Best practices for use of stable isotope mixing models in food-web studies

Abstract

Stable isotope mixing models are increasingly used to quantify consumer diets, but may be misused and misinterpreted. We address major challenges to their effective application. Mixing models have increased rapidly in sophistication. Current models estimate probability distributions of source contributions, have user-friendly interfaces, and incorporate complexities such as variability in isotope signatures, discrimination factors, hierarchical variance structure, covariates, and concentration dependence. For proper implementation of mixing models, we offer the following suggestions. First, mixing models can only be as good as the study and data. Studies should have clear questions, be informed by knowledge of the system, and have strong sampling designs to effectively characterize isotope variability of consumers and resources on proper spatio-temporal scales. Second, studies should use models appropriate for the question and recognize their assumptions and limitations. Decisions about source grouping or incorporation of concentration dependence can influence results. Third, studies should be careful about interpretation of model outputs. Mixing models generally estimate proportions of assimilated resources with substantial uncertainty distributions. Last, common sense, such as graphing data before analyzing, is essential to maximize usefulness of these tools. We hope these suggestions for effective implementation of stable isotope mixing models will aid continued development and application of this field.

Potential of submerged macrophytes to support food webs in lowland agricultural streams

“…All δ 13 C data were normalised for lipid content using formulae provided by Post et al (2007), because the study included a range of consumers with a range in C:N ratios, and thus variation in δ 13 C-depleted lipids which may bias results (Post et al 2007;Phillips et al 2014). We also used concentration-dependent source data in mixing models due to differing C:N ratios of sources which contradicts the model assumption that all sources contribute equal proportions of C and N to consumers (Phillips and Koch 2002).…”

“…Reducing the number of sources increases the discriminatory power of mixing models (Phillips et al 2005(Phillips et al , 2014. The same two-factor ANOVA was also used to compare means of C:N ratios for basal resources, followed by Tukey's post hoc test for multiple comparisons of sources.…”

“…Although consumer diets can vary temporally (eg. Beatty 2006;Reid et al 2008b), this study focussed on the relative importance of macrophytes during their peak growth period, and the tissues analyzed have a short metabolic turnover (Phillips et al 2014), so represent diets during this period.…”

“…The development of modern mixing models has facilitated the use of stable isotopes to examine relative contributions to animal diets from multiple sources (Moore and Semmens 2008;Phillips et al 2014), and recent studies have shown that macrophytes can be an important basal resource in several Australian streams (Deegan and Ganf 2008;Reid et al 2008b;Watson and Barmuta 2011). This study used stable isotope analysis to examine the importance of submerged macrophytes as a basal source for stream food webs in reaches with varying riparian vegetation condition.…”

The role of feeding morphology and competition in governing the diet breadth of sympatric stomatopod crustaceans

“…Seven individuals of each of the two stomatopod species as well as eight potential prey types: alpheid shrimp, clams, crabs, hermit crabs, fish, planktonic crustaceans, snails and worms (6)(7)(8)(9)(10)(11)(12)(13)(14)(15) Bayesian stable isotope mixing models in the program Mix-SIAR v. 3.1.6 [15] were used to estimate the proportion of different prey in the diet for each stomatopod species (reviewed in [16]). Experimentally determined trophic discrimination factors (D, the difference between the predator and prey stable isotope ratios) for stomatopods, hereafter denoted as 'experimental DFs', were used in the mixing models (D 15 …”