Graduate Thesis Or Dissertation
 

Ontogenetic Shifts in Disease Response in Native and Invasive Anuran Species

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  • This dissertation presents a different approach to understanding how amphibians are responding to disease through ontogeny. Although numerous efforts have been conducted to understand host responses to the fungus Batrachochytrium dendrobatidis (Bd), studies have been restricted to distinct developmental stages. This dissertation provides information on host response to Bd across life history transitions in native and invasive anuran species. My dissertation is an effort to understand several aspects of the host-pathogen dynamic in the amphibian- chytrid system from embryos to hatchlings, larvae, and juveniles to reproductive adults. I investigate how previous exposure at early life stages (embryos) carries over to impact host response in later life stages. Then, I explore how the virulence of the pathogen varies according to its origin and how this modifies host response. Finally I explore variation across geographic ranges in size at first reproduction, a life history trait that influences invasion potential, in the American bullfrog (Lithobates catesbeianus).Biodiversity loss threatens ecosystems worldwide and several factors, such as habitat transformation, overexploitation, and pollution contribute to this unprecedented crisis. Additional threats include emerging infectious diseases and the introduction of invasive species, both included as central topics of my dissertation research.I experimentally examined if embryonic exposure of anuran species to the fungus Batrachochytrium dendrobatidis (Bd) produces effects within the same stage. I exposed embryos of three anuran species found in the Willamette Valley, Oregon, to different strains of Bd at particular stages of embryonic development. I found that exposure to Bd resulted in direct effects on embryos; I found an increase in mortality after Bd exposure, and this response was conditioned by the host species, timing of exposure and Bd strain. I followed individuals through the hatching life history transition and into the larval stage. I detected both direct and latent effects of Bd exposure on the anuran larvae. Direct effects were observed in individuals exposed only as larvae, while latent effects were detected in individuals exposed only as embryos. Finally, repeated exposure to Bd as embryos and larvae resulted in species-specific mortality (Chapter 2).Research on variation in host response to pathogens isolated from conspecifics in different distributional ranges is needed to understand how pathogen origin can mediate host response. Chapter 3 explores the susceptibility of wild-caught invasive American bullfrogs to different Bd strains isolated from conspecifics in different distributional ranges. I found larval bullfrogs were susceptible to a novel Bd strain despite it being isolated from conspecifics. The finding of lower infection loads over time suggests bullfrogs are potentially able to clear Bd infection, but this response seems to be strain-specific. In an era of emerging diseases and globalization, understanding the impacts ofnovel strains provides information about the importance of evolutionary relationships between hosts and pathogens.In chapter 4, I studied the next anuran life history transition: reproductive adults. I quantified variation in a key life history trait, size at first reproduction, which contributes to reproductive, and thus invasion, success. I used field sampling and laboratory analysis to determine the minimum reproductive size in an invasive anuran species, the American bullfrog, in the Willamette Valley. I found the minimum reproductive size of bullfrogs is similar to the reported values for bullfrogs in other invaded ranges yet smaller than sizes reported from their native range at similar latitudes. The results obtained by this research may be applied to management actions towards controlling and minimizing the impacts of this invasive species over local species of conservation concern.Chapter 5 summarizes the findings and implications of the studies presented in this dissertation.
  • Keywords: Reproductive Biology, Chytridiomycosis, Chytrid strains, Direct effects, Amphibians, Latent effects
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