Do stable isotopes reflect the food web development in regenerating ecosystems?

We evaluated the use of delta N-15- and delta C-13-values to monitor the development of food web complexity and biodiversity in a regenerating ecosystem. Therefore a model food chain was established feeding cultivated woodlice (Porcellio dilatalus) on a cellulolytic fungus (Chaetomium globosum) grown on cellulose paper. Two diets of different quality (C:N ratios of 54 vs. 200) with different delta N-15- (1.3 parts per thousand vs. 3.1 parts per thousand) but identical delta C-13-values caused low and high dietary stress in animals of treatment A and B, respectively. After an incubation time of 7 weeks amount, elemental and isotopic composition of collected faeces and exuviae as well as woodlice and remaining food were determined. The increase of delta N-13-values of woodlice relative to the diet was 5.7 parts per thousand and 2.5 parts per thousand in treatments A and B, respectively, whereas delta C-13-sfiifts were 1.0 parts per thousand and 1.6 parts per thousand, showing a reverse relationship. Modelling of elemental and isotopic mass balances indicated that faeces recycling explains the unexpected high N-15-enrichments. Moreover, C-13-enrichmentts were positively correlated to the degree of starvation. Considering the effects of starvation and recycling of faeces, stable isotopes represent a useful tool to elucidate trophic interactions in regenerating food webs.