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Abstract
Species redistribution and invasion are becoming increasingly common due to climate change and anthropogenic impacts. Understanding the resultant shifts in host-parasite associations is important for anticipating disruptions to host communities, disease cycles, and conservation efforts. Parasite spillover and spillback can cause host declines or local extinctions, and changing circulation of zoonotic pathogens puts human health at risk as well. Thus far, research in this area has focused on parasite sharing. While valuable as a starting point, moving the field forward requires evidence-based rules for the loss and gain of parasites as hosts change their geographic range. My dissertation addresses this knowledge gap, using introduced mammals as a tractable study system. I explicitly consider hosts as both sources and recipients of parasites by defining four possible parasite fates upon host invasion: retention (carried with the host during invasion), loss (not carried with the host during invasion), acquisition (picked up by the host post-invasion), and non-acquisition (not picked up by the host post-invasion). I first show that both parasite acquisition and retention are predictable and generalizable processes, driven by parasite and host community traits. Hosts are more likely to acquire parasites with higher community prevalence and from established host species to which they are closely related. Further, hosts are more likely to retain specialist parasites that have the potential to infect closely related hosts. These results show some variation across parasite type, which suggests that parasite and host community characteristics both influence parasite acquisition and loss. I further synthesize the distinct yet intertwined processes of acquisition and retention in the context of two ecological theories. I establish that the enemy release hypothesis, whereby invasive hosts are partly successful due to leaving behind parasites, operates in terrestrial mammal systems. Additionally, I provide broad support for the vacated niche hypothesis, demonstrating that a host's loss of a parasite species is frequently accompanied by the acquisition of a related parasite across parasite taxonomy, type, specificity, and transmission mode. In sum, both hypotheses provide valuable frameworks through which to understand and predict changing host-parasite associations. This work provides general rules to help anticipate novel host-parasite associations created by climate change and other anthropogenic disruptions.