Plant Quality Mediates the Response of Disease to Global Environmental Change

Abstract

A major challenge for ecology rests in understanding how direct and indirect effects of abiotic and biotic drivers combine to influence organisms under rapid environmental change. The local environment of both hosts and parasites can have profound impacts on disease dynamics, but the major mechanisms underlying these changes remain largely unresolved. In this dissertation, I combine a series of manipulative experiments to assess the effects of an ongoing and pervasive driver of global environmental change, elevated CO2, on a plant-phytophagous host-parasite system. In Chapter II, I investigated the effects of elevated CO2 on the defensive and nutritional chemistry of milkweeds and the subsequent impacts of those phytochemical changes on monarch tolerance and parasite virulence. I found that high-cardenolide milkweeds lost their medicinal properties under elevated CO2; monarch tolerance to infection decreased, and parasite virulence increased. Declines in foliar medicinal quality were associated with declines in foliar concentrations of lipophilic cardenolides. In Chapter III, I examined how those same phytochemical changes induced by elevated CO2 influence the defensive phenotype of monarchs against predation, e.g. toxin sequestration and flight ability. I found that monarchs maintained the concentration and composition of cardenolides that they sequestered despite changes in the phytochemistry of milkweed under elevated CO2. Additionally, feeding on high cardenolide milkweed was associated with the formation of rounder, thinner wings, which are less efficient at gliding flight. In Chapter IV, I evaluated changes in monarch cellular and humoral immunity in response to phytochemical shifts induced by elevated CO2. I found that the immune enzyme activity of early-instar monarchs declined under parasite infection but was “rescued” by consuming foliage grown under elevated CO2. Additionally, infection and a diet of foliage from elevated CO2 increased the hemocyte concentrations of early-instar monarchs. In late-instar monarchs, the immune response against parasitoids declined on “medicinal” milkweed, suggesting a potential tradeoff between resistance against parasitoids and resistance against agents of disease. Finally, in Chapter V, I examined how elevated CO2 might alter plant resistance traits and regrowth tolerance and the subsequent relationship between these defense strategies. I found that elevated CO2 altered the resistance of regrowth foliage in a species-specific manner. However, the tradeoff between resistance and regrowth tolerance varied only among milkweed species. Taken together, my research illustrates that anthropogenic changes in abiotic and biotic factors operate in complex combinations, and at multiple scales, to influence the outcome of host-parasite interactions in our changing world.