Understanding population dynamics and the biotic and abiotic processes that drive and influence them is one the most fundamental issues in ecology, and is vital for successful ecological management of populations in the face of global environmental change. Species interactions influence population processes, and natural enemies in particular can have important impacts on vital rates, and are thought to be capable of population regulation. This thesis investigated the host-natural enemy interactions and spatio-temporal dynamics of two Lepidoptera-parasitoid-pathogen communities, which were used as model systems in which to explore these issues.
Using multi-year field data, potentially regulatory delayed density-dependent pathogen mortality was demonstrated in both the cyclical Operophtera brumata, but also unexpectedly in the non-cyclical Abraxas grossulariata. In addition, there was evidence that increasing temperature-related climatic conditions negatively influenced the interactions of O. brumata and its pathogen. Immune functioning was investigated in wild populations of the non-cyclical A. grossulariata, and unexpectedly found to be influenced by population density. Evidence consistent with trans-generational immune costs from defence against parasitism were also found. Scale-dependent effects of habitat fragmentation were investigated in the A. grossulariata-natural enemy community, and were found to have direct negative effects on host density at both small and large spatial scales, indirect negative effects on virus mortality at the largest scale, and, unexpectedly, direct positive effects on parasitism at small and medium scales. Finally, it was found that spatial population synchrony in O. brumata at the scale of Britain may be due to spatially correlated environmental processes, but that unlike O. brumata populations within mainland Europe there was no evidence for travelling waves in abundance within British populations, either driven by the mainland European travelling waves or occurring separately. The significance of these findings is discussed in the context of current research, and potential areas for future research are also addressed
