Graduate Thesis Or Dissertation
 

Complexity in host-pathogen dynamics : dispersal uncertainties, metapopulation persistence and the role of a changing climate

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gm80hx90w

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  • Emerging infectious diseases impact both human and wildlife populations. Infectious agents, in particular the aquatic fungus Batrachochytrium dendrobatidis (chytrid), have an influential role in driving global amphibian population declines. The emergence of the chytrid fungus has aspects of both geographic spread as well as climate shifts altering environmental conditions and host-pathogen interactions. My dissertation examines the spatial spread of chytrid by host dispersal at the metapopulation level, as well as how spatial risk from chytrid is associated with the climate. In Chapter 2 of my thesis, I examine preexisting conclusions in the wildlife disease literature on the relationship between disease spread mediated by host dispersal and metapopulation persistence. I show how explicit inclusion of local dynamics and dispersal-induced synchronization alters conclusions derived by previous metapopulation disease models. Contrary to existing models that do not include explicit local dynamics, I find that synchronization increases metapopulation extinction risks and regional persistence is optimized at intermediate dispersal levels when disease transmission rate from external sources are low. However, at high rates of external infections, I come to the similar conclusion that increased dispersal monotonically increases metapopulation persistence. In Chapter 3, I use a spatially explicit, individual-based model to simulation disease spread dynamics in a set of connected mountain yellow-legged frog population. I compare the simulated disease forecasts to field data, and test for the sensitivity of these results to assumptions of host dispersal potential. I find that chytrid is able to spread across the majority of the metapopulation even with assumptions of low host dispersal potential and that metapopulation extinction rate increases with increased host dispersal. In Chapter 4, I examine how chytrid distribution is influenced by climatic variables based on the most comprehensive and up-to-date set of global chytrid surveillance data. Using a machine learning algorithm, I generate predictions showing how chytrid distributions might be expected to change according to IPCC projected scenarios of future climate change. I conclude that chytrid distribution is likely to shift to higher altitudes and latitudes with overall increases in environmental suitability in the high latitudes of the Northern Hemisphere. The chosen input climatic variables yields excellent performance when predicting chytrid occurrence at a site, but no single variable has dominant predictive power. My dissertation provides insight into the applicability of conclusions derived from existing metapopulation disease models to specific conservation contexts. Much research has been invested in the chytrid-amphibian system at the individual and population level, yet how disease management might integrate into conservation planning targeted at the metapopulation level remains largely unknown. My research will form an important part in addressing amphibian conservation in spatially-fragmented, pathogen-ridden landscapes, which is especially important in today’s changing climate.
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