Biotracers are commonly used in food-web studies to estimate consumer diets. Increasingly, multiple biotracer types are combined to provide more-nuanced pictures of consumer resource use, unravel more-complex diet mixtures, and improve the discriminating power of mixing models. However, few studies compare the results of different biotracer types, including the impact of tracer selection choices, and few methods exist for combining multiple types into a single analysis.
We collected three biotracers types (stable isotopes, fatty acids, and amino acids) from the same samples to determine how common galaxias (Galaxias maculatus) and flathead gudgeon (Philypnodon grandiceps) utilize different basal resources (algae, macrophytes, and detritus) using Bayesian mixing models. We then combined subsets of different biotracer types by either combining them directly into one model using generalist priors (all possible two- and three-biotracer type combinations) or by using informative priors based on the results of the other biotracer types. We compared our biotracer mixing model results to independent food webs constructed from gut contents to assess their accuracy.
Our study found that fatty acids and stable isotopes gave the most accurate diet estimates, while amino acids models were the least accurate. Overall model precision depended on the number of individual tracers included. We found that adding biotracer types that produced similar diet estimates in their respective separate models tightened the posterior probability distributions of combined models, while adding ones that disagreed expanded those distributions, as expected. We also demonstrate the utility of a new complementary method that combines information from multiple biotracers using informative priors based on other biotracer types.
Overall, our work shows that choices made during the construction of mixing models can greatly influence resultant diet estimates and we recommend an approach to assessing those choices so as to be most appropriate for an individual study’s objectives and available data.