The emerging infectious disease chytridiomycosis, caused by the Amphibian Chytrid Fungus Batrachochytrium dendrobatidis (Bd), is implicated in widespread and sudden population declines, local extirpations, and species-wide extinctions of amphibians in many parts of the world.  In the neotropics, most such amphibian declines have occurred in mid-to high-elevation sites (>400m asl), in theory because Bd is intolerant to the high temperatures found in lowland tropical forests. However, the lowland (30-140m) La Selva Biological Station, in northeastern Costa Rica, has experienced gradual yet significant population declines over the past three decades. The role of Bd in these declines has not been assessed.  Here, we report the results of a 12-month disease surveillance program at La Selva for three common species of frogs representing a range of reproductive modes and variation in use of aquatic habitats.  We combine standard non-invasive skin swabbing techniques with a quantitative polymerase chain reaction assay (qPCR) to analyze the prevalence and Bd load across an annual cycle.

Results/Conclusions

Our data indicate low overall prevalence (~6% of frogs at the site infected with Bd), and the average Bd load for infected animals was 1020 zoospore equivalents (range: 0.08 to 22418 zoospore equivalents).  Our data indicate strong temporal variation in the prevalence of Bd (ranging from <1% in several months to a peak of ~18% in January), yet no temporal variation in Bd load. Prevalence was negatively correlated with air temperature, and the coolest temperatures experienced by the site appear to manifest the largest variation in prevalence. We find no difference among species in prevalence of infection despite considerable variation in life histories, suggesting that Bd infection is not limited to those species with aquatic life stages. Our data indicate that Bd can attain relatively high prevalence even in lowland forests, and that lowland forests should not necessarily be considered unsusceptible to Bd-associated declines. However, because high temperatures do appear to limit prevalence of Bd, our study does provide support for the hypothesis that regulation of Bd by temperature limits widespread extinctions to cooler montane forests.