Stable N-isotope signatures of central European ants - assessing positions in a trophic gradient

Studies employing stable isotope technology have greatly contributed to understanding trophic relationships of tropical ants, but temperate-zone ants remain under-explored. We studied delta N-15 values of 43 ant species from three subfamilies sampled across central Europe. After statistically accounting for the effects of elevation and geographical location of habitats, which alter the isotopic composition of nitrogen in ecosystems, significant patterns in N-isotope signatures were detected. These signatures hint at differences across ants in the contribution of plant-derived nitrogen obtained via trophobiosis and nectarivory relative to nitrogen obtained via predation and scavenging. In general, Myrmicinae had higher delta N-15 values than Formicinae, in line with a greater relative importance of trophobiosis in the latter. The genus Myrmica scored especially high, indicating predominantly predacious nitrogen sources. Remarkably, also the granivore Messor cf. structor had high delta N-15 values. This suggests that, despite the major portion of food uptake being made up by plant seeds, this ant could derive substantial fractions of its nitrogen budget from feeding on arthropod corpses or vertebrate faeces. Moreover, this highlights that deductions from observed quantities of ingested food on its relative contribution to ants' matter balance should be accompanied by isotope analyses. At the other end of the spectrum, Camponotus and Plagiolepis had low delta N-15 values. In line with multiple field observations, this suggests a contribution of trophobiosis not only to their energy, but also to their nitrogen budget. Formica and Lasius had intermediate N-15 values, which is in agreement with the current view that these ants have mixed diets with a balance between trophobiosis and predation. A possible influence of endosymbiotic bacteria on the isotope signatures of several genera is discussed. This study provides a first application of stable isotope technology to estimate the role of plant-derived nutrients to the nitrogen budget of a larger range of central European ants. Furthermore, it shows that N-isotope analysis is applicable across extended ecological and geographical gradients. Future studies along this line are promising to complement our current understanding of the nutritional ecology of temperate-zone ants.