|Oribatid mites are involved in decomposition processes, formation of soil structure and nutrient cycling in most soils worldwide. The main habitats of oribatid mites are litter and soil but they also occur numerously on tree trunks, dead wood, marine intertidal zones and freshwater habitats. Especially in forest ecosystems they reach high local density and diversity and often dominate arthropod fauna in edaphic and arboreal habitats where they form an important part of the food web. Although oribatid mites are often regarded as a functional group of primary decomposers they feed on a wide range of food resources including lichens, mosses, algae, litter, fungi and nematodes. The exceptionally high proportion of oribatid mite species reproducing via parthenogenesis and the co-occurrence with sexual species in the same habitats make oribatid mites an interesting model group for ecological and evolutionary research.
In this thesis we investigated oribatid mite density, diversity, community structure and the proportion of parthenogenetic individuals across regions, forests types and habitats. Furthermore, we investigated the importance of root-derived resources for soil living oribatid mite communities by root-trenching, and assessed the trophic diversity on the bark of dead wood using stable isotopes.
In Chapter 2 we investigated how oribatid mite communities on the bark of early decaying dead wood are affected by log species (beech vs. oak), forest type (deciduous vs. coniferous) and region (Schorfheide-Chorin, Hainich-Dün and Schwäbische Alb). We further assessed the trophic structure of dead wood communities using stable isotopes (15N and 13C) in one region (Hainich-Dün). The results suggest that dead wood of an early decaying stage is colonized by a mixture of edaphic and arboreal oribatid mite species and only few dead wood specialists. Stable isotope analysis provided evidence that dead wood is colonized by a community of high trophic diversity including lichen, moss, litter and fungal feeders with the latter dominating. Compared to soil, the proportion of parthenogenetic individuals on dead wood was generally low resembling arboreal communities. We showed that oribatid mite assemblages on the bark of dead wood are shaped by log species, the surrounding forest type and the region whereas density was not significantly affected by all three factors. The results indicated that oribatid mite communities on dead wood are shaped by both neutral and niche-based process.
In the second study (Chapter 3) we investigated oribatid mite density, community structure and the percentage of parthenogenetic individuals in four different forest types across three regions in Germany in 2008 and once again in 2011. We compared temporal (inter-annual) fluctuations in population densities between sexually and parthenogenetically reproducing species of oribatid mites. Oribatid mite community structure did not differ significantly between years but varied with forest type and region, indicating low species turnover over time. Temporal fluctuations were significantly higher in parthenogenetic as compared to sexual species. The percentage of parthenogenetic individuals was significantly higher in coniferous as compared to beech forests and significantly higher in Schorfheide-Chorin as compared to Hainich-Dün and Schwäbische Alb. The results indicate that parthenogenetic species flourish if populations are controlled by density-independent factors and dominate at sites were resources are plentiful and easily available, such as coniferous forests, and in regions with more acidic soils and thick organic layers, such as Schorfheide-Chorin. However, historical factors also may have contributed to the increased dominance of parthenogenetic species in the Schorfheide-Chorin, since this region was heavily glaciated, and this may have favoured parthenogenetic species. Overall, our study supports the hypothesis that parthenogenetic species benefit from the lack of density-dependent population control whereas the opposite is true for sexual species.
In the third study (Chapter 4) we tested the significance of root-derived carbon for oribatid mite communities by interrupting the carbon flux from plants into the soil via root-trenching in two regions in Germany (Schorfheide-Chorin and Hainich-Dün). After one year, root-trenching tended to reduce total oribatid mite densities in Hainich-Dün while it had no overall effect in the Schorfheide-Chorin. Root-trenching primarily reduced densities of soil-living oribatid mites in the Hainich-Dün and primarily litter-living species in Schorfheide-Chorin. Oribatid mite community composition of both regions was not significantly affected by root-trenching. The results suggest that in contrast to previous studies only a minor part of the oribatid mite community benefits directly or indirectly from root-derived resources. The different response of oribatid mites following root-trenching in the two regions indicates that the importance of root-derived resources varies with soil structure, being more intense when litter accumulation is low. Possibly, the weak response to reduced belowground resource input was buffered by internal carbon resources of
coarse roots being cut and also by the trophic plasticity of many oribatid mite species.