Abstracts Biological Sciences

Niche and neutral processes in a dry grassland: the example of oribatid mites

by Stefanie Maaß

The overall aim of this dissertation was to evaluate the relative roles of niche and neutral based processes like environmental filtering, niche partitioning, dispersal and biotic interactions as forces structuring soil oribatid mite communities on different scales. We used different methods and theoretical approaches to gain an extensive insight into an oribatid mite community in a dry grassland in Germany but also into published data sets of oribatid mite communities from different habitats. In chapter 2, we conducted a field study based on a small-scale spatially explicit sampling design along an environmental gradient of a hillside in a dry grassland in Brandenburg, Germany. In spring and autumn 2012, we took soil cores (10cm long, Ø 5cm) and cut them into five 2cm slices to account for depth distribution of the species. We measured %water, pH, content of C and N as well as soil structure to account for major environmental variables of the location. We determined the oribatid mite community to species level, measured the body length of each individual and analyzed the stable isotope ratios 12C/13C and 14N/15N to assign species to feeding groups (i.e. fungal feeders/secondary decomposers, predators/scavengers, moss feeders, lichen feeders, etc.). For the first time, we apply niche and neutral based theories to one trophic level only which is an important but mostly ignored prerequisite. We found evidence for environmental filtering with organic matter content (i.e. content of C and N) and soil structure being the major predictors for community variance. When we tested for potential mechanisms behind niche partitioning in terms of the limiting similarity concept we did not find evidence for segregation via body size, trophic niche or depth distribution. However, our data support the hypothesis that temporal segregation based on population dynamics might play an important role and should be investigated in more detail in future studies. In chapter 3, we compiled the available literature about the effects of soil microarthropods on soil aggregation. We found a gap in the literature which might be due to handling issues during experimentation. Still, we were able to discuss potential mechanisms which can be divided into direct and indirect mechanisms. Direct mechanisms include the provision of organic material in form of faecal pellets, molts, etc. which can act as starting points for microaggregates. High local abundance of microarthropods might cause damage via feeding to the hyphal enmeshment of soil aggregates which might decrease soil aggregate stability. Organic material can additionally serve as nutrient source for microorganisms, hence increasing their growth amd increasing soil aggregation formation. Additionally, the selective feeding and transport of microbes on the cuticle of microarthropods indirectly affect soil aggregation. Finally, the interactions between soil microarthropods and bigger or smaller soil fauna need to be studied further as we expect various positive effects on soil aggregation in…