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

Abstract

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 data sets. We identified 10 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.

Amino acid nitrogen and carbon isotope data: Potential and implications for ecological studies

“…Based on the consistency in trophic AA and source AA δ 15 N, an AA‐based TP (TP AA ) equation was developed from a bulk SIR‐based TP equation, as shown in Figure 5a (Chikaraishi et al, 2009 ). The TP AA is a transformative approach of trophic AA δ 15 N for consumer that is internally indexed to baseline component (or nutritional resource) with source AA δ 15 N variable (Kjeldgaard et al, 2021 ; McMahon & McCarthy, 2016 ; Ohkouchi et al, 2017 ; Ramirez et al, 2021 ). It is applicable for whole bodies (or any proteinaceous tissue types) from environmental samples to zooplankton and higher trophic‐level fish and apex predators (Cherel et al, 2019 ; Choi & Shin, 2021 ; Choi et al, 2020 ; Germain et al, 2013 ; Laiz‐Carrión et al, 2019 ; Mompean et al, 2016 ; Richards et al, 2020 ).…”

“…It is very important to obtain proper TDF and β parameters, fitted to AA pair, from lab‐/field‐collected or literature data. Such diverse trials produce more realistic TP estimates matched with feeding habits of the consumer widely known (Kjeldgaard et al, 2021 ; Ledesma et al, 2020 ; Matthews et al, 2020 ; Ohkouchi et al, 2017 ; Ramirez et al, 2021 ; Troina et al, 2021 ). For example, in point of δ 15 N Glx , protists (heterotrophic microconsumers) show quite similar to phytoplankton (microalga Dunaliella ) (Decima & Landry, 2020 ; Decima et al, 2017 ; Gutierrez‐Rodriguez et al, 2014 ).…”

“…and β parameters, fitted to AA pair, from lab-/field-collected or literature data. Such diverse trials produce more realistic TP estimates matched with feeding habits of the consumer widely known (Kjeldgaard et al, 2021;Ledesma et al, 2020;Matthews et al, 2020;Ohkouchi et al, 2017;Ramirez et al, 2021;Troina et al, 2021).…”

Variation in the stable isotope trophic position of the bluefish Pomatomus saltatrix (Linnaeus, 1766) from two Mediterranean sites: insights from a global meta-analysis

“…A ∆ 15 N = 3.4‰ was used (Post, 2002), while trophic levels for baseline forage species in the Mediterranean Sea were taken from Karachle & Stergiou (2008;, with λ = 3.38 for E. encrasicolus (Adriatic Sea) and λ = 3.42 for L. mormyrus (Ionian Sea), assuming basal resources = trophic level 1, primary consumers = trophic level 2, etc. These values were chosen following the recommendations by Mancinelli et al (2013) and Kjeldgaard et al (2021) as they were consistent with previously published isotopic estimations for both E. encrasicolus (e.g., 3.3: Rumolo et al, 2016) and L. mormyrus (3 -3.8: Mancinelli et al, 2016;Signa et al, 2017). Bluefish TP values were calculated for each specimen to consider inter-individual variations in trophic habits; for the global dataset, estimations were made at each location using the mean δ 15 N values reported for bluefish and baseline species.…”

Changes In Argentine Ant Trophic Position As A Function of Time

“…To obtain baseline corrected δ15N values, we subtracted the δ15N value of the plant material from each site from the respective δ15N value of the ants. Although baseline corrections have limitations (Kjeldgaard et al 2021), δ 15 N plant values are based on leaves from three individuals of each of two to three perennial plant species (Table 1) to ensure representative estimates of δ 15 N plant values fo reach site. Prior to performing statistical tests, we con rmed that our data met the assumptions of the models used.…”