The role of size structure in cannibalism and intraguild predation in larval dragonflies

Size structure, the degree to which individuals in a population vary in size, can greatly influence the dynamics of predation within ecological communities. Several mechanisms that can influence size structure, especially in populations of aquatic insects include: overlap between year classes, developmental asynchrony, cohort splitting, and phenological separation between cohorts. In this dissertation, I describe how migration by adult Anax junius dragonflies influences (1) the size structure of larval A. junius assemblages; (2) cannibalism mediated through the effect on size structure; and (3) intraguild predation interactions also mediated through effects on size structure. A. junius larvae are highly voracious and use their relatively large size and visual acuity to assume the role of top predator in fishless ponds. Few studies have explored the implications of migration on interactions among larval odonates; however, this study begins to elucidate how migration can influence size structure and resulting size-structured interactions in communities dominated by the top predator, A. junius. Chapter one introduces the study system and provides conceptual background on the topics addressed in chapters two through six. Chapter two demonstrates the spatio-temporal variability in the size-structure of A. junius assemblages and how differences in size among larvae and temperature can influence cannibalism. Chapter three shows how direct and indirect effects of multiple predators, including non-size structured assemblages of A. junius, can be quantified to explain prey mortality rates. Chapter four explores the role of A. junius size structure on the survival and behavior of prey species. Chapter five addresses the substitutability of two size classes of A. junius for prey mortality in a field setting. Chapter six explores the role of visual and chemical cues in modulating the behavior of two size classes of A. junius larvae. This study demonstrates that (1) migration can influence the size structure of larval A. junius assemblages; (2) cannibalism is most likely in highly size-structured assemblages of A. junius; and (3) the dynamics of intraguild predation interactions are strongly influenced by the degree of size structure in assemblages of A. junius.