, has caused mass amphibian die-offs. While global spread of a hypervirulent lineage of the fungus Batrachochytrium dendrobatidis (BdGPL) causes unprecedented loss of vertebrate diversity by decimating amphibian populations, its impact on amphibian communities is highly variable across regions. Here, we combine field data with in vitro and in vivo trials that demonstrate the presence of a markedly diverse variety of low virulence isolates of BdGPL in northern European amphibian communities. Pre-exposure to some of these low virulence isolates protects against disease following subsequent exposure to highly virulent BdGPL in midwife toads (Alytes obstetricans) and alters infection dynamics of its sister species B. salamandrivorans in newts (Triturus marmoratus), but not in salamanders (Salamandra salamandra). The key role of pathogen virulence in the complex host-pathogen-environment interaction supports efforts to limit pathogen pollution in a globalized world.

The sixth mass extinction is a consequence of complex interplay between multiple stressors with negative impact on biodiversity. We here examine the interaction between two globally widespread anthropogenic drivers of amphibian declines: the fungal disease chytridiomycosis and antifungal use in agriculture. Field monitoring of 26 amphibian ponds in an agricultural landscape shows widespread occurrence of triazole fungicides in the water column throughout the amphibian breeding season, together with a negative correlation between early season application of epoxiconazole and the prevalence of chytrid infections in aquatic newts. While triazole concentrations in the ponds remained below those that inhibit growth of Batrachochytrium dendrobatidis, they bioaccumulated in the newts' skin up to tenfold, resulting in cutaneous growth‐suppressing concentrations. As such, a concentration of epoxiconazole, 10 times below that needed to inhibit fungal growth, prevented chytrid infection in anuran tadpoles. The widespread presence of triazoles may thus alter chytrid dynamics in agricultural landscapes.

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While the amphibian disease chytridiomycosis is causing ongoing population declines and biodiversity losses around the globe, efficient mitigation strategies are lacking. The free‐living zoospores of the causative agents of this disease, the chytrid pathogens Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal), are a potential food source for filter‐feeding micropredators as part of the aquatic food web. While consumption of zoospores can lower environmental pathogen loads, alternative food sources may interfere with pathogen removal rates. We compared the ability of three filter‐feeding zooplankton species, the cladoceran Daphnia magna, the rotifer Brachionus calyciflorus and the ostracod Heterocypris incongruens, to remove Bd zoospores in water and investigated the effect of alternative food sources, the green algae Pseudokirchneriella subcapitata and Chlorella vulgaris, on zoospore ingestion by D. magna. Daphnia magna was the only micropredator candidate that effectively removed Bd zoospores from its environment, with an average removal rate of 1,012 ± 542 GE ind.−1 hr−1 within our test system. High concentrations (1 × 105 cells/ml) of large and easily ingestible P. subcapitata reduced pathogen removal rates, whereas the small and less edible C. vulgaris did not interfere with pathogen removal. Synthesis and applications. We showed that Daphnia spp., which are keystone species in all sorts of aquatic habitats worldwide, are promising target agents for biologically mitigating chytridiomycosis infections and how natural food sources may interfere with this strategy. We also suggest potential management actions for biological disease mitigation, aiming to optimize environmental conditions for these target filter feeders, thereby reducing pathogen densities and eventually infection pressure in amphibian hosts. Examples of such management actions include, but are not limited to, removal of planktivorous fish, habitat restoration, nutrient control or agrochemical regulation in the vicinity of amphibian breeding ponds. Further studies, including field trials, are needed to confirm the effects of pathogen consumption on infection dynamics in natural situations and investigate the impact of intervention actions.