Data from: Widespread variation in stable isotope trophic position estimates: patterns, causes, and potential consequences

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.DESCRIPTIONS FOR "Review_Paper_Data.xlsx" FILE (also included in README.txt file) Sheet 1 - Complete Literature Search 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 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 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 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)