Evolutionary and community ecology of parasitism in <i>Daphnia</i>
Parasites have been postulated to have a wide variety of effects on the ecology and evolution of their hosts. Among other things, it has been suggested that parasites impact the size, evolution and diversity of their host populations, as well as the interactions of their hosts with other species. Yet, in most cases, these roles of parasites remain speculative, lacking strong empirical support. In my dissertation research, I used the freshwater invertebrate Daphnia dentifera and two of its virulent parasites, Metschnikowia bicuspidata and Spirobacillus cienkowskii, to address two fundamental questions regarding host-parasite interactions. First, what factors control parasite epidemics in natural populations? And, second, what are the effects, both ecological and evolutionary, of parasite epidemics on host populations?
My dissertation research has focused on two factors that influence parasite epidemics in natural populations: selective predation and rapid evolution. I found that selective predation by the bluegill sunfish can seasonally restrict parasitism in lake Daphnia populations. Further, I have studied the role of rapid evolution in the termination of parasite epidemics, and have found that rapid evolution has the potential to cause the fade-out of parasite epidemics.
Parasite epidemics often have large effects on host individuals and populations. Both Metschnikowia and Spirobacillus reduce host fecundity and lifespan, and both can have negative population-level impacts. Using an epidemiological model, I show that the size of epidemics and their impact on host populations are greatly affected by the selectivity and intensity of fish predation. Parasite epidemics can also lead to rapid evolution of the host population. Interestingly, I found evidence for both directional and disruptive selection occurring during parasite epidemics.
Overall, this research demonstrates the importance of parasites to the ecological and evolutionary dynamics of their host populations, and the ability of community members to have large impacts on host-parasite interactions. As such, it highlights the importance of considering host-parasite interactions within their community context, and for considering both ecological and evolutionary processes when studying host-parasite interactions.