Transmission and effects of an emerging fungal pathogen (Batrachochytrium dendrobatidis) in a declining amphibian (Rana muscosa).

摘要

Over the past few decades, amphibians have been disappearing around the world and in many cases for unknown reasons. Emerging infectious diseases are becoming increasingly recognized as threats to the persistence of wildlife populations. An emerging infectious disease of amphibians, chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis, recently has been implicated in population declines and possible extinctions throughout the world, including in protected areas. B. dendrobatidis zoospores, the infectious stage of the fungus, infect keratinized cells found in the mouthparts of anuran tadpoles and the skin of postmetamorphic individuals. In this dissertation, I examined transmission and the resulting individual and population-level effects of this pathogen on the mountain yellow-legged frog (Rana muscosa). Although R. muscosa exists primarily on protected federal land in the Sierra Nevada, California, USA, it is in steep decline with multiple threats to its persistence, most recently chytridiomycosis.;We found that B. dendrobatidis transmission can occur within and between the tadpole and postmetamorphic life stages of R. muscosa. Tadpoles remained clinically healthy with infection, but showed a mouthpart pigmentation loss over time. In the laboratory and in some natural environments, I detected a significant positive relationship in the proportion of R. muscosa tadpole hosts that became infected over a few weeks of B. dendrobatidis exposure with increasing numbers of previously infected R. muscosa tadpoles added to their local environment. The density-dependent assumption of transmission, often called mass action, most appropriately characterized B. dendrobatidis transmission in R. muscosa tadpoles.;Infected R. muscosa tadpoles retained B. dendrobatidis through metamorphosis, and then become lethargic, anorexic, and died approximately two weeks after metamorphosis in both the laboratory and the field. Postmetamorphic individuals with chytridiomycosis showed similar effects. Temporal population studies at infected and uninfected lakes in the Sierra Nevada found that chytridiomycosis caused high mortality in postmetamorphic R. muscosa. Together, laboratory and field studies indicate that this emerging infectious disease is the proximate cause of numerous R. muscosa population crashes in the Sierra Nevada. Chytridiomycosis is a serious threat to this already declining species, creating a highly uncertain future for R. muscosa.