Intestinal parasites, acorn masts and population dynamics of Peromyscus

Understanding the factors contributing to temporal variation in population size is of great ecological interest. While a number of studies have examined the role of predators in causing oscillations in prey abundance, few empirical studies have evaluated the role of parasitism.

The white-footed mouse (Peromyscus leucopus) and the deer mouse (Peromyscus Maniculatus) are ecologically similar species that demonstrate temporal fluctuations in population size. Previous studies suggest that population peaks are due to increased food availability during oak acorn masts. However, resource depletion docs not fully explain why populations crash after mast years. One possible explanation is that density dependent transmission of deleterious intestinal pathogens may contribute to post-mast crashes.

This dissertation examined how intestinal parasites and periodic variation in food availability may regulate natural populations of Peromyscus. I first analyzed long-term data on mouse dynamics, and confim1ed acorn mast intensity as the dominant factor contributing to fluctuations in Peromyscus. Predation and abiotic conditions were identified as significant but secondary factors determining mouse abundance, however, a large proportion of the variation in population size remained unexplained. Using a theoretical model l demonstrated that resource depletion alone is insufficient to explain the documented dynamics of Peromyscus populations. Adding a density dependent pathogen to the model provides qualitative agreement with the long-term data. To explore whether the unexplained variation in population dynamics might be a product of host-parasite dynamics, I first tracked the patterns of prevalence and infection intensity of the Peromyscus internal parasite community over three years. Second, I used an antihelminthic drug to remove intestinal nematode and demonstrate that parasite community composition may be structured by within-host competition. Lastly, to test whether population crashes may be influenced by intestinal parasitism, I experimentally manipulated both resource pulses and intestinal nematode infection. I found that the interaction between resource availability and parasitism arc likely to contribute to mouse population dynamics, and more specifically the seasonally crashes of Peromyscus.

In conclusion, I have demonstrated that multiple factors arc important for the dynamics of small mammal populations, and that parasitism may interact with resource pulses to contribute to population fluctuations observed in Peromyscus species.