Trophic diversity and niche partitioning in a species rich predator guild – Natural variations in stable isotope ratios (13C/12C, 15N/14N) of mesostigmatid mites (Acari, Mesostigmata) from Central European beech forests

A large number of predatory mesostigmatid mite species populate forest soils in high densities. The present study investigates the trophic structure of the Mesostigmata community of old growth beech stands in Central Germany and identifies potential prey groups using natural variations in stable isotope ratios (13C/12C and 15N/14N). Data on relative abundances and body mass were included for each of the 40 species studied to analyze functional aspects in Mesostigmata feeding ecology. The results indicate that Mesostigmata predominantly feed on secondary decomposers, whereas primary decomposer and intra-guild prey are of minor importance. Dominant species featured high δ13C signatures suggesting that they predominantly feed on species relying on root derived resources such as bacterial feeding nematodes. Less abundant species where characterized by lower δ13C values suggesting that they predominantly feed on prey relying on litter derived resources such as fungal feeding Collembola. Related taxa often had distinctively different isotope ratios suggesting that trophic niche partitioning facilitates coexistence of morphologically similar species. Unexpectedly, the trophic position of Mesostigmata species was not related to body size reflecting the varying trophic position of their main prey, nematodes and Collembola, suggesting that body size is a poor predictor of trophic position in soil food webs.