DOI: 10.5061/dryad.mpg4f4qxp
title: Data from: Widespread variation in stable isotope trophic position estimates: patterns, causes, and potential consequences

description: Stable isotope analysis is one of the most widely used techniques to
 estimate trophic position and provides fundamental insight into the
 structure and management of ecological communities. To account for the
 effects of geographic variation in isotope levels, trophic position is
 typically estimated relative to an isotope “baseline” (i.e., material
 representing geographic variation) using a methodology such as a formula
 or statistical analysis. There is, however, remarkable variation in the
 baselines and methodologies used to estimate trophic position from stable
 isotopes. The consequences of this lack of standardization are unknown but
 could result in biased or erroneous conclusions. We conducted a literature
 review to quantify the variation in baselines and methodologies used to
 estimate trophic position from stable isotopes. Next, we assessed the
 consequences of this variation on individual species estimates and food
 web structure by extracting published trophic positions and applying
 various baselines and methodologies to existing datasets. We identified
 ten baselines and eight methodologies, the use of which varied by
 ecosystem studied. Moreover, we found that different baselines and
 methodologies yield significantly different trophic position estimates for
 individual species, as well as different conclusions about food web
 structure. Authors should avoid biological interpretations of absolute,
 stand-alone trophic positions (as these are prone to a number of biases).
 We recommend pairing stable isotope analysis with other techniques for
 more robust conclusions. Increased sample size may mitigate some of the
 variation caused by different baselines and methodologies; however, an
 alarmingly large proportion of studies collected only one sample in at
 least one trophic group (41% of all reviewed studies). Authors should
 collect a minimum of five samples per trophic group (but ten for
 best-practices) from as many trophic groups as possible to increase
 statistical power and redundancy in comparisons. When sample size is
 unavoidably constrained, we recommend using compound-specific isotope
 analysis with a taxon-specific trophic discrimination factor, because it
 may be more accurate and require fewer samples to maintain appropriate
 statistical power. Implementing our recommendations will increase the
 robustness and accuracy of conclusions based on stable isotopes, resulting
 in better management decisions and a more accurate understanding of
 ecological communities.
description: DESCRIPTIONS FOR "Review_Paper_Data.xlsx" FILE (also
 included in README.txt file) Sheet 1 - Complete
 Literature Search<br> Title, authors, DOI, year published, abstract,
 and ecosystem type from the complete search on Web of Science. All
 repeated entries were deleted. Ecosystem was specified based on the search
 (aquatic or terrestrial) as described in the manuscript, as well as
 manually if any paper was incorrectly specified. Information was
 downloaded as a txt file from Web of Science. Abstracts were searched for
 species names to create a species list of commonly-studied taxa in the
 literature. Sheet 2 - Reviewed Literature<br>
 This is a list of 200 papers that were reviewed from the overall stable
 isotope literature. Citations for each paper (name in the first column)
 can be found in Appendix S1. Data that were extracted from each paper
 include the baseline & methodology used to estimate trophic
 position from stable isotope data. Sometimes a combination of
 methodologies were used to estimate trophic position, in which case
 "combination" was given as the methodology, and the
 methodologies were given in the "If combination" column. If
 statistics were used as a methodology, the statistics are described in the
 "If statistics" column. Keywords used in the Web of Science
 search are included in the final column. Sheet 3 -
 Extracted Trophic Positions<br> Published trophic position estimates
 from stable isotope data were extracted from the literature. Citations for
 each paper (name in the first column) can be found in Appendix S2.
 Estimated trophic position and the methodology & baseline used to
 calculate each trophic position were extracted from each study. Raw
 numbers were extracted whenever possible, but sometimes only averages were
 given in the study, in which case standard deviation or standard error
 were included whenever available. To simplify the analysis, we included a
 "Simplified baseline" column where the baseline used by authors
 was simplified to trophic group (e.g., primary consumer or basal
 resource). Several papers were conducted over a long time span, so the
 starting and ending years of sampling were included to determine if the
 methodology & baseline used were affected by the year the study
 was conducted. Sheet 4 - Analyzed Datasets<br>
 Nine stable isotope datasets were downloaded, and trophic position was
 estimated using several different methodologies and baselines (whenever
 possible) to determine the effect of methodology and baseline on
 estimates. Information for how each estimate was calculated can be found
 in the manuscript, but raw d15N values were used in each calculation. Raw
 d15N values are included in this file & specified under the column
 "Methodology used to calculate TP". The "Type" column
 includes the classification for each taxa reported in the original study.
 The "Type_plot" column is a simplified classification used in
 the plots in our manuscript wherein the number indicates the trophic group
 and the other part of the classification indicates the type of material
 (e.g., 1.Benthic is a basal resource [trophic group = 1] collected from
 the benthic zone).