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

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.