| dc.description.abstracteng | Soils are complex and heterogeneous habitats for soil animals. Trophic interactions between soil animals are depicted in soil food webs which form an essential component of terrestrial ecosystems. Soil food webs are based on predator-prey interactions and reflect the flux of matter and energy through ecological systems. The soil food web is compartmentalized in distinct energy channels that process energy in different ways. The main energy channels in forest systems are the bacterial, fungal and plant litter energy channel with the bacterial channel probably being the fastest. However, the importance of predator-prey interactions in these channels remains little understood. Especially nematodes are an understudied group in soil food webs since they are small, difficult to determine and also difficult to detect as prey organism.
I investigated the role of nematodes as prey for microarthropods using molecular gut content analysis. Therefore, specific primers for certain prey taxa were used to screen potential predators for presence of the respective prey. With this approach the consumption and distribution of certain prey among many potential predators can be investigated (bottom-up view). By screening many individuals the importance of predator species as antagonists of nematode prey species can be evaluated (top-down view). Thereby, molecular gut content analysis provides the opportunity to investigate predator-prey interactions allowing to trace trophic links between certain prey taxa and higher consumers of the soil food web.
In the first study (Chapter 2) I investigated if nematodes serve as prey for microarthropods. Therefore, the entomopathogenic nematodes Phasmarhabditis hermaphrodita and Steinernema feltiae were used as model organisms to investigate if soil mites, especially species that have been assumed to live as decomposers, include nematodes in their diet. Established molecular markers for the two nematode species were used in these studies. To confirm detection of predation events, I investigated how long nematode DNA can be traced in the gut of the oribatid mite species Steganacarus magnus. In the field I investigated if soil mites preferentially consume dead or living nematode prey and if active predation for nematode prey occurred. The results indicate that nematode DNA can be traced for up to 128 h in the gut of S. magnus confirming good detection of nematode prey during the experiments. However, the detection time of prey DNA varied between nematode species and depended on the exposure time of the nematodes to the mites. Soil mite species consumed the two model nematode species in the laboratory and in the field suggesting that nematodes form part of their regular diet. In the field experiment many ‘classical decomposer’ soil mite species fed on the nematodes P. hermaphrodita and S. feltiae. Living and dead nematodes were consumed indicating that both feeding modes, predation and scavenging, occur. The mites differentially consumed the two nematode species depending on whether they were dead or alive pointing to nematode defence mechanisms that influence this predator-prey interaction.
It is very likely that microarthropods also feed on indigenous nematode species and may significantly impact nematode communities; however, no molecular markers exist to test this assumption. Therefore, I designed specific primers for four free-living bacterial feeding soil nematode species (Acrobeloides buetschlii, Panagrellus redivivus, Plectus minimus and Plectus velox) and established them for molecular gut content analyses of potential microarthropod predators (Chapter 3). The specificity of the molecular markers was confirmed by a non-target test to check for cross-reactions, and the sensitivity was confirmed by a two-fold serial dilution of prey DNA. The newly designed molecular markers amplify sensitively taxon-specific 18S rDNA up to 128 h after ingestion in the gut of a microarthropod predator. The detection time for the respective nematode species varied between nematode species, mite species and time of exposure indicating that these prey detection times differ for every predator-prey interaction. Abundant soil mite and collembolan species were shown to feed on these nematode species in the laboratory and on A. buetschlii and Plectus spp. in the field indicating that indigenous nematodes indeed form part of the diet of soil microarthropods including those previously assumed to live as detritivores. Nematode-predator interactions presumably contribute significantly to the flux of energy from root exudates via bacteria to higher trophic levels.
Forest soils are patchy habitats comprising different microhabitats, such as litter, moss and grass. These microhabitats significantly affect the density and distribution of nematode and microarthropod species, but also contribute to variations in interactions between soil animal species. I investigated if A. buetschlii and Plectus spp. were differentially consumed by fourteen abundant soil mite species including Mesostigmata and Oribatida from litter, moss and grass using molecular gut content analysis (Chapter 4). The mites differentially consumed the two nematode taxa related to their density and the consumption of nematodes differed between the habitats. Our results indicate shifts in trophic niches with changing habitat characteristics which likely contribute to the high diversity of microarthropods in deciduous forests.
By designing and establishing specific nematode markers for molecular gut content analysis to investigate the role of indigenous bacterial feeding nematodes as prey for microarthropods this thesis provides promising tools to investigate how carbon is channelled from roots over bacteria to higher trophic levels. Although the few analyzed nematode taxa only represent a small fraction of the nematode community of temperate forest soils, they were frequently detected as prey of microarthropods - including those previously thought to live as decomposers - suggesting that the impact of microarthropods on nematodes is high. Overall, the results represent a major step forward for the understanding of soil animal food webs and highlight that the decomposer food web is more complex and trophically diverse than previously assumed. | de |