Abstract
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 in AA isotope fractionation in birds. We conducted a controlled CSIA feeding experiment on an avian species, the gentoo penguin (Pygoscelis papua), to examine patterns in individual AA carbon and nitrogen stable isotope fractionation between diet (D) and consumer (C) (Δ13CC-D and Δ15NC-D, respectively). We found that essential AA δ13C values and source AA δ15N 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 nonessential AA Δ13CC-D values 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-Phe equation with the avian-specific TDFGlu-Phe value from our experiment provided estimates that were more ecologically realistic than estimates using a single TDFGlu-Phe of 7.6‰ from the previous literature. Our results provide a quantitative, mechanistic framework for the use of CSIA in nonlethal, archival feathers to study the movement and foraging ecology of avian consumers.
Highlights
Resource acquisition and allocation are fundamental requirements for all animals and significantly influence the behavior of individuals, the ecological and evolutionary trajectories of populations, and the functioning and resilience of entire ecosystems (Paine 1966; Polis and Strong 1996; Tinker et al 2008)
We found that essential amino acids (AA) d13C values and source AA (Phe) d15N values in feathers showed little fractionation between diet and consumer, and provide excellent proxies of d13Cbaseline and d15Nbaseline in a non lethal, archival tissue that is widely used in avian ecology
Nonessential AA D13CC-D values varied significantly according to their biosynthesis pathway and reflected the macromolecule sources being utilized for biosynthesis
Summary
Resource acquisition and allocation are fundamental requirements for all animals and significantly influence the behavior of individuals, the ecological and evolutionary trajectories of populations, and the functioning and resilience of entire ecosystems (Paine 1966; Polis and Strong 1996; Tinker et al 2008). There is, for instance, both theoretical and empirical support for the roles of food chain length and primary producer composition in structuring food web architecture, mediating the relationship between species diversity and ecosystem function, and regulating fisheries productivity and biogeochemical fluxes (Post 2002a; Vander Zanden and Fetzer 2007; Young et al 2013). The ability to quantify food web architecture is critical to understanding ecosystem structure and function, in light of past and future changes in climate and anthropogenic disturbance. Stable isotope analysis (SIA) has become a widely used tool for examining food web architecture across diverse a 2015 The Authors.
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