Compartmentalization and energy channels within the soil animal food web investigated by stable isotope (13C and 15N) and fatty acid analyses
Kompartimentierung und Energie-Kanäle im Bodentier-Nahrungsnetz untersucht mittels Isotopen- und Fettsäuremuster-Analyse
by Melanie Mira Maraun
Date of Examination:2012-02-09
Date of issue:2012-11-12
Advisor:Prof. Dr. Stefan Scheu
Referee:Prof. Dr. Stefan Scheu
Referee:Prof. Dr. Teja Tscharntke
Referee:Prof. Dr. Ulrich Brose
Series:Biodiversity and Ecology Series - B; 7
Persistent Address:
http://dx.doi.org/10.3249/webdoc-3756
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Abstract
English
Forest soil food webs are complex and heterogeneous systems. Trophic relationships are hidden from direct observation due to small size of soil animals, cryptic habitat and complex mixtures of basal resources that are not easily separable. Despite the fundamental role of aboveground-belowground feedbacks and the major importance of decomposition processes for ecosystem functioning, the decomposer subsystem has become only recently the subject of ongoing research. In this thesis, we investigated the trophic compartmentalization of the decomposer food web and traced energy fluxes to different compartments within the food web using stable isotope analyses of δ15N and δ13C and compound specific 13C fatty acid analyses. To improve the applicability of fatty acid analyses for field studies we additionally investigated whether marker fatty acids for specific food sources are transferred to higher trophic levels including predators, and we studied the time span required to detect marker fatty acids in consumers after consumption of a specific food source as well as the time that marker fatty acids of the previous diet can be detected after switching to a different food source. In our first study (Chapter 2) we depicted the trophic compartmentalization of the soil animal food web using the natural variation of δ15N and δ13C in basal resources and soil animals. We showed that the trophic compartment of primary decomposers utilizing leaf litter directly is comparatively small and hypothesized that it is of minor importance for the decomposer food web. The largest compartment comprised secondary decomposers presumably feeding on ectomycorrhizal fungi and predators. Due to similar δ13C signatures of primary decomposers and ectomycorrhizal fungi, we were not able to separate predators preying on primary decomposers from ectomycorrhizal fungal feeders and therefore could not further resolve feeding strategies within this largest compartment of the soil animal food web. By supplying specific and relative markers for bacteria, fungi and plants, fatty acid analysis was potentially applicable to obtain a finer resolution of feeding strategies within trophic compartments. To verify trophic transfer of marker fatty acids from basal resources to higher trophic levels including predators, we conducted a laboratory experiment (Chapter 3) in which we fed two major predators, the centipede Lithobius forficatus and the spider Pardosa lugubris, with the collembolan Heteromurus nitidus kept on different diets, including fungi, bacteria and tree leaves. Marker fatty acids for the respective diets were transferred over three trophic levels to predators; and predators could reliably be assigned to specific basal resources according to their fatty acid profiles, suggesting that fatty acid analysis is adequate for the analysis of whole food webs. In another laboratory experiment (Chapter 4) we investigated physiological parameters of fatty acid metabolism, such as the chronological change of fatty acid biomarkers in collembolans when switched between different food sources and the change of fatty cid biomarkers during starvation. Fatty acids typical for a specific diet were already present in the neutral lipids of consumers after one day, and were still detectable 14 days after switching to a different diet. During starvation, there were only minor changes in fatty acid composition, with marker fatty acids being still detectable in sufficient amounts after 14 days of food deprivation. Hence, fatty acid analyses provide a reliable and integrative measure of dietary composition, even for short and intermediate time intervals. After verifying the applicability of fatty acid analyses for food web analysis, we conducted a field study (Chapter 5) in the frame work of the Swiss Canopy Crane Project, where the tree crowns of a mature temperate forest are labeled with CO2 depleted in 13C. By employing a leaf litter exchange experiment, we were able to separate carbon fluxes originating from aboveground via leaf litter and from belowground via roots/root exudates. Compound specific 13C fatty acid analyses of leaf litter, soil, roots and soil animals in combination with the application of marker fatty acids for specific food sources allowed to separate energy fluxes through major channels of the decomposer food web, such as the ectomycorrhizal vs. saprotrophic fungi channel and the bacterial channel based on leaf litter or root exudates. Our findings suggest that root derived carbon is of major importance for the soil animal food web and that it mainly enters the food web via feeding on ectomycorrhizal fungi. In contrast to previous assumptions that forest soil food webs are mainly supported by the fungal energy channel, we also found considerable fluxes of energy through the bacterial channel, with all investigated predators containing significant amounts of bacterial marker fatty acids. Since systems based on multiple pathways of energy fluxes are assumed to be more stable, the partitioning between the fungal and bacterial channel presumably contributes to food web stability. By identifying trophic compartments and by tracing energy fluxes via different energy channels, results of this thesis represent major advances in the understanding of soil animal food web structure and functioning.
Keywords: Soil animals; fatty acid analysis; stable isotope analysis; 15N; 13C; food web; energy channels; bacteria; fungi
Other Languages
Waldboden-Nahrungsnetze sind komplexe und
heterogene Systeme. Trophische Beziehungen sind aufgrund der
geringen Größe der Bodentiere und deren kryptischer Lebensweise,
sowie des komplexen Gemisches basaler Ressourcen schwer ergründbar.
Das Zersetzersystem wird trotz seiner fundamentalen Rolle in
Zersetzungsprozessen und für das Funktionieren von
Schlagwörter: Bodentiere; Nahrungsnetze; Fettsäuremusteranalyse; Bakterien; Pilze