Trophic relations of decomposters on alpine pastureland

Earthworms are important macrodecomposers and engineers on alpine pastureland. They obviously affect decomposition and soil physical properties through digestion and bioturbation. However, there is little explicit recognition of the complexity of the processes which regulate the interactions of these soil invertebrates with microbes. These interactions determine the food palatability and food selection of these animals and finally the species composition and functional biodiversity. To investigate the feeding ecology of the main decomposer species (earthworms and millipedes) on managed and abandoned alpine pastureland in the Central Alps three approaches will be followed up: In the proposed project (1) we will reveal if the bacterial community in the invertebrate decomposer gut is, both in functional and structural sense, determined by the ingested food (litter type) or if it is species-specific, (2) we will verify the trophic positions of the macrodecomposers by assessing stable isotope shifts between the saprotropic species and their diets and (3) we will test if decomposer species preferentially feed on dwarf shrub litter in their natural habitats on abandoned pastureland as laboratory experiments have already suggested. (1) With the help of DNA-based methods the bacterial gut communities in earthworms will be assessed. The phylogenetic affiliation of the microbial communities within the gut of the saprotrophic species will be determined using denaturing gradient gel electrophoresis (DGGE). The metabolic abilities of the gut microflorae as well as the bulk soil will be analysed with community level physiological profiles (CLPP). (2) In a controlled laboratory experiment the isotope enrichment rates, which are species-specific and diet-related, between the soil animals and their diets will be tested. In a field experiment decomposer species will be offered litter types of different quality (grass and labelled dwarf shrub litter) to evaluate their food preferences. The rationale of this study is to improve the fundamental understanding of diversity-function relationships in decomposition processes under changing conditions at the alpine treeline. The linkage of soil zoological and microbiological approaches and the application of molecular techniques as well as the findings of the stable isotope shift experiment will promise important and innovative data on this subject.

Earthworms and millipedes are important decomposers and engineers on managed and abandoned alpine pastureland. They affect decomposition and soil physical properties through digestion and bioturbation. However, there is little explicit recognition of the processes which regulate the interactions of these soil invertebrates with microbes, although these interactions determine the food palatability and selection of the decomposer animals. In the completed project we investigated (1) the interactions between decomposer species (earthworms and millipedes) and microbes by analysing the gut microflora of these animals and (2) their feeding ecology by conducting several controlled laboratory and field experiments. Most importantly, we were able to demonstrate that (1) the gut microbiota of earthworms (L. rubellus) is diet-related, while the gut microbiota of millipedes (C. fulviceps) is species-specific. (2) earthworms and millipedes feed on dwarf shrub litter in their natural habitat, which supports findings of our previous project where we showed that this litter type, though of poor nutritional quality and palatability, is a potential food source for decomposer species. Furthermore, we tested whether it is possible to molecularly identify soil animals which are important in the process of humus formation. As the soil is an opaque habitat where direct observations are difficult, indirect methods to study soil invertebrates are urgently needed. In a controlled laboratory experiment we were able to show that DNA of decomposer animals can be detected in their faecal pellets, which form a central part of many humus forms. This finding is an important prerequisite for any application of this method in the field. We also investigated the connection between litter quality, microbial community structure and feeding preferences of earthworms. Although experiments and analyses are still ongoing, we can rule out, that polyphenolic compounds play a central role in the food selection of earthworms.