A fundamental goal of ecology and evolutionary biology is to elucidate the factors that determine what proportion of plant biomass is consumed by herbivores. Ecologists have long recognized that predators play a role by suppressing herbivore populations. At the same time, work by evolutionary biologists has shown that the vulnerability of an herbivore species to predators is determined by both plant nutritional quality and the diversity of plant species included in that herbivore's diet. The work funded here seeks to integrate these separate but complementary areas of research. The overarching hypothesis to be tested is that plant nutritional quality and herbivore diet breadth together determine an herbivore's vulnerability to predators. In so doing, plant quality and herbivore diet breadth are predicted to determine the importance of predators in plant protection, as well whether predators act as source of natural selection that influences the evolution of plant nutritional quality. The proposed work tests this hypothesis using an experimental garden of native shrubs and their associated insect community. Genetic lines of plants that vary in nutritional quality will be grown in experimental treatments with herbivores alone and with herbivores and predators together. These plants will be fed upon by one of two species of herbivore: an aphid that feeds only on one plant species or an aphid that feeds on a wide diversity of plants. This experiment will thus measure the effects of predators on herbivores and plants, and how such effects are determined by herbivore diet breadth and plant quality.
The primary broader impact is an initiative to establish a public garden of native shrubs in the University of California-Irvine Arboretum, one of two botanical gardens featuring the native flora of California in Orange County (population > 3 million). This display will provide the public with a hands-on demonstration of the variation in plant traits that determine its nutritional quality for herbivores, as well as information on the natural history of the associated insect community. The garden will also serve as a key resource for independent research and teaching opportunities for undergraduate students.