Patterns of Trophic Shift in δ<SUP>15</SUP>N and δ<SUP>13</SUP>C Through a Cynipid Gall Wasp Community (<I>Neuroterus</I> sp.) in <I>Quercus turbinella</I>

Abstract

The degree of shift in stable isotope ratios between trophic levels, known as trophic shift, can help elucidate trophic interactions in systems not amenable to conventional analyses. Gall wasp 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. Before stable isotopes can be successfully applied to trophic interactions within gall communities, quality estimates of trophic shift between community members are required. In this study, we document the degree of trophic shift in carbon (&dgr;13C) and nitrogen (&dgr;15N) isotopes within a simple cynipid gall wasp community, Neuroterus sp. (Cynipidae), in Quercus turbinella Greene (Fagaceae). The trophic shift in &dgr;15N between Neuroterus sp. and Q. turbinella was much lower than values reported for Cecidomyiid gallers, whereas the shift in &dgr;15N between Neuroterus sp. and its parasitoid, Omyrus sp., was similar to that reported for parasitoids. The trophic shift in &dgr;13C was considerably greater in Neuroterus sp. than previous estimates from other types of herbivores, whereas Omyrus sp. exhibited a trophic shift in &dgr;13C similar to other biological systems. The unusual trophic shift in &dgr;13C in Neuroterus sp. is likely a result of metabolic differences between host and gall tissues. We discuss commonalities in the observed trophic shift of &dgr;13C and &dgr;15N in the Neuroterus sp. community to other biological systems and postulate physiological mechanisms for deviations from reported estimates of trophic shift.