Abstract/Summary

Species interactions have caught ecologists’ attention since the field of ecology first developed. Direct effects between species are readily recognised as the interaction is explicit and easy to detect. In contrast, indirect effects are masked under direct effects and difficult to detect. However, ecologists have been attempting to quantify and understand the role of indirect effects on community structure. In Chapter 1, I reviewed the literature, clarifying the basic definitions of direct and indirect effects, and then apply these concepts to components in my study systems. I employed two different study systems to examine direct (host seed�bean beetle system) and indirect (plant pathogen-plant-insect system) effects in this thesis. In Chapter 2, I explored the direct effects of host quality and maternal effects on preference and performance of two bean beetle species, Callosobruchus maculatus and C. analis. They showed similar and consistent preference, but their performance was species dependent and influenced by both host and maternal effects. In Chapter 3, I investigated the direct effects of host quality and mating status on C. maculatus reproductive costs. I found that host quality affects beetle longevity, and this effect also depends on mating status, which then differs between treatments and sexes. In Chapter 4, I examined the indirect effects emanating from the plant pathogen Botrytis cinerea on two aphid species, Aphis fabae and Acyrthosiphon pisum. I found that A. fabae experiences indirect negative effects while Ac. pisum gains indirect positive effects from B. cinerea infection. In Chapter 5, I investigated the indirect effects of B. cinerea infection on A. fabae and their natural enemies and ant mutualists in the field, where levels of urbanisation varied. My results suggest that the population of A. fabae is influenced by infection, while the population of aphid natural enemies and ant mutualists are instead dependent on urbanisation. In Chapter 6, I investigated the effects of plant pathogens on insect herbivores’ natural enemies using meta-analysis technique. I found no overall effects of plant pathogens on insect natural enemies, but if considered by type of pathogen, I found fungal pathogens cause consistent indirect negative effects, while bacterial pathogens show indirect positive effects on insect natural enemies. No significant effects of viral pathogens were found. In my last Chapter (Chapter 7), I discuss the main findings, contributions and further recommendations of my study.