Patterns and Mechanisms in Parasite Community Structure

Abstract

In this thesis I investigate how parasite communities are structured, and how parasite traits influence the interactions that shape this community structure. I argue that parasite traits are essential in the study of parasite communities. Due to the idiosyncrasies of each parasite species, there is variation in the way a parasite species deal with other parasite species or with the host. Using data for different fish parasite communities I explore different hypotheses regarding the drivers of observed patterns in parasite assemblages. I look at the distribution of parasite species among hosts, and how it changes over host ontogeny, which is view as a parallel to free living communities living in patches of different size and successional stage. As the distribution of the number of parasite species changes, so does the composition and number of individuals present in the hosts. Afterwards I map the network of interactions between parasites within hosts, looking for common ground between host species, and discussing the potential underlying processes behind the presence and absence of interactions between parasite species. Unlike free-living communities, a majority of parasite-to-parasite interactions are mutualistic. Finally, I present results of co-occurrence and nestedness null model analyses, and discuss them in the light of the previous chapters. Utilizing a paleontological tool for stratigraphic data, I replace layers (classically depicting geological strata) by host size and age. This tool is effective in pointing to host or parasite phenomena influencing diversity shifts in parasite communities. This thesis presents novel evidence of interaction networks of marine parasite communities, replicated in different host species, providing strong arguments against considering these communities as random species assemblages. I show that parasite communities reflect familiar trade-offs between colonization and competition as observed in free-living communities, but complicated by the interaction between each parasite species and their host; which indirectly impacts interactions between parasites in the same host individual These indirect interactions occur mostly through manipulations of the hosts immune system and energy resources, while more subtly influencing the availability of patches for other parasites to colonize, through the parasite impact on host birth and mortality rates.