WHAT CAN STABLE ISOTOPES (δ15N AND δ13C) TELL ABOUT THE FOOD WEB OF SOIL MACRO-INVERTEBRATES?

Abstract

Animals reflect the stable isotope ratios (δ15N and δ13C) of their diet, with a slight enrichment in 15N that allows the use of δ15N as a trophic-level indicator. Stable isotope contents were measured for the litter, soil, and macro-invertebrates of three temperate deciduous forest sites, in spring, summer, and autumn, to study the trophic structure of this community. No distinct trophic structure could be derived from measurements of δ13C. In contrast, when the δ15N values of all animal species were grouped together, the hypothesis of an isotopically similar diet was rejected. Therefore, the community spreads over more than one trophic level and was subdivided into detritivores and predators. The potential detritivore food sources in the forest litter and soil showed a variety of isotopic ratios. Despite this fact, the variance of the isotopic ratios of the detritivorous species was not larger than what could be expected from interspecific variability of trophic isotopic enrichment alone. This was also the case for the predators in most of the sample sets. However, in some cases this variance was significantly larger, due to a small number of species with high δ15N values. The δ15N values of the detritivores indicated that the mean δ15N value of their food was close to that measured for the superficial litter layers. The difference in δ15N between detritivores and predators was highly significant and never significantly different from the value expected for one trophic transfer (3.4‰), although often slightly higher. Most of the litter macro-invertebrate community we studied can therefore be described as belonging to two trophic levels, one feeding on the superficial litter layers (or on soil fractions that have a similar δ15N value), and a second trophic level feeding on the first, with some indication of intraguild predation among the predators. Between-site differences of up to 7‰ were found for δ15N in the litter, and the δ15N values of the whole animal community were shifted in accordance with the local value of the litter δ15N. Therefore, the trophic structure must be studied in relation to the local isotopic content of the litter. Seasonal differences in isotopic ratios of the litter or animal samples were neither large nor consistent. These findings indicate similar trophic structure of the communities at the three sites and during the three sampling periods.