Amphibian susceptibility to parasitism in relation to environmental contaminant exposure: a meta-analysis

Amphibians are declining globally, which may have consequences for structure and function of ecosystems. In the natural environment, amphibians are often challenged with multiple threats simultaneously. Two challenges that amphibian species currently face include exposure to environmental contaminants (ECs), and exposure to parasites. The relationship between EC exposure and amphibian susceptibility to parasites has been investigated for multiple amphibian species, parasite types, and ECs. However, the overall effect of EC exposure on amphibian susceptibility to parasites remains unclear. Therefore, we conducted a meta-analysis to summarize the overall effect of EC exposure on amphibian susceptibility to parasitism and to investigate whether moderating factors influenced the effect of EC exposure on amphibian susceptibility to parasitism. We did not detect an overall effect of EC exposure on amphibian susceptibility to parasitism. The effect of ECs on amphibian parasite levels differed depending on EC class, parasite type, the relative order the host was exposed to ECs and parasites, the host's developmental stage when exposed to parasites, and which organism was exposed to ECs. Amphibian parasite levels increased as the total experiment duration increased. The effect of ECs on parasite-associated amphibian mortality differed depending on EC class, with fungicides increasing parasite-associated amphibian mortality relative to the other EC classes tested. We identified two main considerations for future research: to broaden the array of ECs and parasites tested, and to investigate the ontogeny of EC exposure on amphibian susceptibility to parasites. We also implore researchers to publish their raw data to facilitate data collection and analysis for future meta-analyses.