New perspectives on soil animal trophic ecology through the lens of C and N stable isotope ratios of oribatid mites

Abstract

Knowledge of the trophic ecology of soil animals is important for understanding their high alpha diversity as well as their functional role in soil food webs and systems. In the last 20 years, the analysis of natural variations in stable isotope ratios (15N/14N, 13C/12C) has revolutionized our view on soil animal trophic ecology. Here, we review the state of the art of the trophic ecology of a highly abundant and diverse soil animal taxon, oribatid mites (Oribatida), investigated by stable isotope analyses. The review is based on 25 papers reporting stable isotope data of 292 oribatid mite taxa from 30 different sites. Four main findings emerged. (1) Oribatid mites cluster into six trophic groups, i.e. moss feeders, lichen feeders, primary decomposers, fungal feeders/secondary decomposers, predators/scavengers and marine algal feeders, plus one additional group, which incorporates CaCO3 in their cuticle for defence but still belongs to the fungal feeders/secondary decomposers group. (2) Of the 292 species studied 43.7% were classified as fungal feeders/secondary decomposers, 27.0% as primary decomposers and 15.7% as predators/scavengers, only few species include CaCO3 into their skeleton (6.1%), feed on lichens (4.9%), mosses (2.1%) or marine algae (0.7%). (3) In about one-third of the species studied the trophic niche was constant or varied little between sites or habitats, but in two-thirds of the species their trophic niche varied between habitats, with some species even shifting trophic levels, indicating trophic plasticity. (4) When aggregated at higher taxonomic level oribatid mite species clustered in only three instead of six trophic groups. This indicates that species within the same high level taxon often belong to different trophic groups, for example because feeding habits evolved convergently. Therefore, to accurately reflect the trophic ecology of oribatid mites their stable isotope signatures need to be analysed at species level. However, stable isotope analyses also have limitations, e.g. feeding on bacteria and fungi cannot be separated, and the same is true for feeding on ectomycorrhizal and arbuscular mycorrhizal fungi. Other methods such as fatty acid, amino acid and molecular gut content analyses as well as microbiome analyses may complement stable isotope studies and resolve oribatid mite trophic niche differentiation at a higher resolution. This will contribute to a better understanding of the local coexistence of large numbers of species in soil. Finally, we provide perspectives on how to integrate microarthropods into soil food webs using stable isotope and other methods allowing deeper insight into their trophic structure.