Seasonal differences of Batrachochytrium dendrobatitis in persisting populations of Colostethus panamensis in western Panama
Amphibians are declining at an alarming rate. An IUCN assessment shows that one-third or more of described amphibians are threatened with extinction. Many of these declines are due to the disease chytridiomycosis, which is caused by the fungal pathogen, Batrachochytrium dendrobatidis (“Bd”). In Western Panama, an epidemic wave of chytridiomycosis spread from west to east causing mass mortality events and catastrophic losses in amphibian abundance and diversity. One species provides a particularly compelling example of Bd-related declines. The common rocket frog (Colostethus panamensis) experienced a 98% change in abundance within six months of Bd arrival at one high elevation site. Environmental conditions (e.g., temperature) may have contributed to this high rate of mortality. Temperature strongly affects Bd’s growth rates and host immune responses. In the tropics, previous research has demonstrated that prevalence and intensity of infection is greater at higher elevations, where temperatures are consistently low. We investigated persisting C. panamensis populations, microhabitat temperature conditions and pathogen prevalence ~10 years post-initial-outbreak during the dry and wet seasons. Additionally, we used temperature data loggers to characterize microhabitat conditions across seasons and elevations.
We surveyed during four field seasons (2013-2014) and found multiple sites where populations of C. panamensis are persisting following a chytridiomycosis outbreak in Western Panama. Bd was detected in all sites where C. panamensis were sampled but infection prevalence differed among seasons. For example, infection prevalence was 62.71% during the wet season and 24.39% during the dry season. Microhabitat temperatures differed across seasons with higher temperatures in all sites during the dry season compared to the temperatures in the same sites during the wet season. The positive qPCR results suggest that these populations have survived despite the presence of Bd. Furthermore, higher microhabitat temperatures or short daily variations with high temperatures may limit fungal growth during the dry season. Post-decline surveys are critical for conservation of neotropical amphibians as well as other endangered amphibians. Documenting rediscovered sites or species is important for informing conservation and management initiatives. Future studies will focus on decrease of pathogen virulence or increased resistance/tolerance of the host in this disease system in Central America.