Effects of host population size and isolation on parasite dynamics in a naturally fragmented host metapopulation

Parasites, by definition, need hosts in order to survive. Many things may keep a parasite from successfully spreading: hosts may come into contact too rarely, some hosts may be resistant to infection, others may die before the infection is passed on. As a host population gets smaller or decreases in density, the number of opportunities for parasite transmission become increasingly more rare to the point where the parasite may become locally extinct. This, in essence, is the idea of a host population threshold: that there is a certain population size or density below which the parasite cannot persist. Extinction of a parasite in one host population may only be temporary if it can be reintroduced again from a neighboring host population and if by that time the number of hosts has increased again above the threshold number. If there is a whole group of such host populations, the parasite may go extinct at different places at different times but never disappear from the system as a whole. Ecologists would call such a system a metapopulation. Knowing where and when host population thresholds and metapopulations are found for parasites in nature would be of great interest, because it could explain why the risk of becoming infected varies between different places and could give us clues about how to potentially eradicate certain diseases. Yet at this point, we know almost no examples of host population thresholds and parasite metapopulations in wild species. This project will study two rodent species and their parasites (fleas, lice, a virus, and different types of gut parasites) on a group of natural islands, situated within a large lake. This system offers a unique opportunity to look for host population thresholds because the islands differ in size. This means that the number of rodent hosts living on them will also vary to the extent that larger islands may be able to support a particular parasite wheras smaller islands may not. Because islands are separated by considerable expanses of open water, it is also unlikely that rodents and their parasites will move between them very easily. Because of this limit to host movement, this system may also well constitute a metapopulation for certain parasites. I will monitor rodents (by live trapping) and their parasites (by collecting rodent faecal and blood samples) on these islands for two years. Based on these data I will determine whether this system could provide one of the first examples for host population thresholds and metapopulation structure influencing and potentially limiting the distribution of parasite species within a landscape.