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Trophic discrimination of amino acid-specific nitrogen stable isotopes in raptor nestlings: implications for estimating trophic position

Bautista, Anna1; Johnson, Devin2; Henderson, Michael3; Anderson, David3; Booms, Travis4; Williams, Cory1

Published Apr 08, 2025 on Dryad. https://doi.org/10.5061/dryad.wwpzgmstq

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Apr 08, 2025 version files 152.90 KB

Abstract

Bulk stable isotope analysis of carbon and nitrogen is commonly used to assess trophic relationships among organisms in a wide variety of ecosystems. However, compound-specific stable isotope analysis of individual amino acids may be a more accurate approach for resolving food web structure. We compared the applicability of bulk and compound specific methods in an Arctic tundra ecosystem. Specifically, we determined bulk δ13C and δ15N, as well as amino acid specific δ15N, in red blood cells from nestling raptors and muscle samples from common prey species. We subsequently estimated bulk and amino-acid specific trophic discrimination factors (TDFs) for nitrogen in gyrfalcons (Falco rusticolus) using high-precision diet estimates from nest cameras to compare against TDFs in the literature. Although bulk δ15N values of secondary consumers were enriched relative to primary consumers, overlap occurred across groups and greater separation was apparent using δ15N of amino acids. Comparing red blood cell δ15N values to prey muscle δ15N values, we found that bulk TDFs and compound-specific TDFs for trophic amino acids were lower than values from the literature that have typically been used to estimate trophic position. Our findings suggest that while δ15N values of amino acids have the potential to improve our assessment of trophic relationships, the assumption that amino acid specific TDFs remain constant across all consumers and diet types is violated for raptor nestlings, such that consumer-specific TDFs must be calculated to accurately estimate trophic position. Use of “universal” TDFGlu-Phe values will result in an underestimate of nestling trophic level. Diet estimates for nestlings based on stable isotope mixing models that use adult TDFs for will be inaccurate.