Trophic shift in δ15N and δ13C through galling arthropod communities:: Estimates from Quercus turbinella and Salix exigua

Galling arthropod communities have long been a model system for community ecologists, yet much remains to be explored concerning trophic interactions between hosts, herbivores, and natural enemies. While the utilization of stable isotope ratios can help to elucidate complex trophic interactions in such communities, estimates of trophic shift between community members are required before stable isotope analyses can be appropriately employed. In this chapter, we document the degree of trophic shift in carbon (delta C-13) and nitrogen (delta N-15) isotopes within galls and hosts of a cynipid gall wasp (Cynipidae) in Quercus turbinella (Fagaceae), and a gall midge (Cecidomyiidae) and sawfly (Tenthredinidae) on Salix exigua (Salicaceae). We found trophic shift in nitrogen isotopes to be reduced relative to estimates from other systems, while carbon isotopes were considerably enriched. In combination with our current results, we review estimates of trophic shift in gall communities and compare patterns of trophic shift across studies. We discuss physiological mechanisms that determine the distribution of stable isotopes throughout gall communities and their potential effect on estimates of trophic shift (delta C-13 and delta N-15).