Grazing food web view from compound-specific stable isotope analysis of amino acids

Background/Question/Methods

Knowledge of the trophic position (TP) of organisms in food webs allows ecologists to track energy flow and trophic linkages among organisms in complex networks of ecosystems. Compound-specific stable isotope analysis (CSIA) of amino acids has been employed as a relatively new method with the high potential for accurate and precise estimates of the TP of organisms. This CSIA method is constructed based on contrasting isotopic fractionation during fundamental amino acid metabolisms between two common amino acids: glutamic acid (Glu) shows a significant ¹⁵N-enrichment (+8.0‰) whereas phenylalanine (Phe) shows little ¹⁵N-enrichment (+0.4‰), which commonly occurs both in aquatic and in terrestrial organisms. Recently, we have successfully established a general equation for the estimation of trophic position (TP) of organisms: TP = [(δ¹⁵N_Glu – δ¹⁵N_Phe + β)/7.6] + 1, where β represents the isotopic difference between glutamic acid and phenylalanine in aquatic algae (–3.4‰), and terrestrial C3 (+8.4‰) and C4 plants (–0.4‰). In this presentation, we demonstrate an application of this method for multiple species collected from coastal marine (a stony shore) and terrestrial (a fruit farm) environments, to obtain a fine view of the grazing food web structure in these ecosystems.

Results/Conclusions

The isotopic composition of phenylalanine ranged from +3.5 to +8.7‰ for a stony shore and from +3.0 to +17.0‰ for a fruit farm, representing much variation in the isotopic composition of primary producers at the bases of these food webs. However, since this background variation is negated in the principle of this method, a small error (less than 0.2 unit as 1σ) is accompanied with the TP estimated by the CSIA method. Based on the observed TP, we can present the trophic spectra for these species: 0.9-1.2 for macroalgae, 1.7-2.0 for gastropods and echinoids, 2.3-2.6 for crabs, and 2.9-4.6 for fish in the coastal marine; 1.0 for plants, 1.9-2.2 for caterpillars, bees, and butterflies, 2.9-3.1 for wasps and ladybugs, and 3.5-4.0 for hornets in the terrestrial food webs. Thus a fine view of the grazing food web structure in the ecosystems can be obtained from the accurate and precise estimation of TP for multiple species based on the CSIA method, which will be useful in better understanding energy flow and trophic linkages (e.g., trophic niche and function) among organisms in complex networks of ecosystems.