The dynamics of epidemics and the opportunity for host dispersal as a means of escape

Many organisms disperse between habitat patches to avoid the mounting fitness costs of a degrading habitat. An important driver of habitat degradation is parasites which build up during epidemics and reduce host fitness. Therefore, we may expect hosts to disperse early in epidemics to avoid these costs. However, research has found inconsistent relationships between parasite presence and host dispersal in part because it is difficult to track epidemics and the fate of host dispersers that flee them. My dissertation elucidates the relationship between epidemic progression and host dispersal success using theoretical approaches and a tractable host-parasite study system, the dispersing nematode Caenorhabditis elegans and its natural microsporidian parasite Nematocida parisii. In my first chapter, I studied the unique dynamics of epidemics in boom-bust host populations, a pattern of host demographics common to species that undergo dispersal and colonization. Using theoretical simulations, I found that the early stages of epidemics provide hosts with an advantage over their parasites that strongly drives parasite extinction when boom-bust cycles are frequent and/or severe. In my second chapter, I tested whether disease spread is biased away from early host life stages which could protect young host dispersers. In analyzing C. elegans – N. parisii epidemics, I found that early host life stages were indeed protected from infection early in epidemics and found that host dispersers were healthier than the populations they dispersed from. In my third chapter, I tested how the fitness of host dispersers (i.e., their ability to colonize new habitats after dispersal) changed over the course of an epidemic. Using the same C. elegans – N. parisii system, I found that the optimal time for hosts to disperse was pushed earlier by the presence of parasites. Additionally, I found evidence that the costs and benefits of different dispersal strategies are modulated by the host’s level of immunological resistance. Overall, this work advances understanding about the selection pressures on dispersal, a ubiquitous life history trait with deep implications for ecology, evolution, and species persistence.