The use of food resources is one of the most important aspects of ecosystem functioning. Trophic relationships determine fluxes of matter and energy, shape interactions between organisms and ultimately direct the evolution of the species themselves. Competition is a fundamental biotic interaction, and niche partitioning constitutes an important mechanism to allow species coexistence. However, many other factors influence community structuring, and may change or supplant the outcomes of competition. Ants are one of the most abundant, widespread and ecologically relevant terrestrial organisms. On the ground of tropical forests, dozens of species may coexist, which raises the question: how similar are they? Behavioral and environmental mechanisms of coexistence have been proposed for ants, but the use of resources itself is surprisingly understudied, and the trophic niches of most species is unknown. In this thesis, I used three complementary methods, representing a gradient of source-specificity/time-representativity, to describe patterns of resource use in a tropical and a temperate ant community. In the first study, I reviewed the available literature on resource use for the identified tropical species and compared it to field data obtained with baiting. Previous information was scant or inexistent for most species. Ants broadly used most resources available, but with quantitative differences between species. Wasmannia auropunctata has the most specialized niche, using only feces, a new behavior for the species. In the second study, my coauthors and I performed a laboratory experiment to describe fatty acid assimilation in ants. Two temperate ant species (Formica fusca and Myrmica rubra) displayed similar patterns and dynamics in composition, although amounts were influenced by their reproductive status. The main fatty acids (C16:0, C18:0 and C18:1n9) were extensively synthesized from sugars, but we observed some diet-specific ones that might work as biomarkers (C18:2n6, C18:3n3, C18:3n6). The experiment fulfilled a basic knowledge gap and set the ground for application of fatty acid analysis in an ecological context. In the third study, we put together field observations, fatty acid and stable isotope analyses to describe overall patterns of resource use and species’ niches in both communities. We observed a consistent picture of high, and quantitatively equivalent, generalism in both communities, regardless of species richness. Temperate species presented fatty acid patterns distinct from tropical ones, which may be related to environmental factors. Similarities in bait attendance, fatty acid compositions and isotope signatures were all correlated in Brazil, thus all methods corresponded in their characterization of species’ niches to some extent, and were robust enough to detect differences even in a highly generalized community. Method complementarity was particularly important to understand the behavior of the most specialized species. In Germany, no correlations were observed, likely due to the small number of species available. Fatty acid analysis emerges as a powerful tool and may be applied to answer many questions related to resource use in ants, but use of fatty acids as biomarkers seems to be limited. In general, the results of this thesis agreed with the recent view that specialization does not increase with species richness towards the tropics. Several coexistence mechanisms may act to structure ant communities, with trophic niche partitioning playing a relatively small role in the ones we studied. No mechanism appears to be universal and community structure may be better understood on a case-by-case basis, at local scale
