Amino Acid-Specific Nitrogen Isotope Discrimination in Raptor Nestlings: Advancing Trophic Position Estimation

Dr. Julian C. Holbrook; Dr. Aris B. Kian; Prof. Elena V. Petrov

Authors

Dr. Julian C. Holbrook Department of Isotope Science, Arctic Ecology Research Centre, Fairbanks, USA
Dr. Aris B. Kian School of Biological Sciences, University of Copenhagen, Copenhagen, Denmark
Prof. Elena V. Petrov Faculty of Wildlife Management, Siberian Institute of Ecological Studies, Tomsk, Russia

Keywords:

Trophic position, stable isotopes, amino acid, raptors, Gyrfalcon, trophic discrimination factor, Falco rusticolus

Abstract

Background: Stable isotope analysis is a cornerstone of trophic ecology, yet estimating trophic position (TP) accurately is often hindered by variable bulk tissue nitrogen trophic discrimination factors (TDFs). Amino acid-specific compound-specific isotope analysis (AA-CSIA) offers enhanced resolution by differentiating metabolically distinct amino acids, providing a more precise tool for tracing nitrogen flow through food webs. Empirical data on AA-specific TDFs, particularly in developing avian predators, remains limited.

Objectives: This study aimed to determine amino acid-specific nitrogen stable isotope trophic discrimination factors (Δ15NAA) in raptor nestlings under controlled dietary conditions. We evaluated the variability of these TDFs among different amino acids and assessed their implications for refining trophic position estimation in these ecologically significant predators.

Methods: We reared raptor nestlings on a precisely controlled diet with a known isotopic signature. Tissues (e.g., blood, feathers) were collected at various developmental stages. Amino acids were extracted, derivatized, and analyzed for their δ15N values using gas chromatography/combustion/isotope ratio mass spectrometry. Δ15NAA values were calculated by comparing nestling tissue and diet amino acid δ15N values. Trophic positions were then estimated using established AA-CSIA models.

Results: Significant variability in Δ15NAA was observed across different amino acids, with 'trophic' amino acids showing consistent enrichment relative to 'source' amino acids. These empirically derived TDFs allowed for more accurate and less variable trophic position estimates in nestlings compared to those derived from bulk tissue analysis. Our findings provide species-specific TDFs critical for robust trophic modeling.

Conclusion: This study establishes precise amino acid-specific nitrogen trophic discrimination factors for raptor nestlings, significantly enhancing the accuracy of trophic position estimation in these crucial avian predators. Integrating these refined TDFs into ecological studies will improve our understanding of raptor foraging ecology, food web dynamics, and conservation strategies in a changing environment.

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