Species diversity often increases the stability of natural systems at multiple scales. By experimentally manipulating diversity of amphibian species harboring their natural symbiotic microbiomes, we tested the interactive effects of host species diversity and skin microbial diversity on host health. We swabbed wild-caught individual frogs prior and during the experiment and extracted genomic DNA from each swab using MoBio PowerSoil-htp 96-well soil DNA isolation kit. We tested swabs for the amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) in singlicate using Taqman qPCR to determine the infection load of Bd on each individual host. For assessing skin bacterial communities, we PCR-amplified the V4 region of the bacterial 16S rRNA gene using a dual-index approach with the 515F and 806R barcoded-primers. We used a combination of Generalized Linear Mixed Models, Network and Path analyses to identify mechanisms linking host species diversity, microbial diversity, and disease.
Results/Conclusions
We found higher average skin bacterial diversity in multi-host species than single-host species treatments. Contrary to our predictions, however, bacterial diversity per se did not have a direct effect on host immune capacity against the highly pathogenic amphibian fungus Batrachochytrium dendrobatidis (Bd). Furthermore, Bd spillover from tolerant host species was a strong mechanism of pathogen amplification, microbiome impoverishment, and disease for a susceptible host species in diverse assemblages. Finally, abundance of core skin bacteria negatively associated with Bd increased during the experiment in susceptible hosts, indicating that pathogenic infections could filter bacterial communities towards anti-pathogen competitors in the microbiome. Our findings, combined, highlight that microbiome diversity (including Bd) could both impact and be impacted by host species diversity.