[MARMAM] New QFASA Diet Estimation Paper

Bromaghin, Jeffrey jbromaghin at usgs.gov
Thu Jul 14 11:26:28 PDT 2016

Dear colleagues,

I am pleased to announce the publication of a paper that compares options
for scaling fatty acid signatures prior to diet estimation using
quantitative fatty acid signature analysis (QFASA), a method that has been
widely utilized for marine species.


Bromaghin, J. F., S. M. Budge, and G. W. Thiemann. 2016. Should fatty acid
signature proportions sum to 1 for diet estimation? Ecological Research


Knowledge of predator diets, including how diets might change through time
or differ among predators, provides essential insights into their ecology.
Diet estimation therefore remains an active area of research within
quantitative ecology. Quantitative fatty acid signature analysis (QFASA) is
an increasingly common method of diet estimation. QFASA is based on a data
library of prey signatures, which are vectors of proportions summarizing
the fatty acid composition of lipids, and diet is estimated as the mixture
of prey signatures that most closely approximates a predator’s signature.
Diets are typically estimated using proportions from a subset of all fatty
acids that are known to be solely or largely influenced by diet. Given the
subset of fatty acids selected, the current practice is to scale their
proportions to sum to 1.0. However, scaling signature proportions has the
potential to distort the structural relationships within a prey library and
between predators and prey. To investigate that possibility, we compared
the practice of scaling proportions with two alternatives and found that
the traditional scaling can meaningfully bias diet estimators under some
conditions. Two aspects of the prey types that contributed to a
predator’s diet influenced the magnitude of the bias: the degree to which
the sums of unscaled proportions differed among prey types and the
identifiability of prey types within the prey library. We caution
investigators against the routine scaling of signature proportions in QFASA.


Jeffrey F. Bromaghin, PhD
Research Statistician
USGS Alaska Science Center
Marine Ecosystems Office
4210 University Drive
Anchorage, AK 99508
jbromaghin at usgs.gov
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