Feeding biology and diversity of oribatid mites (Oribatida, Acari)

The aim of this study was to understand the regulatory mechanism of the high diversity of soil living oribatid mites (Acari, Oribatida) which appear to coexist without extensive niche differentiation. I focussed on trophic niche differentiation, perturbation and resource quantity and quality as three putative mechanisms. Two food choice experiments, one stable isotope analysis and two field experiments were conducted. Dark pigmented fungi (‘Dematiacea’) are generally preferred as diet by soil microarthropods, such as collembolans and oribatid mites. I investigated the feeding preferences of ten oribatid mite species among eight ‘Dematiacea’ in a food choice experiment. The investigated oribatid mite species differentially fed on dark pigmented fungal species. Since overall oribatid mites preferred two ‘Dematiacea’ (Alternaria alternata and Ulocladium sp.), I conclude that niche differentiation in oribatid mites in respect to dark pigmented fungi is limited, but nevertheless may contribute to the high diversity of this microarthropod group. Mycorrhizal fungi reach high densities in forest soils and are a potential food resource for oribatid mites. I investigated the feeding preferences of three oribatid mite species among six ectomycorrhizal and one ericoid mycorrhizal fungal species in a food choice experiment. Feeding preferences significantly differed between the oribatid mite species. However, overall, oribatid mites preferred two mycorrhizal species, Hymenoscyphus ericae and Boletus badius. This also supports the hypothesis that trophic niche differentiation in oribatid mites occurs, but appears not to be sufficient to solve the enigma of the large number of oribatid mite species in soil. Trophic niche differentiation of 36 oribatid mite species/taxa from four forests was investigated using stable isotope analysis (15N). The oribatid mite species investigated formed a gradient from phytophagous species over primary and secondary decomposer to predators and scavengers (four trophic levels/feeding guilds). These surprisingly large differences in 15N values of oribatid mite species strongly indicate that oribatid mites occupy different trophic niches in the field. This finding clearly supports the view that trophic niche differentiation is an important mechanism for the maintenance of the high number of oribatid mite species in forest ecosystems. Intermediate disturbances may contribute to the coexistence of large numbers of plant and animal species. Therefore, I investigated the effect of three perturbation levels (single perturbation, perturbation once every two months and once every two weeks) on oribatid mite diversity and density in a field experiment. I hypothesised that the species number of oribatid mites is highest at the intermediate disturbance level (one perturbation every two month). In contrast to this hypothesis, oribatid mite diversity and density continually decreased with increasing perturbation level. These results suggest that the great majority of oribatid mite species are sensitive to disturbances irrespective of intensity and frequency. Therefore, the intermediate disturbance hypothesis presumably does not apply for oribatid mites. The effect of the temporary enhancement of resources of different nutrients (wood, glucose & nitrogen & phosphorous (CNP), pet food, wheat bran) on oribatid mite diversity and density was investigated in a field experiment in the Solling (Germany). I hypothesised that the number and the density of oribatid mite species increase with the amount and quality of resources. The results of this experiment did not support this hypothesis, since the diversity of oribatid mites generally did not respond to the experimental treatments. The density of oribatid mites was slightly higher in the CNP-treatment compared to the pet food treatment. However, this increase likely resulted from indirect effects due to the decrease in earthworm density in the CNP-treatment. The results indicate that the oribatid mite community is not limited by the amount or quality of resources but rather by the burrowing and mixing activity of earthworms. Overall, the results of the present study suggest that the coexistence of the large number of oribatid mites in soil cannot be explained by the presence of intermediate levels of disturbances or the amount and quality of resources but rather by the presence of trophic niche differentiation. The comparison of the results of the laboratory food choice experiments and the field study using stable isotopes indicate that oribatid mites feed on a wide range of substrates (when offered in the laboratory) but occupy rather distinct trophic niches in the field. To understand the factors that form these niches, further studies on the feeding biology of oribatid mites are necessary. In the future, recently established techniques such as the analysis of phospholipid fatty acids and the analysis of gut contents by molecular tools may contribute to the understanding of niche differentiation in oribatid mites.