Herbivory and pollination in hemiparasitic plants: Effects of host plant and host-obtained alkaloids

The role of secondary plant compounds in resistance to herbivory has been recognized for several decades, but the effects of secondary compounds on plant-pollinator interactions are only beginning to be explored. The difficulty of manipulating secondary compounds in planta has made detection of effects on multispecies interactions impracticable. Hemiparasitic plants, which take up secondary compounds from their host plants, provide a unique opportunity to study chemically mediated interactions between plants, herbivores and pollinators.; This dissertation addresses several aspects of the interactions between host plants, hemiparasitic plants, herbivores and pollinators. Chapter One examines host effects on hemiparasite ecology. The hemiparasite Castilleja miniata had increased seed set and reduced herbivory when parasitizing Lupinus argenteus than when parasitizing other hosts, but pollinators did not discriminate between C. miniata plants parasitizing different hosts.; To assess the effect of alkaloid uptake on herbivory, pollination, and plant fitness, I grew individuals of the hemiparasite Castilleja indivisa with low-alkaloid or high-alkaloid lines of Lupinus albus . Alkaloid uptake resulted in decreased herbivory, increased pollination, and increased lifetime seed set in the hemiparasite (Chapter Two).; Path analysis demonstrated that alkaloids directly reduced floral herbivory but did not have a direct effect on pollinators (Chapter Three). Rather, alkaloid uptake resulted in increased pollinator visits due to more attractive floral displays from reduced herbivory.; Chapter Four describes the alkaloids transferred from Lupinus hosts to Castilleja hemiparasites. Alkaloids were detected in floral structures but not in nectar, providing an explanation for the direct effect of alkaloids on floral herbivores but not pollinators.; Chapter Five examines heritability of host preference in hemiparasites. Maternal families of Triphysaria pusilla varied in their preference for different hosts, but this preference appeared to be due to maternal environment rather than genetic effects.; Chapter Six presents a review of 'toxic nectar', and introduces the hypothesis that toxic nectar occurs as a consequence of defense against herbivores.; In summary, alkaloid uptake can have a large impact on the ecological interactions between multiple species, and the outcome of multispecies interactions may not be predictable from pairwise interactions.