The ecological interactions of the hemiparasite Rhinanthus minor and its invertibrate herbivores

The hemiparasite Rhinanthus minor is a common component of many northern temperate grasslands. It can have major impacts on ecosystem processes, and is often present at very high densities, therefore constituting an important potential food source for invertebrate herbivores. Thus, the aim of this thesis is to investigate the interaction between this hemiparasite and its invertebrate herbivores, and to explore the various ecological factors which are likely to affect this interaction. In the first series of experiments the thesis explores how the density of the hemiparasite affects the composition of the vegetation, the performance of the hemiparasite and the levels of invertebrate herbivore damage it receives. The results of a field experiment and a greenhouse study demonstrated that hemiparasite density can adversely affect its own performance and survivorship and dramatically change the composition of the vegetation, but surprisingly appeared to have no impacts on the levels of herbivore damage the hemiparasite receives. The second series of experiments investigated the impacts if host identity on the performance of the hemiparasite and how this affects its invertebrate herbivores. The results demonstrated that host identity can have a major impact on the performance of the hemiparasite and its herbivores; however, the indirect effects on the invertebrates appear to be species specific. Thirdly, the thesis examines the effects of multiple host plants on the performance of R. minor and the knock-on effects for its invertebrate herbivores. Experiments demonstrated that multiple hosts are beneficial to R. minor, and that the antiherbivore defensive properties conferred to the hemiparasite by certain host plants are maintained in the presence of a second host species. Finally, the impact of nutrient addition and host plant damage on the performance of the hemiparasite and on the performance of its invertebrate herbivores was examined. The experiments showed that while certain host plants have highly contrasting effects on the performance of the hemiparasite?s herbivores, the addition of nutrients and impact of host plant damage largely remove these differences, while neither factor appeared to affect the performance of the hemiparasite.