Reliability of isotopic fractionation (Delta N-15, Delta C-13) for the delimitation of trophic levels of oribatid mites: Diet strongly affects Delta C-13 but not Delta N-15

摘要

Stable isotope analysis has become an analytical tool of central importance in trophic ecology. The actual degree of isotopic fractionation, however, remains a black-box in most studies. Hence, mean values for trophic enrichment from other taxa are commonly used to delimitate trophic levels. On the other hand, resource composition as well as consumer physiology both influence fractionation patterns. Especially in soil food webs, high variability of isotopic fractionation has been found for both N-15 and C-13. Here, we investigate effects of diet on trophic enrichment in a no-choice feeding, experiment with the oribatid mite model species Archegozetes longisetosus and a set of fungal, animal, algal and plant resources (seeds and vegetative tissues). We found consistent trophic enrichment for nitrogen irrespective of diet (Delta N-15 = 3.9 parts per thousand), but no reliable fractionation for carbon being negative for fungi (Delta C-13 = 1.8 parts per thousand), neutral for algae, and positive for the remaining plant and animal resources (Delta C-13 = 2.3 parts per thousand). The results suggest that delta N-15 is a reliable marker for delimiting the trophic level of oribatid mites and presumably other soil detritivores, while understanding delta C-13 signals needs a priori knowledge about isotopic fractionation with respect to consumed resources. (C) 2016 Published by Elsevier Ltd.