Trophic structure of a macroarthropod litter food web in managed coniferous forest stands: a stable isotope analysis with δ15N and δ13C

We studied the composition of a litter detrital community in a temperate coniferous forest using stable isotopes of nitrogen and carbon. Samples of mineral soil, bulk litter material, macroarthropods and understory plants were collected from ten experimental forest stands. Half of the stands were previously thinned 17-42 years ago, the other half served as controls. Values of δ15N and δ13C were based on the analysis of almost 500 individuals of at least 22 species in 11 arthropod families. The isotopic analysis showed a significant increase in δ15N and δ13C values with soil depth. Isotopic signatures of macroarthropods ranged from −26.51‰ to −20.52‰ for δ13C and −2.85‰ to 5.10‰ for δ15N. All consumers showed levels of 13C enrichment substantially higher than those of primary producers and litter. Predators were generally significantly more 15N enriched than detritivores and herbivores, but their δ13C levels were similar to those of primary consumers. Our data indicate that this community consists of at least 2-3 trophic levels with a considerable amount of variation in the 15N enrichment among detritivores and predators. We suggest that the spread of δ15N values of predators likely reflects the diversity of potential prey among detritivores and a varying degree of intraguild predation among different species. Our findings generally agree closely with the results of similar studies from other forest litter communities. Thinning did not appear to influence the overall isotopic composition of the detrital food web. Extensive omnivory and intraguild predation among litter consumers may buffer long-term effects of thinning on the trophic structure of these species-rich communities.