The Ecological and Evolutionary Consequences of Parasites that Manipulate their Hosts

Infectious disease is prevalent in nature and can significantly alter human, animal, plant and bacteria populations. Parasites are a leading cause of infection, so understanding parasite evolution is vital for disease surveillance, control and limitation. The field of host-parasite relationships is vast; however, there are still unanswered questions, for example, about the effect of other species' interactions and complex transmission processes. Therefore, increasing our knowledge of these topics is beneficial for the field and for limiting the spread of infectious diseases.

In this thesis, I utilise mathematical models to study the evolution of host-parasite relationships when the transmission route is complex within the context of wider community interactions. Trophic parasites will transmit to a definitive predator host via an intermediate prey host. The predators in this system form an important part of the parasite's life cycle. Parasites can harbour manipulation as an adaptive trait and alter the appearance or behaviour of the prey host to increase predation risk. We extend a model of shared parasite infection in a predator-prey system and explore the scenarios where parasites evolve to manipulate their prey hosts to facilitate transmission. My three key research questions in this thesis are as follows:

When are parasites most likely to evolve a higher degree of manipulation? (chapter 2)

When will hosts evolve defence mechanisms against parasitic infection when faced with the threat of predation? (chapter 3)

Do parasites go extinct in regions on cyclic dynamics when stochastic effects are accounted for? (chapter 4)

Our key results are that there exists an important feedback loop between ecological and evolutionary dynamics, emphasised by the fact that population densities are evolutionary drivers. Also, we highlight the significant evolutionary effect of fluctuating dynamics. The theoretical results in this thesis provide important contributions to the field and expand our understanding of host-parasite relationships that include complex transmission routes and other species interactions that can be used as a foundation for future experimental work.