Occurrence of Batrachochytrium dendrobatidis within and between species: A review of influential variables as identified from field studies

In the last two decades a wealth of field studies has documented Bd infection on amphibians worldwide. These studies have demonstrated that the pathogen presence, infection severity and disease impact differ substantially between and within species. However, aside from the influence of temperature, the relative importance of various biotic and abiotic factors driving pathogen occurrence is unclear, leading to potential bias and limitation in our understanding of host-Bd dynamic ecology. Here, we systematically review the Bd literature that investigated environmental, pathogen-related or intrinsic species-specific factors to explain spatial patterns of Bd as measured from field-collected samples. We searched Scopus, Web of Science, and key review papers for fieldbased articles published from 1999 to 2020. We evaluated 102 articles on spatial heterogeneity of Bd occurrence between amphibian populations. Distinct factors have been investigated to influence either prevalence or infection intensity of Bd, the two most common Bd detection metrics. Factors identified in the studies go beyond temperature-associated constraints on Bd, which is commonly assumed to be the most significant environmental driver. Intra-specific variation in genetic response to the pathogen, presence of invasive species or other taxa, salinity, and even the presence of Bd predators can also underpin the observed pathogen occurrence. We also found that studies were poorly representative of the global geographic distribution of amphibians and focused mostly on Least Concern species and common families. Studies on threatened species are consistently underrepresented within different families. Understanding of Bd-host dynamics as well as conservation management need to be shaped to population and landscape scales to consider the intricacy of factors influencing amphibian declines caused by Bd.

Automated summary

Field studies over the past two decades have revealed significant spatial heterogeneity of Bd occurrence in amphibian populations, influenced by a variety of biotic and abiotic factors beyond temperature. Factors such as genetic response to the pathogen, presence of invasive species, and even Bd predators play important roles in driving pathogen occurrence. However, studies have been biased towards certain amphibian species and families, with threatened species consistently being underrepresented, highlighting the need for a more comprehensive and inclusive understanding in Bd-host dynamics and conservation management.