Trophic hierarchies illuminated via amino acid isotopic analysis

Abstract

Food web ecologists have long sought to characterize the trophic niches of animals using stable isotopic analysis. However, distilling trophic position from isotopic composition has been difficult, largely because of the variability associated with trophic discrimination factors (inter-trophic isotopic fractionation and routing). We circumvented much of this variability using compound-specific isotopic analysis (CSIA). We examined the (15)N signatures of amino acids extracted from organisms reared in pure culture at four discrete trophic levels, across two model communities. We calculated the degree of enrichment at each trophic level and found there was a consistent trophic discrimination factor (~7.6‰). The constancy of the CSIA-derived discrimination factor permitted unprecedented accuracy in the measurement of animal trophic position. Conversely, trophic position estimates generated via bulk-(15)N analysis significantly underestimated trophic position, particularly among higher-order consumers. We then examined the trophic hierarchy of a free-roaming arthropod community, revealing the highest trophic position (5.07) and longest food chain ever reported using CSIA. High accuracy in trophic position estimation brings trophic function into sharper focus, providing greater resolution to the analysis of food webs.

Figure 1. Trophic discrimination factors.

Linear regression analysis of trophic discrimination factors (Δ¹⁵N) deriving from A) CSIA analysis, and B) bulk ¹⁵N analysis. Each point represents the trophic discrimination factor of an individual organism. Discrimination factors depicted at trophic levels 2, 3, and 4 represent the isotopic shifts from levels 1 to 2, 2 to 3, and 3 to 4, respectively.

Figure 2. Trophic position estimates, controlled-feeding trials.

Trophic position estimates (mean ± 1σ) from the controlled-feeding trials. A) Plots of observed trophic positions vs. their corresponding known trophic positions. Black circles indicate TP_glu-phe estimates, and gray triangles indicate TP_bulk estimates. The dotted line (y = x) represents perfect agreement between observed and known trophic positions. B) Degree of departure between the observed and known trophic positions (mean ± 1σ).

Figure 3. Trophic position estimates, apple orchard.

Trophic position estimates (mean ± 1σ) from an apple orchard food chain (pictured): apple leaves, apple aphid, hover fly, parasitoid, and hyperparasitoid. Observed trophic positions are plotted against expected trophic positions. Black circles and gray triangles respectively indicate TP_glu-phe and TP_bulk estimates. The dotted line (y = x) represents perfect agreement between observed and expected trophic positions.