Trophic diversity in a Mediterranean food web-Stable isotope analysis of an ant community of an organic citrus grove

Ants as generalist predators and mutualists of herbivores can play an important role in relative stable agroecosystems like plantations. The categorization of the diverse life strategies and traits into ecological groups like trophic levels is essential for a better understanding of food web structures and a better prediction of changes in communities. Stable isotope technology provides simultaneously detection of trophic levels and the ultimate C source of many species. We studied a highly diverse Mediterranean ant community in an organic citrus grove in Tarragona, NE Spain, and analyzed stable isotope contents of 17 species of ants together with dominating plants and important spider and aphid species to establish trophic guilds and detect seasonal changes. The results revealed significant differences between species spanning over a huge range in delta N-15-values of at least 10.67% which is only comparable to a Peruvian tropical forest with a much higher species diversity. The trophic levels of ants reflected most of previous knowledge on predaceous vs. plant feeding habits. Messor harvester ants and Camponotus species had the lowest delta N-15-values. Aphids, smaller spider species, and most other ant genera, including the dominating species Formica rufibarbis and Lasius grandis, had intermediate delta N-15-levels. The large spider Dysdera crocata and the typical Mediterranean ant Pheidole pallidula had higher delta N-15-values, but two specialized predatory ants with very tiny workers had the highest trophic level. We found unexpectedly high delta C-15-values with a high seasonality for several ground-living ant species. The possible role of soil fauna as a second main food resource besides the most commonly analyzed green food chain is discussed. Our results support the hypothesis that the strong seasonality intrinsic to Mediterranean climate and the high heterogeneity of different plant resources and microclimatic conditions in the organically managed plantation are reflected by a notably high trophic diversity of the ant community.