High consistency of trophic niches in soil microarthropod species (Oribatida, Acari) across soil depth and forest type

Abstract

Individuals of species may differ in resource use within and between populations. High intraspecific variation in resource use may hamper the co-existence of species in natural communities. To better understand the intraspecific variation in trophic niches of oribatid mites (Oribatida, Acari), we quantified stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) of 40 Oribatida species that co-occurred in litter and soil of five forest types (European beech, Douglas fir, Norway spruce, two beech–conifer mixed forests) covering a range of environmental conditions. We found that although stable isotopes in litter and soil varied among forest types, δ13C and δ15N values of Oribatida and their trophic niches were remarkably stable between litter and soil, and also among forest types. We considered four trophic guilds of Oribatida representing the guild composition of the regional species pool; notably, trophic niches of Oribatida guilds also did not vary with soil depth. Furthermore, δ13C of Oribatida was more enriched (detrital shift) in European beech than in coniferous forests, but δ15N of Oribatida did not vary among forest types, indicating that basal resources of Oribatida are variable, but trophic positions are highly consistent across forest ecosystems. We conclude that trophic positions of Oribatida species and guilds are consistent across different forest types, and Oribatida species occupy virtually identical trophic niches irrespective of the soil depth they are colonizing. Overall, the results suggest that low intraspecific variability facilitates Oribatida niche differentiation and species coexistence.