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Title: Carbon and nitrogen isotope fractionation of amino acids in an avian marine predator, the gentoo penguin ( Pygoscelis papua )

Compound‐specific stable isotope analysis (CSIA) of amino acids (AA) has rapidly become a powerful tool in studies of food web architecture, resource use, and biogeochemical cycling. However, applications to avian ecology have been limited because no controlled studies have examined the patterns inAAisotope fractionation in birds. We conducted a controlledCSIAfeeding experiment on an avian species, the gentoo penguin (Pygoscelis papua), to examine patterns in individualAAcarbon and nitrogen stable isotope fractionation between diet (D) and consumer (C) (Δ13CC‐Dand Δ15NC‐D, respectively). We found that essentialAAδ13C values and sourceAAδ15N values in feathers showed minimal trophic fractionation between diet and consumer, providing independent but complimentary archival proxies for primary producers and nitrogen sources respectively, at the base of food webs supporting penguins. Variations in nonessentialAAΔ13CC‐Dvalues reflected differences in macromolecule sources used for biosynthesis (e.g., protein vs. lipids) and provided a metric to assess resource utilization. The avian‐specific nitrogen trophic discrimination factor (TDFGlu‐Phe= 3.5 ± 0.4‰) that we calculated from the difference in trophic fractionation (Δ15NC‐D) of glutamic acid and phenylalanine was significantly lower than the conventional literature value of 7.6‰. Trophic positions of five species of wild penguins calculated using a multi‐TDFGlu‐Pheequation with the avian‐specificTDFGlu‐Phevalue from our experiment provided estimates that were more ecologically realistic than estimates using a singleTDFGlu‐Pheof 7.6‰ from the previous literature. Our results provide a quantitative, mechanistic framework for the use ofCSIAin nonlethal, archival feathers to study the movement and foraging ecology of avian consumers.

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Author(s) / Creator(s):
 ;  ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Ecology and Evolution
Medium: X Size: p. 1278-1290
["p. 1278-1290"]
Sponsoring Org:
National Science Foundation
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  1. Rationale

    Ecologists increasingly determine the δ15N values of amino acids (AA) in animal tissue; “source” AA typically exhibit minor variation between diet and consumer, while “trophic” AA have increased δ15N values in consumers. Thus, trophic‐source δ15N offsets (i.e., Δ15NT‐S) reflect trophic position in a food web. However, even minor variations in δ15Nsource AAvalues may influence the magnitude of offset that represents a trophic step, known as the trophic discrimination factor (i.e., TDFT‐S). Diet digestibility and protein content can influence the δ15N values of bulk animal tissue, but the effects of these factors on AA Δ15NT‐Sand TDFT‐Sin mammals are unknown.


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    Diets were less digestible and included more protein in Experiment A than in Experiment B. As a result, the mice in Experiment A probably oxidized more AA, resulting in greater Δ15NConsumer‐Dietvalues. However, the similar responses of Δ15NT‐Sand of TDFT‐Sto diet variation suggest that if diet samples are available, Δ15NT‐Saccurately tracks trophic position. If diet samples are not available, the patterns presented here provide a basis to interpret Δ15NT‐Svalues. The trophic‐source offset of Pro‐Lys did not vary across diets, and therefore may be more reliable for omnivores than other offsets (e.g., Glu‐Phe).

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  2. Abstract

    Compound‐specific stable isotope analysis of individual amino acids (CSIA‐AA) has emerged as a transformative approach to estimate consumer trophic positions (TPCSIA) that are internally indexed to primary producer nitrogen isotope baselines. Central to accurate TPCSIAestimation is an understanding of beta (β) values—the differences between trophic and source AA δ15N values in the primary producers at the base of a consumers’ food web. Growing evidence suggests higher taxonomic and tissue‐specificβvalue variability than typically appreciated.

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