Trophic hierarchies revealed via amino acid isotopic analysis

Background/Question/Methods

To better predict the effects of biodiversity loss on ecosystem functioning, ecologists have endeavored to characterize the trophic roles of species using stable isotopic analysis. Despite the potential of isotopic methods to illuminate trophic function, accurate estimates of lifetime feeding tendencies have remained elusive. A relatively new approach—referred to as compound-specific isotopic analysis (CSIA)—has emerged, centering on the measurement of ¹⁵N:¹⁴N ratios in amino acids (glutamic acid and phenylalanine). CSIA has recently been used to generate trophic position estimates among animal species; however, the validity of such estimates is dependent on the consistency of the trophic enrichment factor (TEF). To address whether the TEF may scale with trophic level, we created model communities with discrete trophic groups (a basal resource, strict herbivores, strict predators, and predators of strict predators), and used CSIA to analyze the organisms representing these trophic levels. We then tested the method using wild specimens from a terrestrial ecosystem.

Results/Conclusions

We provide the first evidence that the TEF remains consistent across a wide range of trophic levels. Based on these findings and previous studies in aquatic and marine systems, the TEF (+7.6 ‰) appears to be a universal, non-scaling enrichment factor. Employing this parameter within formulae for trophic position estimation, we derived highly accurate estimates of animal trophic positions. These estimates permitted the assembly of trophic hierarchies, revealing the highest trophic position (5.07) and thus longest food chain ever measured with verifiable accuracy. Our findings indicate that CSIA is a valid empirical approach to the estimation of trophic positions among global fauna.