Evaluating 15N/14N and 13C/12C isotope ratio analysis to investigate trophic relationships of elaterid larvae (Coleoptera: Elateridae)

Carbon and nitrogen isotope ratios in consumer tissues can be used to analyse the diet and trophic level of soil animals. However, life history traits may significantly influence stable isotope patterns. We evaluated in a series of experiments how stable isotope ratios of carbon (C-13/C-12) and nitrogen (N-15/N-14) at natural abundance can be used to study the diet and trophic position of long-lived macro-invertebrates, elaterid larvae, which are major below-ground herbivores. Small, but significant differences in delta C-13 signatures were found between the larvaes' anterior and posterior body segments, whereas exuvia reflected the body's overall isotopic composition. The species-specific trophic shift (+/- SE) in delta N-15 for Agriotes obscurus and Agriotes sputator (1.62 +/- 0.24 parts per thousand and 1.08 +/- 0.27 parts per thousand, respectively) was significantly lower than mean enrichment estimates reported in the literature, showing the limited applicability of such generalised estimates in studies of invertebrate trophic ecology. To avoid false-positive assignments to two trophic levels due to variation in delta N-15 values, a minimum sample size of three and five individuals for A. obscurus and A. sputator, respectively, was needed to reduce this risk to below alpha = 5%. Keeping elaterid larvae for up to 128 days without food did not affect their isotopic signatures, in contrast to previous studies on starving animals. Switching wireworms to isotopically different diets induced changes in their isotopic signatures within 2 weeks. Changes, however, were significant only when the isotopic difference between diets was large. We conclude that experimental studies evaluating how specific life history traits affect stable isotope signatures in consumers have to precede any interpretation of stable isotope data gathered in the field. (c) 2006 Elsevier Ltd. All rights reserved.