Date of Graduation


Document Type


Degree Name

Doctor of Philosophy in Biology (PhD)

Degree Level



Biological Sciences


Adam M. Siepielski

Committee Member

Marlis R. Douglas

Second Committee Member

Kristian M. Forbes

Third Committee Member

Ashley Dowling

Fourth Committee Member

William J. Etges


community ecology, disease ecology, food web, host, parasite, species interaction


Parasitism is one of the most common life history strategies employed in nature, yet the effects of parasites are often thought to be minimal, and the vast majority of studies fail to consider parasites and their effects on host organisms. This is likely a problem, as the magnitude of parasite-mediated effects on their hosts can be quite large. Additionally, the effects of parasites are known to extend beyond the host to affect other species interactions. I used a series of approaches to gain a more integral understanding of host-parasite interactions by studying (1) the effects of parasites on biotic interactions that hosts engage in, (2) how biotic interactions such as predation and competition can affect host immune defense, and (3) how abiotic and biotic factors within the local environment affecting the host can further mediate parasitism dynamics. Specifically, in Chapter 1 I conducted a phylogenetically informed meta-analysis of the effects of parasites on species interactions (i.e., predation, competition, mutualism, and reproduction). I found that despite a strong overall negative effect on species interactions, the effects of parasites surprisingly ranged from being strongly beneficial to strongly deleterious on host species interactions. In Chapter 2 I used larval damselflies and their dominant fish predator to test how cascading effects of predators on host competitive interactions and resource acquisition affected a critical component of damselfly immune function, the phenoloxidase (PO) cascade. I found that neither direct density-mediated effects, indirect, trait-mediated effects, nor combined effects of predators via natural selection affected total PO activity. Instead, PO levels increased with resource availability, implying resource limitation. Finally, in Chapter 3 I used two field experiments and a detailed observational study to investigate how host, abiotic, and biotic factors within the local environment affected the relationships between damselfly (Enallagma spp.) hosts and their water mite (Arrenururs spp.) ectoparasites. I found that parasitism was species-specific and did not vary with host density or host condition (i.e., immune function). Instead, parasitism was largely predicted by abiotic factors (i.e., pH). Collectively, my results indicate that parasites are key players in the complex web of species interactions that compose food webs. Furthermore, host-parasite interactions are mediated by many of the same ecological factors as other species interactions, which has implications for parasitism dynamics within ecological communities. Future studies of food webs must incorporate parasites into their experimental and theoretical designs, and future studies of host-parasite interactions must expand beyond the focal relationship and consider the ecology of both the host and parasite.