Trickle-up economics: effects of resource availability on herbivores, enemies and ecological networks

Variation in the abiotic environment can have dramatic and ramifying effects across trophic levels. One ubiquitous source of environmental heterogeneity is availability and quality of soil resources, which alters a suite of plant quality traits including leaf toughness, leaf water content, and leaf Nitrogen, as well as investment in anti-herbivore defense. Such physical and chemical changes in plants have been found to shape herbivore performance and feeding behavior, as well as mediate interactions between herbivores and their natural enemies. While many studies have examined effects of plant quality traits on herbivores, most of these have focused on agricultural systems or restricted their work to the laboratory; few studies have explored effects of abiotic resource heterogeneity in the natural systems, and many fewer have extended their work to the third trophic level. In the present study, I investigate how plant-herbivore-parasitoid interactions vary across natural heterogeneity in soil resources and, subsequently, in plant quality. I use network analysis to describe emergent properties of a chaparral shrub-caterpillar-parasitoid system in Northern California, across a natural soil mosaic of low resource serpentine and higher resource non-serpentine soils. Specifically, I ask 1) how does tritrophic network structure differ across a heterogeneous soil environment; 2) does variation in abundance and functional specialization of generalist species drive these differences; and 3) how might plant traits interact with predation to alter abundance and host preference of herbivores.